Chapter 12

Drying and Control of Moisture Content and Dimensional Changes

William T. Simpson

In the living tree, wood contains large quantities of water. As green wood dries, most of the water is removed. The moisture remaining in the wood tends to come to equilibrium with the relative humidity of the surrounding air. Correct drying, handling, and storage of wood will minimize moisture content changes that might occur after drying when the wood is in service and such changes are undesirable. If moisture content is controlled within reasonable limits by such methods, major problems from dimensional changes can usually be avoided.

The discussion in this chapter is concerned with moisture content determination, recommended moisture content values, drying methods, methods of calculating dimensional changes, design factors affecting such changes in structures, and moisture content control during transit, storage, and construction. Data on green moisture content, fiber saturation point, shrinkage, and equilibrium moisture content are given with information on other physical properties in Chapter 3.

Wood in service is virtually always undergoing at least slight changes in moisture content. Changes in response to daily humidity changes are small and usually of no consequence. Changes that occur as a result of seasonal variation, although gradual, tend to be of more concern. Protective coatings can retard dimensional changes in wood but do not prevent them. In general, no significant dimensional changes will occur if wood is fabricated or installed at a moisture content corresponding to the average atmospheric conditions to which it will be exposed. When incompletely dried material is used in construction, some minor dimensional changes can be tolerated if the proper design is used.

Determination of Moisture Content

The amount of moisture in wood is ordinarily expressed as a percentage of the weight of the wood when ovendry. Four methods of determining moisture content are covered in ASTM D4442. Two of these—the oven-drying and the electrical methods—are described in this chapter.

The oven-drying method has been the most universally accepted method for determining moisture content, but it is slow and necessitates cutting the wood. In addition, the oven-drying method may give values slightly greater than true moisture content with woods containing volatile extractives. The electrical method is rapid, does not require cutting the wood, and can be used on wood in place in a structure. However, considerable care must be taken to use and interpret the results correctly. Use of the electrical method is generally limited to moisture content values less than 30%.

Oven-Drying Method

In the oven-drying method, specimens are taken from representative boards or pieces of a quantity of lumber. With lumber, the specimens should be obtained at least 500 mm (20 in.) from the end of the pieces. They should be free from knots and other irregularities, such as bark and pitch pockets. Specimens from lumber should be full cross sections and 25 mm (1 in.) long. Specimens from larger items may be representative sectors of such sections or subdivided increment borer or auger chip samples. Convenient amounts of chips and particles can be selected at random from larger batches, with care taken to ensure that the sample is representative of the batch. Veneer samples should be selected from four or five locations in a sheet to ensure that the sample average will accurately indicate the average of the sheet.

Each specimen should be weighed immediately, before any drying or reabsorption of moisture has taken place. If the specimen cannot be weighed immediately, it should be placed in a plastic bag or tightly wrapped in metal foil to protect it from moisture change until it can be weighed. After weighing, the specimen is placed in an oven heated to 101°C to 105°C (214°F to 221°F) and kept there until no appreciable weight change occurs in 4-h weighing intervals. A lumber section 25 mm (1 in.) along the grain will reach a constant weight in 12 to 48 h. Smaller specimens will take less time. The constant or ovendry weight and the weight of the specimen when cut are used to determine the percentage of moisture content using the formula

(12–1)

Electrical Method

The electrical method of determining the moisture content of wood uses the relationships between moisture content and measurable electrical properties of wood, such as conductivity (or its inverse, resistivity), dielectric constant, or power-loss factor. These properties vary in a definite and predictable way with changing moisture content, but correlations are not perfect. Therefore, moisture determinations using electrical methods are always subject to some uncertainty.

Electric moisture meters are available commercially and are based on each of these properties and identified by the property measured. Conductance-type (or resistance) meters measure moisture content in terms of the direct current conductance of the specimen. Dielectric-type meters are of two types. Those based principally on dielectric constant are called capacitance or capacitive admittance meters; those based on loss factor are called power-loss meters.

The principal advantages of the electrical method compared with the oven-drying method are speed and convenience. Only a few seconds are required for the determination, and the piece of wood being tested is not cut or damaged, except for driving electrode needle points into the wood when using conductance-type meters. Thus, the electrical method is adaptable to rapid sorting of lumber on the basis of moisture content, measuring the moisture content of wood installed in a building, or establishing the moisture content of a quantity of lumber or other wood items, when used in accordance with ASTM D4442.

For conductance meters, needle electrodes of various lengths are driven into the wood. There are two general types of electrodes: insulated and uninsulated. Uninsulated electrodes will sense the highest moisture content along their length (highest conductance). Moisture gradients between the surface and the interior can lead to confusion. If the wood is wetter near the center than the surface, which is typical for drying wood, the reading will correspond to the depth of the tip of the insulated electrodes. If a meter reading increases as the electrodes are being driven in, then the moisture gradient is typical. In this case, the pins should be driven in about one-fifth to one-fourth the thickness of the wood to reflect the average moisture content of the entire piece. Dried or partially dried wood sometimes regains moisture in the surface fibers, and the surface moisture content is greater than the interior. In this case, the meter with the uninsulated pins will read the higher moisture content surface, possibly causing a significant deviation from the average moisture content. To guard against this problem, electrodes with insulated shanks have been developed. They measure moisture content of only the wood at the tips of the electrodes.

Dielectric-type meters are fitted with surface contact electrodes designed for the type of specimen material being tested. The electric field from these electrodes penetrates well into the specimen, but with a strength that decreases rapidly with depth of penetration. For this reason, the readings of dielectric meters are influenced predominantly by the surface layers of the specimen, and the material near midthickness may not be adequately represented in the meter reading if there is a moisture content gradient.

To obtain accurate moisture content values, each instrument should be used in accordance with its manufacturer’s instructions. The electrodes should be appropriate for the material being tested and properly oriented according to meter manufacturer’s instructions. The readings should be carefully taken as soon as possible after inserting the electrode. A species correction supplied with the instrument should be applied when appropriate. Temperature corrections should then be made if the temperature of the wood differs considerably from the temperature of calibration used by the manufacturer.

Approximate corrections for conductance-type (resistance) meters are made by adding or subtracting about 0.5% for each 5.6°C (10°F) the wood temperature differs from the calibration temperature. The correction factors are added to the readings for temperatures less than the calibration temperature and subtracted from the readings for temperatures greater than the calibration temperature. Temperature corrections for dielectric meters are rather complex and are best made from published charts (James 1988).

Although some meters have scales that go up to 120%, the range of moisture content that can be measured reliably is 4% to about 30% for commercial dielectric meters and about 6% to 30% for resistance meters. The precision of the individual meter readings decreases near the limits of these ranges. Readings greater than 30% must be considered only qualitative. When the meter is properly used on a quantity of lumber dried to a reasonably constant moisture content below fiber saturation, the average moisture content from the corrected meter readings should be within 1% of the true average.

Recommended Moisture Content

Wood should be installed at moisture content levels as close as possible to the average moisture content it will experience in service. This minimizes the seasonal variation in moisture content and dimension after installation, avoiding problems such as floor buckling or cracks in furniture. The in-service moisture content of exterior wood (siding, wood trim) primarily depends on the outdoor relative humidity and exposure to rain or sun. The in-service moisture content of interior wood primarily depends on indoor relative humidity, which in turn is a complex function of moisture sources, ventilation rate, dehumidification (for example, air conditioning), and outdoor humidity conditions.

The recommended values for interior wood presented in this chapter are based on measurements in well-ventilated buildings without unusual moisture sources and without air conditioning. In air-conditioned buildings, moisture conditions depend to a great extent on the proper sizing of the air-conditioning equipment. Wood installed in basements or over a crawl space may experience a moisture content greater than the range provided, and wood in insulated walls or roofs and attics may experience a moisture content greater or less than the range. Nevertheless, the recommended values for installation provide a useful guideline.

Timbers

Ideally, solid timbers should be dried to the average moisture content they will reach in service. Although this optimum is possible with lumber less than 76 mm (3 in.) thick, it is seldom practical to obtain fully dried timbers, thick joists, and planks. When thick solid members are used, some shrinkage of the assembly should be expected. In the case of built-up assemblies, such as roof trusses, it may be necessary to tighten bolts or other fastenings occasionally to maintain full bearing of the connectors as the members shrink.

Lumber

The recommended moisture content of wood should be matched as closely as is practical to the equilibrium moisture content (EMC) conditions in service. Table 12–1 shows the EMC conditions in outdoor exposure in various U.S. cities for each month. The EMC data are based on the average relative humidity and temperature data (30 or more years) available from the National Climatic Data Center of the National Oceanic and Atmospheric Administration. The relative humidity data were the average of the morning and afternoon values, and in most cases would be representative of the EMC attained by the wood. However, in some locations, early morning relative humidity may occasionally reach 100%. Under these conditions, condensation may occur and the surface fibers of wood will exceed the EMC. The moisture content requirements are more exacting for finished lumber and wood products used inside heated and air-conditioned buildings than those for lumber used outdoors or in unheated buildings. For general areas of the United States, the recommended moisture content values for wood used inside heated buildings are shown in Figure 12–1. Values and tolerances for both interior and exterior uses of wood in various forms are given in Table 12–2. If the average moisture content is within 1% of that recommended and all pieces fall within the individual limits, the entire lot is probably satisfactory.

General commercial practice is to kiln dry wood for some products, such as flooring and furniture, to a slightly lower moisture content than service conditions demand, anticipating a moderate increase in moisture content during processing and construction. This practice is intended to ensure uniform distribution of moisture among the individual pieces. Common grades of softwood lumber and softwood dimension lumber are not normally dried to the moisture content values indicated in Table 12–2. Dry lumber, as defined in the American Softwood Lumber Standard, has a maximum moisture content of 19%. Some industry grading rules provide for an even lower maximum. For example, to be grade marked KD 15, the maximum moisture content permitted is generally 15%.

Glued Wood Products

When veneers are bonded with cold-setting adhesives to make plywood, they absorb comparatively large quantities of moisture. To keep the final moisture content low and to minimize redrying of the plywood, the initial moisture content of the veneer should be as low as practical. However, very dry veneer is brittle and difficult to handle without damage, so the minimum practical moisture content is about 4%. Freshly glued plywood intended for interior service should be dried to the moisture content values given in Table 12–2.

Table 12–1. Equilibrium moisture content of wood, exposed to outdoor atmosphere, in several U.S. locations in 1997

Figure 12–1. Recommended average moisture content for interior use of wood products in various areas of the United States.

Hot-pressed plywood and other board products, such as particleboard and hardboard, usually do not have the same moisture content as lumber. The high temperatures used in hot presses cause these products to assume a lower moisture content for a given relative humidity. Because this lower equilibrium moisture content varies widely, depending on the specific type of hot-pressed product, it is recommended that such products be conditioned at 30% to 40% relative humidity for interior use and 65% for exterior use.

Lumber used in the manufacture of large laminated members should be dried to a moisture content slightly less than the moisture content expected in service so that moisture absorbed from the adhesive will not cause the moisture content of the product to exceed the service value. The range of moisture content between laminations assembled into a single member should not exceed 5 percentage points. Although laminated members are often massive and respond rather slowly to changes in environmental conditions, it is desirable to follow the recommendations in Table 12–2 for moisture content at time of installation.

Drying of Wood

Drying is required for wood to be used in most products. Dried lumber has many advantages over green lumber for producers and consumers. Removal of excess water reduces weight, thus shipping and handling costs. Proper drying confines shrinking and swelling of wood in use to manageable amounts under all but extreme conditions of relative humidity or flooding. As wood dries, most of its strength properties increase, as well as its electrical and thermal insulating properties. Properly dried lumber can be cut to precise dimensions and machined more easily and efficiently; wood parts can be more securely fitted and fastened together with nails, screws, bolts, and adhesives; warping, splitting, checking, and other harmful effects of uncontrolled drying are largely eliminated; and paint, varnish, and other finishes are more effectively applied and maintained. Wood must be relatively dry before it can be glued or treated with decaypreventing and fire-retardant chemicals.

The key to successful and efficient drying is control of the drying process. Timely application of optimum or at least adequate temperature, relative humidity, and air circulation conditions is critical. Uncontrolled drying leads to drying defects that can adversely affect the serviceability and economics of the product. The usual strategy is to dry as fast as the particular species, thickness, and end-product requirements allow without damaging the wood. Slower drying can be uneconomical as well as introduce the risk of stain.

Softwood lumber intended for framing in construction is usually targeted for drying to an average moisture content of 15%, not to exceed 19%. Softwood lumber for many other uses is dried to a low moisture content, 10% to 12% for many appearance grades to as low as 7% to 9% for furniture, cabinets, and millwork. Hardwood lumber for framing in construction, although not in common use, should also be dried to an average moisture content of 15%, not to exceed 19%. Hardwood lumber for furniture, cabinets, and millwork is usually dried to 6% to 8% moisture content.

Table 12–2. Recommended moisture content values for various wood items at time of installation

Lumber drying is usually accomplished by some combination of air drying, accelerated air drying or pre-drying, and kiln drying. Wood species, lumber thickness, economics, and end use are often the main factors in determining the details of the drying process.

Air Drying

The main purpose of air drying lumber is to evaporate as much of the water as possible before end use or transfer to a dry kiln. Air drying usually extends until wood moisture content is as low as 20% to 25%, at which time the lumber is transferred to a dry kiln if final drying to a lower moisture content is required. Sometimes, depending on a mill’s scheduling, air drying may be cut short at a higher moisture content before the wood is sent to the dry kiln. Air drying saves energy costs and reduces required dry kiln capacity. Limitations of air drying are generally associated with uncontrolled drying. The drying rate is very slow during the cold winter months. At other times, hot, dry winds may increase degrade and volume losses as a result of severe surface checking and end splitting. Warm, humid periods with little air movement may encourage the growth of fungal stains, as well as aggravate chemical stains. Another limitation of air drying is the high cost of carrying a large inventory of high value lumber for extended periods. Air drying time to 20% to 25% moisture content varies widely, depending on species, thickness, location, and the time of year the lumber is stacked. Some examples of extremes for 25-mm- (1-in.-) thick lumber are 15 to 30 days for some of the low density species, such as pine, spruce, red alder, and soft maple, stacked in favorable locations and favorable times of the year, to 200 to 300 days for slow drying species, such as sinker hemlock and pine, oak, and birch, in northern locations and stacked at unfavorable times of the year. Details of important air drying considerations, such as lumber stacking and air drying yard layout, are covered in Air Drying of Lumber: A Guide to Industry Practices (Rietz and Page 1971).

Accelerated Air Drying and Pre-Drying

The limitations of air drying have led to increased use of technology that reduces drying time and introduces some control into drying from green to 20% to 25% moisture content. Accelerated air drying involves the use of fans to force air through lumber piles in a shed. This protects the lumber from the elements and improves air circulation compared with air drying. Small amounts of heat are sometimes used to reduce relative humidity and slightly increase temperature. Pre-dryers take this acceleration and control a step further by providing control of both temperature and relative humidity and providing forced air circulation in a completely enclosed compartment. Typical conditions in a pre-dryer are 27°C to 38°C (80°F to 100°F) and 65% to 85% relative humidity.

Kiln Drying

In kiln drying, higher temperatures and faster air circulation are used to increase drying rate considerably. Specific kiln schedules have been developed to control temperature and relative humidity in accordance with the moisture content and stress situation within the wood, thus minimizing shrinkage-caused defects.

Drying Mechanism

Water in wood normally moves from high to low zones of moisture content, which means that the surface of the wood must be drier than the interior if moisture is to be removed. Drying can be broken down into two phases: movement of water from the interior to the surface of the wood and evaporation of water from the surface. The surface fibers of most species reach moisture equilibrium with the surrounding air soon after drying begins. This is the beginning of the development of a typical moisture gradient (Fig. 12–2), that is, the difference in moisture content between the inner and outer portions of a board. If air circulation is too slow, a longer time is required for the surfaces of the wood to reach moisture equilibrium. This is one reason why air circulation is so important in kiln drying. If air circulation is too slow, drying is also slower than necessary and mold could develop on the surface of lumber. If drying is too fast, electrical energy in running the fans is wasted, and in certain species, surface checking and other drying defects can develop if relative humidity and air velocity are not coordinated.

Figure 12–2. Typical moisture gradient in lumber during drying at time increasing from t₁ to t₃.

Water moves through the interior of wood as a liquid or vapor through various air passageways in the cellular structure of the wood, as well as through the wood cell walls. Moisture moves in these passageways in all directions, both across and with the grain. In general, lighter species dry faster than heavier species because the structure of lighter wood contains more openings per unit volume and moisture moves through air faster than through wood cell walls. Water moves by two main mechanisms: capillary action (liquid) and diffusion of bound water (vapor). Capillary action causes free water to flow through cell cavities and the small passageways that connect adjacent cell cavities. Diffusion of bound water moves moisture from areas of high concentration to areas of low concentration. Diffusion in the longitudinal direction is about 10 to 15 times faster than radial or tangential diffusion, and radial diffusion is somewhat faster than tangential diffusion. This explains why flatsawn lumber generally dries faster than quartersawn lumber. Although longitudinal diffusion is much faster than diffusion across the grain, it generally is not of practical importance in lumber that is many times longer than it is thick.

Because chemical extractives in heartwood plug up passageways, moisture generally moves more freely in sapwood than in heartwood; thus, sapwood generally dries faster than heartwood. However, the heartwood of many species is lower in moisture content than is the sapwood and can reach final moisture content as fast.

The rate at which moisture moves in wood depends on the relative humidity of the surrounding air, the steepness of the moisture gradient, and the temperature of the wood. The lower the relative humidity, the greater the capillary flow. Low relative humidity also stimulates diffusion by lowering the moisture content at the surface, thereby steepening the moisture gradient and increasing the diffusion rate. The greater the temperature of the wood, the faster moisture will move from the wetter interior to the drier surface. If relative humidity is too low in the early stages of drying, excessive shrinkage may occur, resulting in surface and end checking. If the temperature is too high, collapse, honeycomb, or strength reduction can occur.

Drying Stresses

Drying stresses are the main cause of nonstain-related drying defects. Understanding these stresses provides a means for minimizing and recognizing the damage they can cause. The cause of drying stresses in the differential shrinkage between the outer part of a board (the shell) and the interior part (the core) can also cause drying defects. Early in drying, the fibers in the shell dry first and begin to shrink. However, the core has not yet begun to dry and shrink; consequently, the core prevents the shell from shrinking. Thus, the shell goes into tension and the core into compression (Fig. 12–3). If the shell dries too rapidly, it is stressed beyond the elastic limit and dries in a permanently stretched (set) condition without attaining full shrinkage. Sometimes surface cracks, or checks, occur during this initial stage of drying, and they can be a serious defect for many uses. As drying progresses, the core begins to dry and attempts to shrink. However, the shell is set in a permanently expanded condition and prevents normal shrinkage of the core. This causes the stresses to reverse; the core goes into tension and the shell into compression. The change in the shell and core stresses and in the moisture content level during drying is shown in Figure 12–4. These internal tension stresses may be severe enough to cause internal cracks (honeycomb).

Figure 12–3. End view of board showing development of drying stresses (a) early and (b) later in drying.

Differential shrinkage caused by differences in radial, tangential, and longitudinal shrinkage is a major cause of warp. The distortions shown in Figure 3–3 in Chapter 3 are due to differential shrinkage. When juvenile or reaction wood is present on one edge or face of a board and normal wood is present on the opposite side, the difference in their longitudinal shrinkage can also cause warp.

Dry Kilns

Most dry kilns are thermally insulated compartments designed for a batch process in which the kiln is completely loaded with lumber in one operation and the lumber remains stationary during the entire drying cycle. Temperature and relative humidity are kept as uniform as possible throughout the kiln and can be controlled over a wide range. Temperature and relative humidity are changed as the wood dries based on a schedule that takes into account the moisture content and/or the drying rate of the lumber. All dry kilns use some type of forced-air circulation, with air moving through the lumber perpendicular to the length of the lumber and parallel to the spacers (stickers) that separate each layer of lumber in a stack.

Figure 12–4. Moisture-stress relationship during six stages of kiln drying 50-mm- (2-in.-) thick red oak.

Three general types of kilns are in common use. One is the track-loaded type (Fig. 12–5), where lumber is stacked on kiln trucks that are rolled in and out of the kiln on tracks. The majority of softwood lumber in the United States is dried in this kiln type. Another major type is the package-loaded kiln (Fig 12–6), where individual stacks of lumber are fork-lifted into place in the kiln. This type of kiln is commonly used for drying hardwood lumber. These kilns are most commonly heated with steam, although softwood lumber kilns are sometimes directly heated. A third common type of kiln, usually package loaded, is the dehumidification kiln. Instead of venting humid air to remove water, as the other two types of kilns do, water is removed by condensation on cold dehumidifier coils (Fig. 12–7).

Kiln Schedules

A kiln schedule is a carefully developed compromise between the need to dry lumber as fast as possible for economic efficiency and the need to avoid severe drying conditions that will lead to drying defects. A kiln schedule is a series of temperatures and relative humidities that are applied at various stages of drying. In most schedules, the temperature is gradually increased and the relative humidity decreased. The schedule for Southern Pine structural lumber is an exception to this general rule. This is lumber usually dried at a constant temperature and relative humidity. Temperatures are chosen to strike this compromise of a satisfactory drying rate and avoidance of objectionable drying defects. The stresses that develop during drying are the limiting factor in determining the kiln schedule. The schedule must be developed so that the drying stresses do not exceed the strength of the wood at any given temperature and moisture content. Otherwise, the wood will crack either on the surface or internally or be crushed by forces that collapse the wood cells. Wood generally becomes stronger as the moisture content decreases, and to a lesser extent, it becomes weaker as temperature increases. The net result is that as wood dries it becomes stronger because of the decreasing moisture content and can tolerate higher drying temperatures and lower relative humidities without cracking. This is a fortunate circumstance because as wood dries, its drying rate decreases at any given temperature, and the ability to increase drying temperature helps maintain a reasonably fast drying rate. Thus, rapid drying is achieved in kilns by the use of temperatures as high as possible and relative humidities as low as possible.

Figure 12–5. Lineshaft, double-track, compartment kiln with alternately opposing fans. Vents are over fan shaft between fans. Vent on high pressure side of fans becomes fresh air inlet when direction of circulation is reversed.

Drying schedules vary by species, thickness, grade, and end use of lumber. There are two general types of kiln schedules: moisture content schedules and time-based schedules. Most hardwood lumber is dried by moisture content schedules. This means that the temperature and relative humidity conditions are changed according to various moisture content levels attained by the lumber during drying. A typical hardwood schedule might begin at 49°C (120°F) and 80% relative humidity when the lumber is green. By the time the lumber has reached 15% moisture content, the temperature is as high as 82°C (180°F). A typical hardwood drying schedule is shown in Table 12–3. Some method of monitoring moisture content during drying is required for schedules based on moisture content. One common method is the use of short kiln samples that are periodically weighed, usually manually but potentially remotely with load cells. Alternatively, electrodes are imbedded in sample boards to sense the change in electrical conductivity with moisture content. This system is limited to moisture content values less than 30%.

Figure 12–6. Package-loaded kiln with fans connected directly to motors.

Softwood kiln schedules generally differ from hardwood schedules in that changes in kiln temperature and relative humidity are made at predetermined times rather than moisture content levels. Examples of time-based schedules, both conventional temperature (<100°C (<212°F)) and high temperature (>110°C (>230°F )), are given in Table 12–3.

Drying Defects

Most drying defects or problems that develop in wood products during drying can be classified as fracture or distortion, warp, or discoloration. Defects in any one of these categories are caused by an interaction of wood properties with processing factors. Wood shrinkage is mainly responsible for wood ruptures and distortion of shape. Cell structure and chemical extractives in wood contribute to defects associated with uneven moisture content, undesirable color, and undesirable surface texture. Drying temperature is the most important processing factor because it can be responsible for defects in each category.

Fracture or Distortion

Surface checks occur early in drying when the shell of a board is stressed in tension enough to fracture the wood. These checks occur most often on the face of flatsawn boards and are illustrated in Figure 12–8. End checks (Fig. 12–9) are similar to surface checks but appear on the ends of boards. End checks occur because the rapid longitudinal movement of moisture causes the board end to dry very quickly and develop high stresses, therefore fracturing. End coatings, either on the log or freshly sawn lumber, are an effective preventative measure. Collapse is a distortion, flattening, or crushing of wood cells. In severe cases (Fig. 12–10), collapse usually shows up as grooves or corrugations, a washboarding effect. Less severe collapse shows up as excessive thickness shrinkage and may not be a serious problem. Honeycomb (Fig. 12–11) is an internal crack that occurs in the later stages of kiln drying when the core of a board is in tension. It is caused when the core is still at a relatively high moisture content and drying temperatures are too high for too long during this critical drying period. Nondestructive testing methods, using speed of sound, have been found to be effective in detecting the presence of these cracks in dried lumber. Knots may loosen during drying because of the unequal shrinkage between the knot and the surrounding wood (Fig. 12–12).

Fig. 12–7. A typical dehumidification kiln (top) and dehumidification drying system (bottom).

Warp

Warp in lumber is any deviation of the face or edge of a board from flatness or any edge that is not at right angles to the adjacent face or edge. Warp can be traced to two causes: (a) differences between radial, tangential, and longitudinal shrinkage in the piece as it dries or (b) growth stresses. Warp is aggravated by irregular or distorted grain and the presence of abnormal types of wood, such as juvenile and reaction wood. The six major types of warp are bow, crook, twist, oval, diamond, and cup (Fig. 12–13).

Discoloration

The use of dried wood products can be impaired by discoloration, particularly when the end use requires a clear, natural finish. Unwanted discoloration can develop in the tree, during storage of logs and green lumber, or during drying. There are two general types of discoloration: chemical and fungal.

Chemical discoloration is the result of oxidative and enzymatic reactions with chemical constituents in wood. Discolorations range from pinkish, bluish, and yellowish hues through gray and reddish brown to dark brown shades. Brown stain in pines and darkening in many hardwoods is a common problem when drying temperatures are too high (Fig. 12–14). A deep grayish-brown chemical discoloration can occur in many hardwood species if initial drying is too slow (Fig. 12–15).

Fungal stains, often referred to as blue or sap stain, are caused by fungi that grow in the sapwood (Fig. 12–16). Blue-stain fungi do not cause decay of the sapwood, and fungi generally do not grow in heartwood. Blue stain can develop if initial drying is too slow.

Another common type of stain develops under stickers (Fig. 12–17). This stain results from contact of the sticker with the board. Sticker stains (sometimes called shadow) are imprints of the sticker that are darker or lighter than the wood between the stickers and can be caused by either chemical or fungal action, or both.

Moisture Content of Dried Lumber

Although widely used, the trade terms “shipping dry,” “air dry,” and “kiln dry” may not have identical meanings as to moisture content in the different producing regions. Despite the wide variations in the use of these terms, they are sometimes used to describe dried lumber. The following statements, which are not exact definitions, outline these categories.

Shipping Dry

Shipping dry means lumber that has been partially dried to prevent stain or mold during brief periods of transit; ideally the outer 3.2 mm (1/8 in.) is dried to 25% or less moisture content.

Table 12–3. Typical dry kiln schedules for lumber

Air Dry

Air dry means lumber that has been dried by exposure to the air outdoors or in a shed or by forced circulation of air that has not been heated above 49°C (120°F ). Commercial air-dry stock generally has an average moisture content low enough for rapid kiln drying or rough construction use. Moisture content is generally in the range of 20% to 25% for dense hardwoods and 15% to 20% for softwoods and low-density hardwoods. Extended exposure can bring standard 19- and 38-mm (nominal 1- and 2-in.) lumber within one or two percentage points of the average exterior equilibrium moisture content of the region. For much of the United States, the minimum moisture content of thoroughly air-dried lumber is 12% to 15%.

Figure 12–8. Surface checking on Douglas Fir dimension lumber.

Figure 12–9. End checking in oak lumber.

Figure 12–10. Severe collapse in western redcedar.

Figure 12–11. Board machined into millwork shows honeycomb (top). Surface of planed red oak board shows no honeycomb (bottom).

Figure 12–12. Loose knot in Southern Pine.

Kiln Dry

Kiln dry means lumber that has been dried in a kiln or by some special drying method to an average moisture content specified or understood to be suitable for a certain use. The average moisture content should have upper and lower tolerance limits, and all values should fall within these limits. Kiln-dried softwood dimension lumber generally has an average moisture content of 19% or less; the average moisture content for many other softwood uses is 10% to 20%. Hardwood and softwood lumber for furniture, cabinetry, and millwork usually has a final moisture content of 6% to 8% and can be specified to be free of drying stresses. The importance of suitable moisture content values is recognized, and provisions covering them are now incorporated in some softwood standards as grading rules. Moisture content values in the general grading rules may or may not be suitable for a specific use; if not, a special moisture content specification should be made.

Figure 12–13. Various types of warp that can develop in boards during drying.

Figure 12–14. Brown sapwood stain in Southern Pine lumber.

Figure 12–15. Gray sapwood stain in southern red oak that was dried green with humid, low temperature conditions and poor air circulation.

Figure 12–16. Sap stain in Southern Pine. Color ranges from bluish gray to black.

Figure 12–17. Sticker stain in sapwood of sugar maple after planing.

Moisture Control During Transit and Storage

Lumber and other wood items may change in moisture content and dimension while awaiting shipment, during fabrication, in transit, and in storage.

When standard 19-mm (nominal 1-in.) dry softwood lumber is shipped in tightly closed boxcars, shipping containers, or trucks or in packages with complete and intact wrappers, average moisture content changes for a package can generally be held to 0.2% or less per month. In holds or between decks of ships, dry material usually adsorbs about 1.5% moisture during normal shipping periods. If green material is included in the cargo, the moisture regain of the dry lumber may be doubled. On the top deck, if unprotected from the elements, the moisture regain can be as much as 7%. When standard 19-mm (nominal 1-in.) softwood lumber, kiln dried to 8% or less, is piled solid under a good pile roof in a yard in humid weather, average moisture content of a pile can increase at the rate of about 2% per month during the first 45 days. An absorption rate of about 1% per month can then be sustained throughout a humid season. Comparable initial and sustaining absorption rates are about 1% per month in open (roofed) sheds and 0.3% per month in closed sheds. Stock that was piled for a year in an open shed in a western location increased 2.7% on the inside of solid piles and 3.5% on the outside of the piles. All stock that has been manufactured in any way should be protected from precipitation and spray, because water that gets into a solid pile tends to be absorbed by the wood instead of evaporating. The extent to which additional control of the storage environment is required depends upon the use to which the wood will be put and the corresponding moisture content recommendations. The moisture content of all stock should be determined when it is received. If moisture content is not as specified or required, stickered storage in an appropriate condition could ultimately bring the stock within the desired moisture content range. If a large degree of moisture change is required, the stock must be redried.

Plywood and Structural Items

Green or partially dried lumber and timbers should be open piled on stickers and protected from sunshine and precipitation by a tight roof. Framing lumber and plywood with 20% or less moisture content can be solid piled in a shed that provides good protection against sunshine and direct or wind-driven precipitation. However, a better practice for stock with greater than 12% moisture content is the use of stickered piling to bring moisture content more in line with the moisture content in use. Dry lumber can be piled solid in the open for relatively short periods, but at least a minimum pile cover of waterproofed paper should be used whenever possible. Because it is difficult to keep rain out completely, storing solid-piled lumber in the open for long periods is not recommended. If framing lumber must be stored in the open for a long time, it should be piled on stickers over good supports and the piles should be roofed. Solid-piled material that has become wet again should also be re-piled on stickers.

Table 12–4. Amount by which temperature of storage area must be increased above outside temperature to maintain equilibrium moisture content

Finish and Factory Lumber

Such kiln-dried items as exterior finish, siding, and exterior millwork should be stored in a closed but unheated shed. They should be placed on supports raised above the floor, at least 150 mm (6 in.) high if the floor is paved or 300 mm (12 in.) if not paved. Interior trim, flooring, cabinet work, and lumber for processing into furniture should be stored in a room or closed shed where relative humidity is controlled. Kiln-dried and machined hardwood dimension or softwood cut stock should also be stored under controlled humidity conditions.

Dried and machined hardwood dimension or softwood lumber intended for remanufacture should also be stored under controlled humidity conditions. Under uncontrolled conditions, the ends of such stock may attain a greater moisture content than the balance of the length. Then, when the stock is straight-line ripped or jointed before edge gluing, subsequent shrinkage will cause splitting or open glue joints at the ends of panels. The simplest way to reduce relative humidity in storage areas of all sizes is to heat the space to a temperature slightly greater than that of the outside air. Dehumidifiers can be used in small, well-enclosed spaces.

If the heating method is used, and there is no source of moisture except that contained in the air, the equilibrium moisture content can be maintained by increasing the temperature of the storage area greater than the outside temperature by the amounts shown in Table 12–4. When a dehumidifier is used, the average temperature in the storage space should be known or controlled. Table 3–4 in Chapter 3 should be used to select the proper relative humidity to give the desired average moisture content. Wood in a factory awaiting or following manufacture can become too dry if the area is heated to 21°C (70°F) or greater when the outdoor temperature is low. This often occurs in the northern United States during the winter. Under such circumstances, exposed ends and surfaces of boards or cut pieces will tend to dry to the low equilibrium moisture content condition, causing shrinkage and warp. In addition, an equilibrium moisture content of 4% or more below the moisture content of the core of freshly crosscut boards can cause end checking. Simple remedies are to cover piles of partially manufactured items with plastic film and lower the shop temperature during nonwork hours. Increased control can be obtained in critical shop and storage areas by humidification. In warm weather, cooling can increase relative humidity and dehumidification may be necessary.

Dimensional Changes in Wood

Dry wood undergoes small changes in dimension with normal changes in relative humidity. More humid air will cause slight swelling, and drier air will cause slight shrinkage. These changes are considerably smaller than those involved with shrinkage from the green condition. Equation (12–2) can be used to approximate dimensional changes caused by shrinking and swelling by using the total shrinkage coefficient from green to ovendry. However, the equation assumes that the shrinkage-moisture content relationship is linear. Figure 3–4 (Ch. 3) shows that this is not the case, so some error is introduced. The error is in the direction of underestimating dimensional change, by about 5% of the true change. Many changes of moisture content in use are over the small moisture content range of 6% to 14%, where the shrinkage-moisture content relationship is linear (Ch. 3, Fig. 3–4). Therefore, a set of shrinkage coefficients based on the linear portion of the shrinkage-moisture content curve has been developed (Table 12–5). Approximate changes in dimension can be estimated by a simple formula that involves a dimensional change coefficient, from Table 12–5, when moisture content remains within the range of normal use. (Dimensional changes are further discussed in Chs. 3 and 6.)

Estimation Using Dimensional Change Coefficient

The change in dimension within the moisture content limits of 6% to 14% can be estimated satisfactorily by using a dimensional change coefficient based on the dimension at 10% moisture content:

(12–2)

where ∆D is change in dimension, D_I dimension in units of length at start of change, C_T dimensional change coefficient tangential direction (for radial direction, use C_R), M_F moisture content (%) at end of change, and M_I moisture content (%) at start of change.

Values for C_T and C_R, derived from total shrinkage values, are given in Table 12–5. When M_F < M_I, the quantity (M_F − M_I) will be negative, indicating a decrease in dimension; when greater, it will be positive, indicating an increase in dimension.

As an example, assuming the width of a flat-grained white fir board is 232 mm (9.15 in.) at 8% moisture content, its change in width at 11% moisture content is estimated as

∆D = 232[0.00245(11 − 8)]
= 232(0.00735)
= 1.705 mm

∆D = 9.15[0.00245(11 − 8)]
= 9.15[0.00735]
= 0.06725 or 0.067 in.

Then, dimension at end of change

DI + ∆D = 232 + 1.7	(= 9.15 + 0.067)
= 233.7 mm	(= 9.217 in.)

The thickness of the same board at 11% moisture content can be estimated by using the coefficient C_R = 0.00112.

Table 12–5. Coefficients for dimensional change as a result of shrinking or swelling within moisture content limits of 6% to 14% (C_T = dimensional change coefficient for tangential direction; C_R = radial direction)

Because commercial lumber is often not perfectly flatsawn or quartersawn, this procedure will probably overestimate width shrinkage and underestimate thickness shrinkage. Note also that if both a size change and the percentage of moisture content are known, Equation (12–2) can be used to calculate the original moisture content.

Calculation Based on Green Dimensions

Approximate dimensional changes associated with moisture content changes greater than 6% to 14% , or when one moisture value is outside of those limits, can be calculated by

(12–3)

where S_T is tangential shrinkage (%) from green to ovendry (Ch. 3, Tables 3–5 and 3–6) (use radial shrinkageS_R when appropriate).

Neither M_I nor M_F should exceed 30%, the assumed moisture content value when shrinkage starts for most species.

Design Factors Affecting Dimensional Change

Framing Lumber in House Construction

Ideally, house framing lumber should be dried to the moisture content it will reach in use, thus minimizing future dimensional changes as a result of frame shrinkage. This ideal condition is difficult to achieve, but some drying and shrinkage of the frame may take place without being visible or causing serious defects after the house is completed. If, at the time the wall and ceiling finish is applied, the moisture content of the framing lumber is not more than about 5% above that which it will reach in service, there will be little or no evidence of defects caused by shrinkage of the frame. In heated houses in cold climates, joists over heated basements, studs, and ceiling joists may reach a moisture content as low as 6% to 7% (Table 12–2). In mild climates, the minimum moisture content will be greater.

The most common signs of excessive shrinkage are cracks in plastered walls, truss rise, open joints, and nail pops in drywall construction; distortion of door openings; uneven floors; and loosening of joints and fastenings. The extent of vertical shrinkage after the house is completed is proportional to the depth of wood used as supports in a horizontal position, such as girders, floor joists, and plates. After all, shrinkage occurs primarily in the width of members, not the length.

Thorough consideration should be given to the type of framing best suited to the whole building structure. Methods should be chosen that will minimize or balance the use of wood across the grain in vertical supports. These involve variations in floor, wall, and ceiling framing. The factors involved and details of construction are covered extensively in Wood-Frame House Construction (Sherwood and Stroh 1991).

Heavy Timber Construction

In heavy timber construction, a certain amount of shrinkage is to be expected. A column that bears directly on a wood girder can result in a structure settling as a result of the perpendicular-to-grain shrinkage of the girder. If not provided for in the design, shrinkage may cause weakening of the joints or uneven floors or both. One means of eliminating part of the shrinkage in mill buildings and similar structures is to use metal post caps; the metal in the post cap separates the upper column from the lower column. The same thing is accomplished by bolting wood corbels to the side of the lower column to support the girders.

When joist hangers are installed, the top of the joist should be above the top of the girder; otherwise, when the joist shrinks in the stirrup, the floor over the girder will be higher than that bearing upon the joist. Heavy planking used for flooring should be near 12% moisture content to minimize openings between boards as they approach moisture equilibrium. When standard 38- or 64-mm (nominal 2- or 3-in.) joists are nailed together to provide a laminated floor of greater depth for heavy design loads, the joist material should be somewhat less than 12% moisture content if the building is to be heated.

Interior Finish

The normal seasonal changes in the moisture content of interior finish are not enough to cause serious dimensional change if the woodwork is carefully designed. Large members, such as ornamental beams, cornices, newel posts, stair stringers, and handrails, should be built up from comparatively small pieces. Wide door and window trim and base should be hollow-backed. Backband trim, if mitered at the corners, should be glued and splined before erection; otherwise butt joints should be used for the wide faces. Large, solid pieces, such as wood paneling, should be designed and installed so that the panels are free to move across the grain. Narrow widths are preferable.

Flooring

Flooring is usually dried to the moisture content expected in service so that shrinking and swelling are minimized and buckling or large gaps between boards do not occur. For basement, large hall, or gymnasium floors, however, enough space should be left around the edges to allow for some expansion.

Wood Care and Installation During Construction

Lumber and Trusses

Although it should be, lumber is often not protected from the weather at construction sites. Lumber is commonly placed on the ground in open areas near the building site as bulked and strapped packages. Supports under such packages are useful to prevent wetting from mud and ground water and should elevate the packages at least 150 mm (6 in.) off the ground. The packages should also be covered with plastic tarpaulins for protection from rain.

Lumber that is green or nearly green should be piled in stickers under a roof for additional drying before it is built into the structure. The same procedure is required for lumber that has been treated with a waterborne preservative but not fully redried. Prefabricated building parts, such as roof trusses, sometimes lie unprotected on the ground at the building site. In warm, rainy weather, moisture regain can result in fungal staining. Wetting of the lumber also results in swelling, and subsequent shrinkage of the framing may contribute to structural distortions. Extended storage of lumber at moisture contents greater than 20% without drying can allow decay to develop.

If framing lumber has a greater moisture content when installed than that recommended in Table 12–2, shrinkage can be expected. Framing lumber, even thoroughly air-dried stock, will generally have a moisture content greater than that recommended when it is delivered to the building site. If carelessly handled in storage at the site, the lumber can take up more moisture. Builders can schedule their work so an appreciable amount of drying can take place during the early stages of construction. This minimizes the effects of additional drying and shrinkage after completion. When the house has been framed, sheathed, and roofed, the framing is so exposed that in time it can dry to a lower moisture content than would ordinarily be expected in yard-dried lumber. The application of the wall and ceiling finish is delayed while wiring and plumbing are installed. If this delay is about 30 days in warm, dry weather, framing lumber should lose enough moisture so that any additional drying in place will be relatively unimportant. In cool, damp weather, or if wet lumber is used, the period of exposure should be extended. Checking moisture content of door and window headers and floor and ceiling joists at this time with an electric moisture meter is good practice. When these members approach an average of 12% moisture content, interior finish and trim can normally be installed. Closing the house and using the heating system will hasten the rate of drying.

Before wall finish is applied, the frame should be examined and defects that may have developed during drying, such as warped or distorted studs, shrinkage of lintels over openings, or loosened joints, should be corrected.

Exterior Trim and Millwork

Exterior trim, such as cornice and rake mouldings, fascia boards, and soffit material, is normally installed before the shingles are laid. Trim, siding, and window and door frames should be protected on the site by storing in the house or garage until time of installation. Although items such as window frames and sashes are usually treated with some type of water-repellent preservative to resist absorption of water, they should be stored in a protected area if they cannot be installed soon after delivery. Wood siding is often received in packaged form and can ordinarily remain in the package until installation.

Finished Flooring

Cracks develop in flooring if it absorbs moisture either before or after it is laid, then shrinks when the building is heated. Such cracks can be greatly reduced by observing the following practices:

• Specify flooring manufactured according to association rules and sold by dealers that protect it properly during storage and delivery.
• Do not allow flooring to be delivered before masonry and plastering are completed and fully dry, unless a dry storage space is available.
• Install the heating plant before flooring is delivered.
• Break open flooring bundles and expose all sides of flooring to the atmosphere inside the structure.
• Close up the house at night and increase the temperature about 8°C (15°F) greater than the outdoor temperature for about 3 days before laying the floor.
• If the house is not occupied immediately after the floor is laid, keep the house closed at night or during damp weather and supply some heat if necessary.

Better and smoother sanding and finishing can be done when the house is warm and the wood has been kept dry.

Interior Finish

In a building under construction, average relative humidity will be greater than that in an occupied house because of the moisture that evaporates from wet concrete, brickwork, plaster, and even the structural wood members. The average temperature will be lower because workers prefer a lower temperature than is common in an occupied house. Under such conditions, the finish tends to have greater moisture content during construction than it will have during occupancy.

Before the interior finish is delivered, the outside doors and windows should be hung in place so that they can be kept closed at night. In this way, conditions of the interior can be held as close as possible to the higher temperature and lower humidity that ordinarily prevail during the day. Such protection may be sufficient during dry warm weather, but during damp or cool weather, it is highly desirable that some heat be maintained in the house, particularly at night. Whenever possible, the heating plant should be placed in the house before the interior trim is installed, to be available for supplying the necessary heat. Portable heaters can also be used. The temperature during the night should be maintained about 8°C (15°F) greater than the outside temperature but should not be allowed to drop below about 21°C (70°F) during the summer or 17°C (62°F) when the outside temperature is below freezing.

After buildings have thoroughly dried, less heat is needed, but unoccupied houses, new or old, should not be allowed to stand without some heat during the winter. A temperature of about 8°C (15°F) greater than the outside temperature and above freezing at all times will keep the woodwork, finish, and other parts of the house from being affected by dampness or frost.

Plastering

During a plastering operation in a moderate-sized, six-room house, approximately 450 kg (1,000 lb) of water are used, all of which must be dissipated before the house is ready for the interior finish. Adequate ventilation to remove the evaporated moisture will keep it from being absorbed by the framework. In houses plastered in cold weather, the excess moisture can also cause paint to blister on exterior finish and siding.

During warm, dry weather, with the windows wide open, the moisture will be gone within a week after the final coat of plaster is applied. During damp, cold weather, the heating system or portable heaters are used to prevent freezing of plaster and to hasten its drying. Adequate ventilation should be provided at all times of the year because a large volume of air is required to carry away the amount of water involved. Even in the coldest weather, the windows on the side of the house away from the prevailing winds should be opened 50 to 75 mm (2 to 3 in.), preferably from the top.

References

ASTM. [current edition]. Direct moisture content measurement of wood and wood-based materials. ASTM D4442–92. West Conshohocken, PA: American Society for Testing and Materials.

Forest Products Laboratory. 1961. Wood floors for dwellings. Agric. Handb. 204. Washington, DC: U.S. Department of Agriculture.

Forest Products Laboratory. 1972. Methods of controlling humidity in woodworking plants. Res. Note FPL–RN–0218. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.

James, W.L. 1988. Electric moisture meters for wood. Gen. Tech. Rep. FPL–GTR–6. Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory.

McMillen, J.M.; Wengert, E.M. 1978. Drying eastern hardwood lumber. Agric. Handb. 528. Washington, DC: U.S. Department of Agriculture.

Rietz, R.C. 1978. Storage of lumber. Agric. Handb. 531. Washington, DC: U.S. Department of Agriculture.

Rietz, R.C.; Page, R.H. 1971. Air drying of lumber: A guide to industry practices. Agric. Handb. 402. Washington, DC: U.S. Department of Agriculture.

Sherwood, G.E.; Stroh, R.C. 1991. Wood-Frame House Construction. USDA Agric. Handb. 73. Washington, DC: U.S. Department of Agriculture.

Simpson, W.T. 1989. Drying wood: a review. Drying Technology. An International Journal, Pt. 1. 2(2): 235–265, Pt. 2, 2(3): 353–368.

Simpson, W.T., ed. 1991. Dry kiln operator’s manual. Agric. Handb. 188. Washington, DC: U.S. Department of Agriculture.

USDC. 1970. American softwood lumber standard. NBS Voluntary Prod. Stand. PS 20–70; Washington, DC: U.S. Department of Commerce.