This wall has one base plate and two cap plates.
BEYOND ALL QUESTIONS OF STYLE AND intended use for your greenhouse (see chapter 2 for more information on these), your choices for greenhouse construction are quite simple: You can, like many people, buy a kit, or you can design and build a structure yourself (or with a contractor), incorporating an eclectic style, perhaps, and making use of materials close to hand. If you decide to design and build a greenhouse on your own, you’ll need an understanding of basic building techniques ranging from framing and hanging doors and windows to installing insulation.
If you decide to purchase a kit greenhouse, the structure may come partially or completely assembled on the back of a truck or it may be delivered in boxed parts. For some greenhouses, you might have to assemble the jigsaw puzzle of aluminum struts and braces for the frame, bolting together pieces of the predrilled frame or even drilling the holes yourself and cutting parts to length. For other kits, the walls, roof, and ends come preassembled; all you may have to do is bolt together the sides and top. In still other cases, all that is required is installing the glazing. Most kit greenhouse suppliers have clearly illustrated building manuals to help you, and some include videos, a hot line, or even access to a local expert to help you assemble the project.
Before you start building any kit greenhouse, read the instructions carefully, watch any instructional videos provided, and do not cut anything unless the instructions tell you to do so. Before you begin any fastening, lay out the parts on a flat surface according to their placement in the finished structure, checking the part numbers carefully as you do so. Snapping together or fastening any parts before you’ve identified and organized all pieces may result in a join that is difficult to undo.
In general, for kit greenhouses you’ll probably need to lay a special polyfoam sill moisture barrier/insulation on the foundation. Then you’ll have to install the sill or base plate over the polyfoam and the bolts in the concrete foundation and bolt the sill plate to the foundation, which sandwiches the insulation and moisture barrier between the frame and concrete. After this, you’ll have to assemble the walls and stand them on the sill or base plate. Some kits require assembling the wall first because the bottom of the frame acts as the sill plate. On larger greenhouses, you may lay the base plate and erect each framed wall one at a time, tying the entire unit together with longitudinals and a ridge bar.
As you assemble, remember to check that the structure is square and to install any bracing as you work. If you spread out your construction over two days and tighten bolts on day one, you’ll usually find that the following morning you can go back and get another half-turn on each bolt — but be careful not to tighten bolts so much that you risk crushing or damaging the structure.
In many kits, the door is pre-hung. For some, however, you may have to hang the door(s) or install sliding doors. In general, sliding doors are made to run on tracks and if you install them properly, they should provide years of trouble-free operation if you lubricate the bottom track and roller bearings occasionally.
Assembling larger greenhouse structures may require you to set up supports and support posts fastened to the foundation plate. You should determine the location of these before the foundation is poured so that you can insert bolts or tubing into the wet concrete in the correct places for the supports to be attached to them. Note that if your kit is for a large greenhouse, you may need the help of four or five people to erect the structure, one frame wall at a time. The largest greenhouse structures may require a crane to hoist the frames into place. If you are building a sizeable structure, determine these needs well before you start construction.
Unless you are an avid woodworker who will get as much pleasure out of making a greenhouse as you will out of working in it, you may decide that building your own conventional greenhouse is not for you. Yet many benefits can come with building your own structure. For example, you might make decisions and adjustments for your particular situation: You can decide to install triple-glazed glass or polycarbonate on the north wall for better insulation or make a snug-fitting door to keep winter drafts at bay, or you can construct better and lighter vents that can easily be opened automatically. If you build the structure yourself, you can modify plans along the way or, preferably, decide to introduce particular elements at the outset. Refer to the plans for conventional greenhouses in chapter 13 to find a structure that suits your growing needs and think of the ways that you could modify the one you decide to build.
As stated in chapter 6, if you decide to build a conventional greenhouse yourself using red cedar or a high-quality wood, you’ll have to decide on the glazing long before you start work on the greenhouse because the glazing will determine what type of framing you’ll need. Glass, for instance, must be set into the greenhouse frame and fastened in place. Polycarbonate, on the other hand, can be fastened over the framework. Remember that some design modifications beyond glazing require that you make adjustments in the framework. For a precision job such as building your own structure with extensive variations or modifications on a conventional design, you’ll probably need a good woodworking shop with quality tools and plenty of time (see the box below). But you can build a conventional structure such as a Gothic arches greenhouse or make small modifications to a conventional greenhouse using standard tools.
If you asked a plumber, a mason, and a carpenter to design and build a greenhouse from scratch, it is most likely that each would build it from the materials that he or she knows best. The plumber might build a Quonset-style or hoop house out of metal or PVC pipes. The mason would build a house that incorporates a lot of brickwork or stonework, and a carpenter would build it out of wood. Because many homeowners are most familiar with working in wood, it often becomes the material of choice.
The specific number and type of tools will depend on the greenhouse kit, but will likely include:
To design and build a wooden structure, you need to know standard lumber sizes (see chapter 6 for more information on standard lumber) and you need to incorporate these sizes into your design and construction.
For example, suppose your design calls for a 4-foot-9-inch-tall wooden structural wall. You can cut studs to 4 feet 41⁄4 inches and use a sill plate and double cap to get 4 feet 9 inches, but this wastes 3 feet 71⁄2 inches of each stud. If you modify the wall to 4 feet 41⁄2 inches, you can cut a stud in half and use a sill plate and double cap plate to arrive at 4 feet 41⁄2 inches. For any wall height under 4 feet 41⁄2 inches, you can get two pieces of support lumber from every stud. By sizing your structure for standard framing lumber lengths, you will reduce the amount of waste and keep down the cost of the construction.
If you are very careful to install vapor barriers, regular building lumber is adequate for a greenhouse given the caveats regarding longevity and rot resistance outlined in chapter 6. It is easily available and reasonably priced, and can be worked with few tools. But you must know how to build a wall, frame a door, and square the structure properly before you can successfully build a greenhouse.
If you plan to build a 4-foot by 12-foot wall, you could go to the lumber store and buy a bunch of 2×4s and have at it. But if you work out your framing and lumber needs on paper first, you can figure out exactly how many pieces of lumber and what lengths are required. If the wall is to be 12 feet long, the top and bottom plates will be 12-footers, so you can put two (or three if you are using a double cap plate) 12-foot-long 2×4s on your list. Next you’ll need to figure the spacing of studs on the top and bottom plates. By putting together both 12-foot 2×4s and using a square, you can quickly mark out the location of all the 4-foot-long vertical studs. Each stud is placed 16 inches on center; you’ll need 11 of them. (Cut six 8-foot studs in half to get what you need and use the scraps for doubling the corners.) Where walls join at the corners, the framing of one of the walls must be three studs (41⁄2 inches) thick because the adjoining wall will overlap by 31⁄2 inches (the width of a stud) and you’ll need the remaining 1 inch to provide a nailing surface for the interior sheathing. While you can actually frame the ends to be overlapped with three studs, it is more economical to use two studs with 2×4 scraps sandwiched between as shown in the illustration at right.
Some walls, especially those that are to be heavily insulated, may be made of 2×6 lumber. A wall will then be 6 inches wide instead of 4 inches wide (of course, the actual dimension of the lumber is 51⁄2 inches wide). This will allow for 51⁄2 inches of insulation. If you decide to make your walls 6 inches thick, you can space the vertical supports 20 inches on center rather than 16 inches on center.
This wall has one base plate and two cap plates.
Frame a corner using two or three studs to ensure that the interior sheathing can be nailed to the corner. You can use blocking made from scrap pieces of lumber to space the studs the correct distance from the corner.
If you intend to build the entire greenhouse yourself (especially using a wood such as redwood, cedar, or teak), you may require specialized power woodworking tools such as a planer or table saw to cut the lumber accurately to size. You can often rent specialized tools or have a commercial woodworking shop make the special parts for you. Lengths can be cut with a handsaw in a miter box or with a chop box.
In your lumber calculations, account for window and door framing if a wall is to have them. Lay out the rough opening for these — that is, the space the door and its frame will occupy in the wall. Most 3-foot doors have a rough opening width of 371⁄2 inches, allowing 36 inches for the door itself and 3⁄4 inch on either side of the door for the frame. When you lay out the wall to include a door, notice that the two studs inside the door frame are cut shorter than the two outside of it. These studs, known as jacks, filler, or trimmer studs, support the door header, which is secured on top of the jacks to offer structural integrity to the opening so that the weight of the wall is supported. The header is made of two 2×8s or 2×10s, with 1⁄2-inch plywood sandwiched between to match the 31⁄2-inch width of the studs.
A window is framed similarly, but in addition to a header, it has a sill, a piece like the header attached to the bottom of the window opening. The sill is supported by lumber called cripple studs. If a window to be installed is 3 feet wide, the rough opening will be 371⁄2 to 39 inches wide, depending on the type of window.
Note that when the wall is laid out, the bottom plate, known as the base or sole plate, is not cut from the doorway until the door is framed in and the wall is erected. This ensures that the wall and doorway remain square. Note also that inserting a window or a door in a wall does not disrupt the continuity of the 16-inches-on-center stud spacing from end to end.
A door is framed by making a rough opening 371⁄2 inches wide. Two jacks (shortened studs) support the door header. A window has beneath it a piece similar to a header, which is supported on cripple studs.
On a custom greenhouse, the ridgepole is often at the back of the structure. When this is the case, the end walls are usually constructed as shown, sloping down to a knee wall at the front.
Generally, the end walls of a greenhouse are peaked, though the peak may be located toward one side of the structure (at the back of a lean-to greenhouse, for instance) or in the middle of the roof. Many home-built greenhouses have a vertical wall and ridge at the rear and the glazing sloping from the back wall almost to a knee wall at the front. This type of design makes building the roof a bit easier.
If the ridge is to be in the middle of the greenhouse and the greenhouse is to be taller than 8 feet, you should construct stud walls for the end walls (see Building a Lumber Wall) and build the triangular roof section on top of it. If your greenhouse is to be less than 8 feet tall, you can build the entire end wall section from 2×4 studs.
To build an end wall with insulation for a lean-to-style greenhouse (see the illustration at bottom left), first set a base plate and lay out your studs at 16 inches on center. Next, lay out the height of the front knee wall and the tall rear wall and cut two studs to these measurements. You can then estimate the slope of the roof and cut all the rest of the wall studs. An end wall with a center ridge is built in a similar manner, although I like to ensure that the ridgepole is supported on doubled 2×4s.
Building a glazed end wall is a little more difficult. First, you’ll have to determine what type of glazing you are going to use. Glass can be recessed into the wood supports. Polycarbonate can be bolted onto the outside of the wood supports. You’ll also need to determine the width of your glazing and set your vertical studs to accommodate this width. For example, if your glazing is 2-foot-wide polycarbonate panels, you will set your studs 201⁄2 inches on center. The additional half-inch allows for an expansion gap between the glazing panels. At the corners the first support might be set a little nearer to the edge of the greenhouse so that the glazing can cover the wood right to the corner.
If the structure is 8 feet tall or less (A), end walls can be constructed entirely of 2×4 studs. If it’s taller than 8 feet (B), construct stud walls with a triangular roof section on top.
Once your end walls are in place, the next step is to install the ridgepole or beam. It can be a 2×6 for a small greenhouse that is 8 feet to 10 feet long, but for larger greenhouses, it should be a 2×8 or 2×10. If your greenhouse is very long (more than 16 feet), you may have to install a center post to support the ridgepole or use a long laminated beam or maybe a steel I-bar. I’ve installed a center post in my 20-foot-long cool greenhouse.
Construction of the ridgepole
Building the roof is challenging because you want to be sure the glazing fits tightly on the roof so that there’s no risk of it sliding off the edge. This means that both ends of each rafter beam must be carefully shaped. Typically, if the roof has an overhang or eave, a builder will cut a triangular piece, called a bird’s-mouth cut, from the lower end of the rafter to ensure that the rafter will sit more firmly on the vertical support wall. If there is no overhang, the bottom of the rafter will have to be cut level, sometimes known as a seat cut. Most greenhouses do not have eaves and use a level cut at the bottom.
For a lean-to greenhouse (as shown in the illustration above), the top of each rafter will need to be cut vertically with a bird’s-mouth cut to allow it to sit on the rear wall. If the ridge is in the middle of the greenhouse, the top of each rafter should be cut vertically and butted against the matching rafter on the opposite side of the greenhouse. When installing rafters, always install the two opposing rafters before moving on to the next pair. If you install all the rafters on one side before installing those on the other side, you may end up bending the ridgepole slightly, causing misalignment of your greenhouse roof.
Two possible cuts for roof rafters
Laying out rafters takes a certain amount of knowledge and skill. You’ll need to measure and cut carefully or, as Mr. Worrall, my old woodworking teacher, used to say, measure twice, cut once. The best way to cut each rafter is to determine all measurements while the rafter is on the ground. First, determine the pitch of the roof. Pitch is expressed in terms of 12 horizontal inches. Thus, a 4:12 pitch (read “four twelve”) rises 4 inches for every 12 horizontal inches. Next, determine the span of the roof (the horizontal distance from the wall to the ridge). The above illustration, which shows a 6:12 slope (6 inches of slope for every 12 inches), let’s us visualize the pitch of a roof.
To arrive at rafter measurements for a roof with a 4:12 pitch, measure the width of your greenhouse. Let’s say the width is 12 feet with the ridge in the middle of the span. You can lay out the rafters in two ways. The first method uses the Pythagorean theorem, which holds that the square of the length of the hypotenuse of a right triangle is equal to the square of the base plus the square of the height. In other words, if the base is a, the height is b, and the hypotenuse is c, a2 + b2 = c2. If the base is 6 feet and the height is 4 feet, the hypotenuse is 62 + 42 = 52. The square root of 52 = c = 7.211 inches or 7 feet 21⁄2 inches. You can measure this on the rafter and then determine where the vertical cut at the ridge should be made and where the bird’s-mouth or seat cut should be made.
Another method for laying out the rafters is to use a framing square. For a 6:12 slope, the square is placed on the upper end of the rafter with 12 inches along the base and 6 inches on the vertical. To find the right length, the framing square can be walked down the rafter to the bottom, where you can mark for either a bird’s-mouth or seat cut. Note that because it slopes, the rafter is longer than the horizontal distance along the top of the wall and part of it will butt against the wall. For a greenhouse, I like to cut the seat so that it fits flush with the vertical edge of the wall to ensure that the glazing can fit without a large overhang or a gap between the wall glazing and the roof glazing.
Most of custom greenhouse construction is straightforward stud-and-nail or screw construction using butt or lap joints. At times, however, it calls for incorporating other joints. One such case is the construction of a window. For the corners, you can use a lap joint or a miter joint with or without a biscuit depending on your expertise as a carpenter. A biscuit is a flat, oval piece of compressed wood about 2 inches long and 1⁄8 inch thick that is inserted in matching mortises in the two pieces of wood to be joined. Once the pieces are joined, the biscuit is invisible. Glue is usually added to the biscuit and the mortises before inserting it so that when the two pieces of wood are brought together, the wet biscuit will expand and the join will be even stronger. Using biscuits helps to form a perfectly aligned joint.
In most situations, joints for a greenhouse structure can be simple lap joints. But in some cases — for example, where three pieces of wood need to be joined — a lap joint may be difficult to make or it may leave very little material on the original wood. In these cases, you’ll have to modify the lap joint and perhaps glue it to increase its strength. Screwing or nailing corner joints is always fraught with potential difficulties when large sheets of glass or other glazing must be fitted. Some common problems are glass slipping and breaking and glass not fitting in corners that aren’t exactly square.
Instead of nailing or screwing your greenhouse together, you can glue it together with epoxy. This allows you to build the greenhouse without worrying about whether nails or screws will touch the glazing. You can also use epoxy as a coating to protect the wood. Once set, it is virtually waterproof. Epoxy does not contain any UV inhibitors, however, so if you decide to coat the wood with this material, you should apply a couple of layers of UV-resistant varnish such as Epifanes or Schooner to protect against UV degradation.
Epoxy construction is not used only for wood. It will bind fiberglass, aluminum, steel, many plastics, cement, and even granite, and can be an invaluable tool in greenhouse construction.
Epoxy is a glue that is usually colorless, although some epoxies appear to have a slight sheen. The source of its spectacular strength comes from the cross-linked molecular chains that are formed in the course of mixing in specific ratios its two parts, resin and accelerator (from one can or tube) and a hardener (in a separate can or tube). While it can be a wonder glue, it’s important to note that epoxy is often difficult to work with.
Most manufacturers make a “fast” hardener that sets up within 10 minutes or so and a “moderate” or “slow” hardener to be used for bigger jobs that might take 20 to 30 minutes or more to set up. Typically, epoxy with a fast hardener is sufficient for small jobs, allowing you to put together the parts, clamp them, and walk away. If you are mixing large batches of epoxy, you should use the slowest hardener to allow you the time to use the mixture before it sets up in the pot. Some hardeners such as Epiglass from Interlux are designed to allow you to adjust the cure rate by mixing fast, moderate, or slow hardener. By mixing them carefully, you can get a cure rate that varies from about 10 minutes to 30 minutes or more.
Always mix resin/accelerator and hardener in the proportions recommended by the epoxy manufacturer. These proportions vary by brand. One of the most significant causes of epoxy failure is incorrect mixing of these two ingredients. Most manufacturers supply mini-pumps for measuring with their gallon or quart cans, but if you don’t have a pump, you can use a measuring cup or a measuring stick dipped in a cup of resin/accelerator measured against another stick dipped in a cup of hardener. Important: Don’t mix resins and hardeners from different manufacturers — don’t combine, for example, Epiglass resin and MAS hardener. These may have different properties and may not work together. Along with combining the two ingredients in the correct ratio, you’ll need to make sure they are thoroughly mixed. In the process of stirring, scrape down the sides of the pot and make certain all material from the bottom is incorporated before adding thickening agents to help prevent the epoxy from running out of the joints. A thoroughly mixed resin will be an opaque, whitish color. If the epoxy is poorly mixed, it won’t set up properly and you’ll have to scrape it off the surfaces you wanted to join before you can apply a new mixture.
Sanding epoxies can be bad for your health because of the dust it produces. At the minimum, wear a dust mask.
Before you begin mixing, determine how much epoxy you’ll need and mix only that amount. Mixing big batches often leads to tossing out unused resin. Note that the interaction of the resin and hardener produces heat, and if the mixture gets too hot, it will begin to harden in the mixing container before you get a chance to use it. In some cases, the heat generated will melt the mixing cup or container and has even been known to start a fire. For this reason, never toss unused, unset epoxy into the trash. Set it outside on a nonflammable surface until it has set up, then dispose of it properly.
Before mixing epoxy, ensure that the surfaces to be glued are clean and free of dust and sanding residues, waxes, and oils. Professionals generally wipe surfaces with a solvent such as Dupont’s Prep-Sol or acetone. If the solvent beads up, it indicates that additional contaminants are present and the surface should be cleaned again. Use 40- to 60-grit sandpaper on wood to roughen the surface for a good bond. If you are gluing teak, wipe the surface with a solvent such as acetone to remove any teak oil before gluing.
If you are using epoxy for a simple wood joint, coat both surfaces you want to join, wait a minute or two to allow the glue to penetrate the wood, and press together the two surfaces. Clamping or screwing the pieces after you’ve applied the epoxy will further strengthen the join as the glue sets; however, beware of clamping it too tight; if you squeeze most of the epoxy out of the joint, it is likely to fail. After the epoxy has set, use a chisel to clean excess glue off the surfaces of the wood.
It’s all right if your application skills produce a job that’s a bit sloppy. Epoxy works best in sloppy joints where the glue is plentiful. To mask errors, simply mix a little wood filler or fine sawdust into the epoxy and set up the joint in the usual way. The beauty of epoxy is that it both fills small holes and gives a strong joint. In fact, experiments have shown that the wood will fail before the epoxy in a joint does.
The hardener speed and the temperature at the time of using the glue govern the cure time of an epoxy, although the amount of epoxy used and humidity also come into play. By using a fast hardener — the fastest I know of are MAS Fast Hardener and the newest variety of Epiglass hardener, both with a pot life of 7 to 10 minutes (pot life is the time it takes for the epoxy to set up in the container in which you mix it) — and allowing the epoxy to set in a temperature of around 75°F to 80°F, the cure time can be so short that you may not have time to set up the job properly. Remember that pot life is shorter than the amount of time it takes spread epoxy to set. A mixture tends to “kick” sooner in a deep pot than it will when spread thinly over a flat area. To extend the pot life of your resin, pour the epoxy into a shallower pot. You can also extend the pot life by half an hour or more by using a slower hardener and working in 50°F to 60°F temperatures. The amount of time the resin mixture stays in liquid form is known as the open time. This is the total amount of time that you have to work with the glue. Trying to use resin after it has started to cure produces a result that looks like a painted surface that has been disturbed when partly dry. In addition, the partially dry epoxy will not create a strong bond.
Sanding epoxies can be bad for your health because of the dust it produces. When undertaking the job, at the minimum wear a dust mask, though it’s best to wear a respirator. Epoxies can be sanded with normal sanding tools, but you must be careful not to over-sand the soft wood around a hard epoxy finish, which is easy to do. In most cases, epoxies should be left to cure for about 24 hours before they are sanded, although some fast-curing epoxies may be sanded after 6 or 8 hours.
There are myriad details in the design and building of a greenhouse, particularly concerning the roof construction, the way glazing is attached to the rafters and ridge, the way the greenhouse is attached to the foundation, and the construction of doors and windows. Although some of these may be specific to a certain design, many are common to most greenhouses.
When working with epoxy, you should protect your eyes with safety goggles and avoid breathing the vapors. If you’re not working in a well-ventilated area, wear a respirator. Some epoxies contain volatile organic compounds (VOCs) and phenols, which can make you quite ill. If you are doing any sanding, wear a respirator to ensure that you do not breathe in sanding dust. If you get any epoxy in your eye, most manufacturers recommend immediately flushing it with running water for at least 15 minutes. Epoxy fillers such as microballoons used to thicken the glue mixture (which can be fairly runny) are stirred into mixed epoxy. Because these fillers usually come in the form of powders, to prevent inhaling them, you should wear a respirator (or, at minimum, a dust mask) when working with them.
Epoxy is nasty stuff to get on your skin. In fact, it can be so sensitizing to your skin that you break out in a rash as soon as somebody opens a can of the material. When working with it, always use a barrier cream on any exposed skin and wear disposable gloves. Avoid direct skin contact with resin and hardener and wash thoroughly with soap and water after the job is done. Even though many professionals wash with solvent, I don’t recommend doing so because the dilution of most solvents allows them to penetrate the skin very easily. Instead, use plenty of soap and water — and I’ve found that Hand and Tool Towels from Bostick are ideal for removing any resin that does get on the skin. Packaged in a can, these towels are handy to have around your workplace and home.
After the job, wash tools with acetone or white vinegar or (carefully) use the manufacturer’s suggested cleaner. Don’t discard containers of unused epoxy mixtures until the epoxy in them has completely set. Remember that heat is given off when epoxy sets up and if the mixture sets up in a container, it can emit smoke or even combust. Containers of unused, unset epoxy mix should be set on the ground outside the shop (never in a trash can) until the mixture cures and you can safely dispose of them.
If you have questions about the toxicity of any materials that you are working with, you can obtain a Material Safety Data Sheet (MSDS) from the manufacturer. MSDSs are required for all epoxies and provide details on all toxic materials in the resin and hardener.
There are a number of details related to the construction of the rafters. If the rafters call for cutting a joint so that each rafter comes flush to the top of the vertical wall, some way must be devised to keep the glazing from sliding right off the framework. In between the rafters you’ll need to set a filler piece for the glazing to rest on before adding the blocking to keep the glazing in place. On top of the blocking and glazing, add a trim piece with a bead of caulking near the top to keep water out of the joint. In addition, some edging is required at the bottom of the glazing to keep out rainwater.
There are also certain construction details related to the ridgepole. While I’ve chosen to support the ridgepole of my long greenhouse with a double vertical center pole, a single vertical support would work just as well. The glazing (polycarbonate in the illustration below) is laid on the rafters and held in place by a cap piece on each rafter. At the ridge the glazing is fitted under a covering board mitered to suit the angle of the roof. This covering board is either epoxied or caulked at the mitered, vertical joint so that it remains watertight. In fact, by sloping the upper edge of the covering board and caulking it to the glazing, I have found that it stays watertight in almost all conditions.
Keeping the water out of your greenhouse starts at the top of the structure, with the roof and ridgepole assembly and the covering that overlaps the glazing. At the bottom of the roof glazing, the cap pieces are tapered to allow water to run off and all joints are horizontal. Caulking and flashing are used extensively to ensure that water stays on the outside of the structure.
Insulation added to a foundation (A) significantly improves heat retention in the greenhouse. In warmer climates, you may not need insulation (B).
The glazing is also caulked at the bottom of the sidewall to keep out moisture. In addition, the glazing rests on hard rubber pads to help absorb its weight and to give it a little room to expand and contract. If the foundation is insulated, you should make sure that the weight of the structure rests on the concrete portion of the foundation instead of the portion over the insulation. Because the greenhouse is set back from the edge of the foundation, it’s important to add some flashing to direct water away from the wood.
A single window is not too difficult to build; however, even the simplest window can be cumbersome to operate if appropriate materials are not used. For example, in my heated greenhouse, I used 1×11⁄2 stock to frame the 24-inch by 18-inch windows in the structure. After a season of use, however, I found that the lightweight wood frame was distorted where the weight of the glass pulled down on the corners and where the hinges, located nearer the middle of the frame, held it to the framing member. The cure was to increase the size of the horizontal frame members to better support the weight of the glazing.
One difficulty in making windows is accommodating the size of the glazing. If you use double-pane glass, it will probably be at least 3⁄8 inch thick. This requires that the wooden part of the structure is at least 1 inch thick in order for there to be sufficient material on either side of the glazing. In addition, because the glass weighs a fair amount, your structure must be quite strong.
Fastening the glazing in place can also be problematical. For the single-pane window shown in (A) in the illustration above, the wooden frame is routed or sawed to suit the thickness of the glass. The corners are mitered and attached with biscuits to increase their strength. The glass is inserted before the framing is fully closed and is held tightly in place by the frame. With this method, if the glass in the window breaks, you’ll have to take apart the frame to insert a new piece of glass. Yet there is no tricky nailing of trim or quarter round to hold the glass in place.
In (B) in the illustration, you can see how a piece of quarter-round wooden trim is inserted to hold the glass in place. I have found, to my chagrin, that it is very easy to break the glass when nailing quarter round in place.
Windows can be constructed in a variety of ways. (A) shows glazing set in a routed frame with mitered corners. (B) shows mitered corners and a piece of quarter round to hold glazing in place. (C) shows a frame with lap joints. (D) and (E) show construction for double-wall glass or polycarbonate.
Making windows for double-pane glass or polycarbonate introduces more difficulties in terms of the frame thickness. Different methods are used to hold double-pane or polycarbonate in place, mostly involving greatly increasing the thickness of the framing material.
To make a window, first determine how much lumber you’ll need. Let’s say you are going to make a 24-inch by 18-inch window for the side wall of the greenhouse. You’ll use 11⁄2×3 stock. (Because double-pane glass is 3⁄8 inch thick, you’ll need a substantial frame thickness.) Calculate the perimeter of the window to make sure you buy the right amount of stock. For a 24-inch by 18-inch window, multiply 24 inches by 2 lengths and 18 inches by 2 widths and add them together:
(24 × 2) + (18 × 2) = 48 + 36 = 84 inches or 7 feet
If you buy an 8-foot length of stock, you’ll have a little extra for the corners, depending on the joints you decide to use. In this case, let’s say you’ll use simple lap joints.
You’ll also have to determine the size of the opening for the glass. If the stock is 3 inches wide (actually 21⁄2) and you want the glass to overlap the frame by 1 inch, the opening will be:
24 – (2 × 2.5) = 19 inches tall and
18 – (2 × 2.5) = 13 inches wide
But you’ll need some overlap to hold the glass in place. If you allow 1 inch of overlap, the final size of the opening for the glass is 21 inches by 15 inches. This size requires that you specially order the glass, which you should do before beginning the window construction. If you use single-pane glass, it will be 1⁄8 inch thick, but for better insulation value, go with double-pane glass at 3⁄8 inch thick. If you were to use true 1-inch stock for the window frame, you could easily take out 3⁄8 inch of material and have some meat left over, but if you use standard 1-inch lumber, which is actually I inch, and cut 3⁄8 inch of material from it, you’ll end up with 3⁄8 inch of material left to support the glass. This means you’ll have to screw a cap to the window frame to hold the glass in place.
Once you cut from the wood the 1-inch by 3⁄8-inch piece for the glass by running it through the table saw or routing it, you’ll end up with an L-shaped piece of wood.
Cut the wood to length to get two 24-inch pieces and two 18-inch pieces, then cut lap joints in each corner. Remember that the laps on the shorter sides will be opposite the laps on the longer sides. With everything cut, you can glue up the frame. I prefer epoxy, but any waterproof glue will do the job. You may want to add a couple of 5⁄8-inch wood screws in each corner, but it’s not necessary. Check that the frame is square by measuring the diagonals and using a framing square. The glass is square, and if your frame is slightly off, you’ll have to do some chiseling. If you plan on painting the frame, now is the time to do it, before you install the glass. If you plan to use caulking, set it in place on the painted wood, then install the glass.
Next, you’ll need to make a cap piece. This can be made from 1⁄2-inch by 2-inch wood tapered on each side and screwed to the frame to hold the glazing in place. Remember to paint the cap piece before attaching it.
The last job is to install the hinges and any closing hardware. I prefer to do this after the glass is in place, but you may want to cut the hinge recess before you install the glass. The completed window is now ready to be installed in the framed opening.
A typical window frame with recessed glazing. Cutting a 1-inch by 3⁄8-inch piece of wood from the frame leaves an L-shaped frame to accept the glass (A). (B) shows the measurements and construction of the frame.
Because I have a great deal of experience designing boats, I look upon overhead windows and vents as similar to hatches on a boat. Both are intended to keep water from entering the structure. The following illustrations show my design for an opening window in the roof of my greenhouse. The method for constructing a window like this is similar to that for regular windows, but in this case, the window frame is larger than the opening and fits over a lip that stands about 1 inch high. When water runs under the hatch, it is this lip that keeps the water from running down inside the greenhouse.
The lip is made of 3⁄4-inch by 3-inch stock and runs around the inside perimeter of the opening. To make the hatch lighter, the glazing for the window can be made of single- or double-wall polycarbonate screwed directly to a wooden frame. Although this type of window is relatively light and simple to lift, it racks easily, the corner joints can break, and it does not retain heat as well as a hatch cover with double-pane glass. For a more substantial hatch cover, make a window as you would for a side wall (see the previous instructions) and screw it to the hatch frame that fits around the lip.
The lip is high enough to come within 1⁄8 inch of the underside of the window frame. On top of the lip, a strip of soft foam butts against the frame and keeps the entire structure airtight. This style of window can be made to suit any size opening and keeps it airtight as well as waterproof. The major problem that I have encountered is window weight (especially with double-pane glass windows; mine weigh about 9 pounds each). This heavier weight makes it difficult to use automatic window openers, even though some are advertised as being able to lift heavy windows. The heavy-duty window opener shown in chapter 4 would probably work well with this hatch, however.
This style of overhead window keeps rain out of your greenhouse.
In most cases, you’ll buy a door for your greenhouse. Commercially made doors often come with premade trim and a casing requiring a rough opening (371⁄2 inches for a 36-inch-wide door). You can make your own door, however, referring to plans in a number of books on carpentry.
For outdoor protection, it’s best to paint a door. With a glass insert, the door adds light to the interior of the greenhouse and a screen insert aids in air circulation, but if you plan to set a door into an unglazed end wall or a north wall, you might want to make it of solid wood. If the door is to be the outer entry to an air lock, you can add rigid foam insulation to the frame to increase its insulation value and then cover this with 1⁄8-inch or 1⁄4-inch waterproof plywood to help prevent rot.
When I was building my freestanding greenhouse, one evening I set the large pieces of 34-inch by 76-inch glass glazing on top of the framework and stopped work for the day. In the morning, I found that one of the sheets of glass had been blown out of its frame and had slid off the greenhouse. Fortunately, it landed on a pile of leaves and didn’t break. Since that experience, I have come to the conclusion that the glazing of any greenhouse should always be firmly fastened to the greenhouse frame!
There are several ways to fasten glass, polycarbonate, or fiberglass glazing to the rafters. The first, most important step is to make sure that your rafters are suitably spaced for the kind of glazing you’ll be installing. For example, for glass panes that are 34 inches wide, while you might assume that the rafters should be spaced 34 inches on center, there should actually be 3⁄8 inch of space between each pane to allow for expansion and to fasten the cap piece in place. This means the rafters should be spaced 343⁄8 inches apart.
If your glazing is 48-inch-wide polycarbonate panels and you intend to fasten these directly to the rafters, you’ll need to space the rafters 48 inches apart. If you plan to use a commercially made cap piece, locate the rafters at a distance determined by the manufacturer of the cap so that the glazing will fit into or under each cap piece. Make sure that you check this distance before you start building.
Fiberglass glazing is often overlapped when installed. Thus, if your fiberglass panel is 48 inches wide, space your rafters 461⁄2 inches on center to ensure that each panel fully overlaps the rafter.
Glazing on the roof can be installed in three ways, depending on the kind of glazing used: In (A) and (B), glazing is installed on top of rafters. In (C) — best for double-pane glass — glazing is flush to rafters and supported with blocking set below rafters.
The illustration above shows how several types of glazing are fastened to rafters (A shows polycarbonate panels, B shows single-pane glass, and C shows double-pane glass). If you find that a 3⁄4-inch-wide rafter is not sufficient to support a large sheet of polycarbonate or glass, you can add an extra support along the edge of the rafter. The first and second drawings (A and B) show this support mounted at the top of the rafter, while the third (C) shows it set slightly below the top edge, with the glazing set flush to the top of the rafter. While method C requires more precision when you’re building the greenhouse, it does allow you a full 11⁄2 inches on which to screw or nail down the cap, whereas methods A and B allow only 1⁄8 to 1⁄4 inch between the glazing panels to insert a nail or screw. I prefer to use methods A and B for polycarbonate glazing and method C for glass.
Installing Glass If you use method C to install glass glazing, spacing rafters and supports to match exactly the width of the glass panes and mounting blocking on each rafter to support the panes, it’s important that the entire greenhouse frame is very rigid. If it flexes at all, the glass will crack. In general, the more flexible the greenhouse frame, the less suitable it is for glass glazing. Polycarbonate’s “give” better accommodates this flexing and poses less risk of injury if it breaks.
Because glass is a fragile material, it must be installed very carefully. Snagging a sheet of glass on a nail or screw, bending it as you lay it in place, or trying to install it on a windy day can easily lead to picking glass out of your greenhouse frame and beds.
To prevent water from getting under the glass, mount it on waterproof caulking such as isobutyl or polybutylene. (See Caulking and Weather Stripping in chapter 7 for more information on using caulking with greenhouse glazing.) After the caulking is applied, lower each glass pane into place and wedge it along the bottom and sides with hard rubber blocks. Once the pane is in place, I use a piece of quarter round or a cap piece over the glazing to hold it. You can also use good-quality caulking such as isobutyl or polybutylene or even silicone in place of a cap piece, if you prefer. If you use caulking, however, be aware that it may be difficult to remove the glass should it be necessary to replace a pane.
When installing the quarter round to hold the glass in place, take care that nails holding the quarter round to the greenhouse frame do not touch the glass. It’s preferable to use a pneumatic finish nail gun for this job rather than a hammer to prevent glass and nail contact, or, best of all, you could screw into predrilled holes. When attaching beading around the glass pane, insert the nails parallel to the glass surface to keep them clear of the glass. Of course, if you’re uncomfortable nailing the quarter round in place, you can use glazier’s points and putty, but a new greenhouse has a lot of glass and applying that much putty is not my idea of fun.
Glass is installed on neoprene rubber blocks to allow it to expand and move slightly when the greenhouse structure flexes.
Installing Polycarbonate Panels According to the manufacturers, you should adopt some general practices when working with polycarbonate panels. Store the panels in a cool, dry spot, preferably indoors, and leave their protective coverings in place to prevent scratches and damage; you can remove them after the installation is complete. Seal the ends of the sheets with the special tape or U-shaped channels supplied by the manufacturer to prevent insects and moisture from getting inside the greenhouse once the panels are installed. When cutting panels, do it carefully and slowly, using a circular saw with a hollow ground blade and 10 to 12 teeth per inch, a jigsaw with a fine-toothed blade, or a lino cutting knife. (See the box Cutting Polycarbonate, below.)
In order to allow the polycarbonate sheets to move when they expand, any holes you drill for fasteners should be slightly oversized. According to the International Greenhouse Company Website (see Resources), you should drill a 3⁄16-inch hole for a 1⁄4-inch screw, and any holes near the edge of the sheet should be at least twice the fastener diameter or 1⁄2 inch, whichever is greater.
Also to allow for expansion, leave a space between panels when you install them. Some manufacturers suggest a space of up to 1⁄8 inch per 3 feet of width or length where temperatures may vary by up to 100 degrees. Bronze-tinted panels require 30 percent more expansion space. After installation, use a clean sponge or cloth to wash the panels with mild soap and water (do not use ammonia-based cleaners, which will deteriorate the glazing), and rinse them thoroughly.
There are several ways to fasten polycarbonate glazing to the greenhouse roof and walls. The easiest is to use a polycarbonate extrusion that looks like a sideways letter H. This piece is fastened to the frame and the polycarbonate panels fit into it. There are a number of variations on this extrusion, all made of polycarbonate.
Another method involves laying a rubber or butyl gasket on the greenhouse frame. The polycarbonate panel is laid on top of the gasket and a rubber gasket and metal bar cap are screwed in place over the joint.
Note that polycarbonate panels should always be installed with the ribs running vertically, to help water run off them. Most manufacturers suggest that you drill 1⁄4-inch weep holes in the bottom end cap every 12 inches or so to allow an escape for moisture that has drained to the bottom of the panels.
Panels can be screwed directly to a wooden structure, but where the panels butt together, the joint should be covered with a rafter bar. When screws are inserted through glazing, they impose a point load on the glazing. By installing a rafter bar over the glazing and screwing through it, the point load is spread over a larger area. The rafter bar also eliminates screw holes near the edge of the panel. The International Greenhouse Company recommends very specific locations for screws when you are installing multiwall polycarbonate panels. When polycarbonate is installed on aluminum-framed greenhouses, the multiwall material is slotted directly into the extruded frame, which supplies a builtin expansion gap.
The following installation information comes with permission from Suntuf, Inc., manufacturer of square, corrugated polycarbonate panels. While the instructions are specific to its sheets, the information is applicable to all other polycarbonates.
To cut polycarbonate panels, use a circular saw, a utility knife, and a straightedge. Reverse a plywood blade in the circular saw, then run the saw through the sheet at a slow speed. Support the polycarbonate sheets as you cut them to prevent vibration. Using this method, up to five panels can be stacked and cut at one time.
Install panels with the label side facing up toward the sky (sheet corrugations at each side will then face down). Closure strips or caps for the joints between the glass are made of either wood or foam. Foam closures are recommended because they can be stretched slightly to align with the panels and form a compression fit. Suntuf fasteners with neoprene washers are formulated to be compatible with Suntuf polycarbonate panels. Whatever the brand of polycarbonate panels you choose, be sure to use only the manufacturer’s recommended fasteners to install them. Predrill all fastener holes oversized — with a 3/16-inch drill bit — to accommodate the thermal movement of the panel. Failure to accommodate for thermal movement will cause the sheet to buckle after it’s installed.
Attach fasteners on the crown of every other rib in the horizontal direction. In high wind areas, fasten on the crown of every rib. Use 2-inch fasteners (special long screws with load-distributing heads) for all roof applications. Do NOT overtighten fasteners, which will cause the rubber washers to compress. Tip: Back out the fastener one-half turn after the neoprene washer touches the panel. When installed properly, the neoprene washer should just touch the panel.
Avoid contact between the panels and any chemicals, paints, adhesives, or other synthetic materials that are incompatible with polycarbonate. Never use glass cleaners with ammonia or ammonia-based products to clean the panels. Instead, clean them with a lukewarm, soapy solution using a soft cloth or sponge. Do not use abrasive brushes; these will mark the surface.
Installing Fiberglass Panels Greenhouse fiberglass comes in rolls usually 50 or 100 feet long and 4 feet wide. Flat fiberglass is flexible and is easily cut to size, although you should wear gloves and a dust mask when working with the material. Some types of greenhouse fiberglass are not particularly smooth and you can get painful fiberglass splinters in your fingers if you don’t wear gloves when installing them.
When fitting fiberglass to the greenhouse roof, flat panels should be overlapped and carefully screwed or nailed in place. The rafters for fiberglass should be 231⁄2 inches on center to allow enough space to overlap the glazing at the edges. A bead of caulking at the overlap is usually enough to prevent rainwater from penetrating the covering. Next, install a long batten over the overlap and screw through the batten to make sure that the load is evenly distributed along the edge of the material or use screws or nails that pass through felt washers backed with steel washers to help distribute the load. This will help prevent the material from tearing away from nail heads in high winds.
Install corrugated fiberglass by screwing it to specially designed spacers that match the corrugations in the panels. Screws should be installed through the ridges, not in the valleys, where they can cause leaking. When screwing down the material, drill oversized holes through the sheet and use large felt washers backed with metal washers to hold the fiberglass without tearing through the sheeting.
Overlap fiberglass when screwing the caps in place to ensure that there will be no leaks (A). Because fiberglass can flex, it should be screwed down around the edges, with screws about 8 to 10 inches apart. Edges are fastened down using long battens and screws (B). If you use corrugated fiberglass, make sure that you position filler pieces at the top and bottom of the panels to ensure that there is no heat loss at these points.
Installing Acrylic Panels Because acrylic expands a great deal, you’ll need to allow for this expansion when fastening the material to the wooden structure of the greenhouse: All drilled holes should be oversized and large washers should be used to hold the material in place; panels can also be fastened with battens screwed into place. If you are building a kit greenhouse, the acrylic glazing will fit into the extruded aluminum frame and is held in place with either flexible stripping or special clips that are pressed into place.
Also helpful in allowing for the expansion of acrylic sheets is the installation of support bars across them. In most other ways, the installation of acrylic sheets is very similar to the installation of polycarbonate panels; refer to the instructions in Installing Polycarbonate Panels.
Wall glazing is installed in a manner similar to that for roof glazing. As for the roof, confirm the size of glazing and the required support spacing before you build your greenhouse. The illustration at right (representing a view from above) shows how wall glazing is attached. In the first illustration (A), polycarbonate glazing is attached using a cap piece milled from 3⁄4-inch by 11⁄2-inch stock. In the second drawing (B), glass glazing is recessed into the support, which requires cutting or routing out part of the support to allow the glass to fit. Once in place, the glass is held by a trim piece that is either nailed or screwed to the upright stud. The third illustration (C) shows an arrangement for either polycarbonate or double-pane glass. Because the side wall provides only 11⁄2 to 2 inches of attaching surface (depending on the size stud you use), you might want to add additional supports as shown. They can provide more integrity if you live in a windy area or one that is hurricane-prone, although they block more incoming light.
Installing wall glazing. (A) shows a method for double-wall polycarbonate, (B) shows a method for single-pane glass, and (C) shows a method for either polycarbonate or double-pane glass.
Installing polyethylene (plastic) sheeting to a wood-frame greenhouse may require simply laying the sheeting over the top of the greenhouse and fastening it in place. In my experience, however, the best way to attach plastic sheeting to a greenhouse frame is to lay furring strips over the uprights in the frame or any supports and nail or screw the sheeting through the furring strips. This method helps to prevent the plastic from ripping off the nails in strong winds.
Note that using a commercial-grade covering on a PVC hoop house that you have built yourself will invalidate the manufacturer’s warranty. You might use construction-grade polyethylene as a covering. The easiest way to cover a hoop house with a PVC pipe frame is to nail or screw the plastic to the baseboard on the perimeter of the greenhouse through wood or furring strips, though with only the bottom fastened to the greenhouse, the top tends to flap around in the wind. I’ve solved this problem by tying lengths of string from the baseboard on one side of the structure to the corresponding baseboard on the other side. This eliminates the flapping and cuts down on tearing of the material. Construction polyethylene tends to tear along the fold lines, but any rips can easily be repaired with clear polyethylene tape or, in a pinch, with duct tape.