Because the craft of timber framing has been with us for centuries, there are a multitude of frame designs available. Culture, climate, timber supply, available tools, and building purpose have all affected frame design in different regions of the world. This chapter concentrates on the frame design of the project house, although many aspects of this frame apply to other styles of frame design as well.
Bents and bays are very important to timber-framed buildings. The house has three bays — which are the areas between lines of structural supports — and there are four crossframes or bents that define these bays. Each bent is composed of a pair of two-story posts connected by a girding beam or crossbeam that supports the second floor, which together form an H. Diagonal braces are added to these bents to stiffen them laterally. The interior bents may also have an additional single-story (or prick) post to support second-floor loads. It is also important to note that each bent is numbered, starting from the west, for identification purposes.
Sills are the bottom-most part of the wooden house frame. They serve to spread the building’s weight over the foundation, to tie the top of the masonry (as opposed to poured concrete) foundation together, to join the frame at its base, and to prevent the surrounding soil from pushing in the basement wall. Most sills were rectangular rather than square in cross section and were laid with the broad faces flat. These plans call for 8×9 sill beams, a common sill size. All of the first-floor vertical posts are mortised into the sills. Joists may also be notched into the sills and the exterior wall planks are nailed to them. The longitudinal sills in the project house are 36 feet long, and the transverse sills (which span the building’s width) are 18 feet long. Although it is possible to purchase an 8×9 timber that is 36 feet long, you would do better to make each longitudinal sill from two shorter timbers that are scarfed or spliced together. Timbers under 20 feet long are cheaper and easier to procure and handle.
Condensation often makes foundations damp and moisture makes wood rot, so a rot-resistant wood species should be used for sills. Commercially pressure-treated timbers are one solution. Old framers used naturally rot-resistant wood species (see Chapter 4). Throughout North America you can find one or more of these rot-resistant species, which often cost less than pressure-treated timbers. In my area of the Northeast, for example, black cherry is a good choice for sills.
Two 8×10 sill girders (also laid flat) cross the basement or crawlspace to support floor joists and any above-basement interior posts and to strengthen the sills at the bents. Basement posts or crawl-space piers support these girders and prevent them from sagging.
Floor joists span the space between the sills and the sill girders. They transfer the loads of the flooring and objects on the flooring to the rest of the frame. On this particular frame, the joists span the 14- and 8-foot bays and notch into the sill girders and end sills. In many old frames, the floor joists for the first floor were logs, usually with the bark removed, that were only hewn flat on the top side to save time and money. Most frames today are sawn, so the drawings show rectangular joists, although log joists could be used as well. At least one joist in each bay should be mortised and tenoned in place to keep the cross sills and girders from bowing sideways as they season. The number and spacing of the floor joists can vary depending on the thickness of the flooring material and the desired stiffness of the floor, but my recommendations should meet most needs.
This section shows an end crossframe (numbers 1 and 4).
This section shows crossframe 2; 3 is a mirror image of 2.
HOW BRACES WORK
Some houses had a more complex floor system with a summer beam that spanned between girding beams. Floor joists then spanned from the outside wall girt to the summer, which was a shorter distance. Although the use of a summer beam meant more joinery work, it might use less board feet of material as the joists could be considerably smaller.
The chimney mass and basement stairs need openings in this floor system. By making the center bay the width of these elements, the framing necessary around these openings is simplified — the headers are simply joists.
Open-plan houses that lack the stiffening of interior walls require framed braces. Braces in timber-framed buildings are designed to work best in compression, not tension. In effect, any one brace can only resist force (such as wind) from one direction, so you should always use a pair of opposing braces. They can be placed virtually anywhere in the bent — up to the girding beam, down to the sill or girding beam, or in some combination thereof — as long as they work in opposition.
If you choose to build the hall-and-parlor house as a plank-on-timber frame, braces could be (and historically often were) omitted — exposed timbers and braces were not fashionable in houses after about 1800. Builders boxed in timbers with beaded edge boards. In English-style timber-stud houses (see Chapter 1), however, braces were common and necessary; they were buried in the wall cavity with the studs. But braces were difficult to conceal in plank-on-timber houses. Like plywood in modern houses, planking braced the building from the wind. The interior walls of these houses were also planks, and builders nailed them against the sides of timbers in the floor and ceiling to provide intermediate bracing. Houses without framed braces required temporary braces until the plank sheathing was attached. I recommend using braces where the drawings indicate them.
The girding beams are the heart of the second floor—they tie the front and back wall posts together and support the floor joists. They are often called by other names: girders, girts, girths, chimney girts, end girts, crossbeams, tie beams, and simply beams. Beams 2 and 3 carry the greatest loads of all the girding beams, so I have shortened their span by adding an additional one-story post.
Horizontal girts run between the posts on the exterior walls. These girts serve as nailers for wall planking and the upstairs flooring and help stiffen the posts at the second-floor level. The second-floor joists span between the girding beams. As with the first floor, openings can be framed without headers and trimmers, and at least one joist in each bay should be mortised and tenoned to keep the girding beams from bowing sideways. If you wish, you could use conventional floor joists (2×8s or greater) set in notches with plaster or wallboard applied below to create a space to conceal plumbing and wiring. Concealed joists became common after about 1800.
The two plates tie the four bents together at the post tops and support both the attic floor and the roof. Plates may be the most important timbers in a frame. Because plates are continuous, they tie the frame longitudinally and, with their braces to the posts, they brace the frame longitudinally as well. Plates also stiffen the junction of the wall and roof planes. Traditionally, all buildings had plates, even stone-walled structures.
The English tying joint was used to join four timbers to connect the walls, crossframes, and roof. Early builders often used a tapered, flared, or gunstock post, referred to by the English as a jowled post, which was hewn or sawn from the flared butt of a tree with the flare placed up. The extra thickness at the top allows for two tenons to secure the plate and tie beam. The heart of the tying system is a lap dovetail joint where the tie beam lies over the plate. The truss or principal rafter then tenons into the top of the tie beam. This tying joint entered the builder’s vocabulary in England around 1250 AD (see Hewett [1969]) and was a virtual standard thereafter. Though similar joints may be seen elsewhere in Europe, it seems that only the English examples have the lap dovetail joinery. The tying joint survived in America until the mid-nineteenth century.
When old-growth forests were common in this country, a plate could be a single timber 60 feet long on large buildings! On this frame, the plates are 36 feet long overall. This is the place to use two 36-foot timbers if you can find them. If not, scarf two pieces as with the long sills for each plate. The scarf should be in the short center bay so that it does not pose a structural problem.
Plates stiffen and tie the frame longitudinally; the attic floor framing stiffens and ties the top of the frame across its width (transversely). Over each exterior post in each bent a tie beam with a pair of braces down to the posts maintains the spacing of the plates: This is the tying joint. Traditionally, the tying joint was the most important joint in a frame. In our frame, the tying joint is not as important as it could be because the attic floor joists are parallel to the tie beams and reinforce them. As in old English frames, the plans show a dovetail where the tie beam laps over the plate. You can also use a jowled or gunstock post to further strengthen the tying joint to resist uplift, which is a good idea if you live in tornado, hurricane, or earthquake country. Note that the tie beams and joists notch over and extend past the plate. The extension strengthens their tying capability and provides a solidly framed cornice and overhang. Cornices of this size were typical of the late colonial, Federal, and Greek Revival periods, and the timber framing was integral to their support. You may have seen on some old houses a substantial cornice on the front but only a small one on the rear, which was a cost-saver. Cornices provide both visual and practical advantages. Even without fancy decorative elements, they dress up a house by creating a border and shadowline between the roof and wall. More importantly, they add to the life of the house by keeping the roof water runoff from draining down the walls and windows.
TYING JOINT WITH A STRAIGHT POST
TYING JOINT WITH A JOWLED POST
The attic joists and tie beams extend past the plate 1 foot to provide framing for the cornice.
Rafters tenon into tie beams between each bay to create trusses. Between these trusses are the common rafters.
A 2×12 raising plate is nailed to the projecting tie beams and joists. Nail rafters to this over every other attic joist.
Although the rest of the frame is 18 feet wide, the overhang makes the attic joists 20 feet long. As with the first and second floors, summer beams between the tie beams are an alternate way to frame the attic floor and thus cut the joists’ length in half. Summers involve a bit more joinery work but eliminate a number of 20-footers. If you do use summers, be sure the tie beams receive support from either a central post or a stud wall — otherwise tie beams cannot span the width of the house and support a summer beam at the center.
The attic floor joists are a full 2 inches thick and can be 8,9, or 10 inches wide. I suggest that the attic floor be insulated with fiberglass batts between the joists so the framing is concealed. The 10-inch depth of the joists was required to meet energy codes, not necessarily structural needs. Historically, joists were often 2×7s or 2×8s. If you use summer beams for the attic floor, shallower joists can be used (check your local insulation requirements) and the tie beams and summers can be partially exposed in the ceiling if you prefer to see more timber.
Each tie beam has mortises in its top for a rafter couple (a pair of rafters joined at their peak). Together, the three timbers form a rigid triangle or truss that stiffens the roof at bay intervals. On the projecting joist ends and between the truss rafter couples, a wide plank (called the raising or false plate — see Buchanan [1976] for more information) is nailed flat. The raising plate serves as a base for the intermediate rafters and makes it easier to lay the attic flooring — you won’t have to notch and fit the boards against the rafters. The intermediate rafters—which have the same dimensions as the truss rafters — are nailed to the raising plate. Most traditional builders placed a rafter directly above every other floor joist, as I have here. Also note that all rafters are mortised and tenoned at the apex and that the rafters taper in depth from butt to peak. Most old frames show the same taper: Small trees were sawn following their natural taper for economy. The roof could use smaller-diameter sawn timbers or logs hewn on the top if desired. If you choose the latter, the gable end rafters require two flattened sides because the wall planking attaches to them.
The roof pitch varied considerably in old timber frames. I used a 9- in-12 pitch, which is based on the 3-4-5 triangle. This pitch allows for headroom in the attic and was a common and attractive pitch.
I chose a common rafter system for the roof, which is the simplest, most economical approach to roof framing. The tapered rafters are mortised and tenoned at the peak. There is no ridgebeam.
Many timber framers today tend to construct bents that are entire cross sections of the building that include the rafters, much like slices of bread are to a loaf. Short members of one bay’s length connect these bents. In the past, however, timber-framed buildings were thought of in three dimensions and in layers. There were still bents, but they were not complete cross sections of the whole building. Each layer tied the building in a direction opposite to adjoining layers — the girding beams, for instance, link the building across its width, the plates above tie the building lengthwise, and the tie beams above the plates join the building across its width again. The building’s strength depended not on the fastening but on the overlapping layers, much like masonry: On a brick or stone building, the mortar doesn’t hold the building together. The overlapping or bonding of the courses of stone or brick binds the wall as a unit. A mason wouldn’t stack all the bricks directly above one another and then expect the mortar to hold everything. In this frame, most of the pegs could be removed (although I don’t recommend it) and the frame would still stand because of the frame layers. The sills, plates, and tie beams are the most important members in this system.
