types of joinery
The joinery in your frame not only connects and secures the timbers, but also plays a role in transferring the loads and stresses of your house from one frame to another. All of the many types of joints are designed for specific functions. Some of the mos
Birdsmouth. A complex cut made at the tail end, or bottom, of a rafter that allows the timber to extend over and past the wall top plate, providing a greater bearing and attachment surface.
Butt. One of the least complicated joints, in which mating pieces are square-cut and simply butted against one another. Because this joint has little inherent strength and depends on gravity or fasteners to remain in place, it is typically limited to intersections that are not subject to movement or strong opposing forces, such as where timber posts rest atop horizontal beams.
Dovetail. A commonly used joint that includes a fan-shaped tusk or tenon that drops into and interlocks in a similarly shaped pocket cut. The wedge-like shape of this extremely strong joint prevents the interlocked timbers from shifting or separating from one another.
Lap. A joint in which the ends of two timbers are cut at matching angles and simply overlaid, or “overlapped,” then fastened to each other. Because the wood grain direction of the mating pieces is parallel, these joints are easily concealed and often invisible. Lap joints are typically used to extend, or lengthen, timbers in long horizontal runs.
Mortise & Tenon. A frequently used joint in timber framing, it includes a male end (tenon) cut onto the end of one timber that fits into a square-cut matching female receptacle (mortise). Like many timber frame joints, it is often locked in place by the addition of hardwood dowels, or pegs, called trunnels (“tree nails”).
Pocket Cut. Similar to a mortise, this joint receptacle typically is “open” in two dimensions; cut into the side or top face of a timber, it is designed to receive an identically shaped tenon or tusk formed at the end of a mating timber.
Step-Lapped Rafter Seat. An improved type of birdsmouth and overlapping joint, it typically includes complimentary cuts in the rafter and plate to resist downward and outward thrust, as well as side-to-side movement.
Shoulder. A ledge cut into the face of a joint; this added facet increases a beam’s load-carrying capacity by transferring downward force directly to the post while the joint’s tenon resists the lateral load, or tension.
Tongue & Fork. A specialized joint often used to connect the upper ends of rafters that meet to form a peak, or gable. One timber end is cut in an open U-shaped configuration (the fork), and a single tongue formed on the intersecting timber closely fits into the space between the fork ends.
of wood needed for a timber frame would take several years under cover, most timber framers use green, or unseasoned, timber to build their frames. Green timber is easier to work with tools because some woods become very hard when dry.
Green timber will shrink as it dries, and timber frame builders must calculate shrinkage into a frame’s design. While good joinery design is the best precaution against excessive shrinkage, shrinking timbers are sometimes held in place by the use of drawboring. In drawboring, peg holes are drilled slightly offset, so that the peg must take on a curve to follow the holes. In doing so, the peg acts as a spring, pulling the joint tight as it shrinks.
As the green timbers in your home dry and shrink, they will develop surface cracks or splits. These cracks are called checks. Checks do not affect the timber’s performance but merely indicate that the timber is seasoning. Most timber framers use proven strategies to minimize checking, such as applying paraffin to the end grain of timbers to slow the drying process. Other common strategies include working with a certain cut of wood or species of wood, especially in prominent locations in the frame. You can help minimize the checking in your frame by not overheating your home during the first few winters you live in it and by making sure the air in your home has adequate humidity.
USING RECYCLED MATERIALS
More and more, people are salvaging old timbers from old barns, homes and industrial buildings. Salvaged timbers that have been standing for several decades or centuries are already dry and will most likely not shrink any further. While it’s difficult to predict where a green timber will check, any checking that has occurred in old timbers is visible and can be hidden in a new home. Recycled timbers are often re-sawn to remove any twist or warp that occurred as they dried.
The work involved in salvaging recycled timbers makes them slightly more expensive than green timbers. After the old structure is dismantled, the timbers must be re-worked. First, nails are removed. Then the exterior weathered wood is skimmed and planed smooth. However, if the builder wishes to retain the original handhewn finish, the timbers are steam cleaned instead of planed. Drier wood is more difficult to work, which also adds to the cost.