LEARN MORE ABOUT TIMBER FRAMING

Perhaps it's a just a difference of definition; many people use the terms "timber frame(ing)" and "post and beam" interchangeably. As the terms themselves illustrate though, they don't translate exactly. One is a structural unit; the other is a collection of parts of the whole. The terms are not used in the same way; we may build a post and beam barn, or house, or studio, or we may build a timber frame, which will be the structural center-piece of the building. People raise timber frames, and build post and beams. It is a small difference, but a significant one.

What's most important is that people interested in timber structures educate themselves in the different ways of joining timber, and choose a joining system and frame style that fulfills the structural needs and aesthetics of the spaces they are creating.

While both systems can and do use mechanical connections - and there is ample historical evidence for the use of metal mechanical fastening within ancient timber frames - post and beam structures tend to rely more heavily on mechanical connections and enclosure systems for their structural integrity. For us, the essential difference between "timber framing" and "post and beam" lies in the connections of the structural members, their cooperative work in providing a structural framework capable of sustaining loads whatever the enclosure system may be, and the attention and care given to the aesthetics of the work - the chamfer sizes and stops as well as the overall relationships between the spaces, the timbers defining them, and the entire frame.

Timber framed structures rely on the partnerships inherent in their design: joinery that accounts for loads, and the physical properties of the material used; timbers sized to take the loads expected; bracing appropriate for the lateral loads anticipated. Most timber framers also pay considerable attention to how the frame will look and feel to a person entering the space. We may use a 5x8 timber when a 5x7 will do according to load analysis, because it looks better. We may use a brace with a 36" leg length when a 30" one will do, for the same reason.

The fundamental joint of timber framing is the mortise and tenon. In its basic form it is simply a hole in one timber and a corresponding projection or tongue in another, to be inserted and pegged through both. The size of the mortise and the tenon are determined by the loads placed on them, and the wood species used.

Housings, pockets, and shelves are used to help bear loads, fix pieces in location, and can help control a timber that may roll a bit as it dries. They are also used to help the joint appear "clean" as shrinking occurs.

Tie joints, dovetails, wedged through tenons, and splines are used in places where a tensile, or pulling force occurs. These joints are especially demanding of accurate engineering on the desk and careful choice in the shop; trees grow in resistance to gravity, not some force trying to pull them into the air, and generally don't like being stretched.

The tie-joint is a classic, having reached the pinnacle of development in 13th century England and remained virtually unchanged since. It is used to secure the foundation, so to speak, of the roof and upper structure of a frame. It ties the post and eave plate of one wall to each other and to their partners on the opposite wall, and provides a secured apex for cross-lateral braces.

The dovetail joint is often used to tie together girts and rafters bearing floor joists and roof purlins. Discretion must be used, though, since the joint cuts into the working part of the bearing member. Dovetails are most commonly used in combination with whole-piece housings, since a bare-shouldered dovetailed piece may check and crack at the point that the dovetail leaves the timber.

A through-tenon will need to be longer in pine than oak, to hold the same force, and may not be practical for some designs. For example, in a king post truss of any appreciable span, without rafter struts, the tenon of the king post needs to extend below the chord/tie beam, affecting or effecting the aesthetics of the truss.

Splines are used where timbers intersect at 180 degrees to a post, rafter, or girt, and provide strength and control of dimensions virtually impossible with tenons. Protect your posts!

Some frames require timber longer than is available. Scarf and lap joints are used to "splice" timber to create a longer piece. A lap joint is generally used in situation where there is continual support under the joint; as a sill bearing directly on a foundation.

Scarf joints are used higher in the structure, in eave plates and purlin plates, for instance. The lengths and conformations of scarf joints depend on the dimensions of the timber used, and the forces exerted on them. There are many types and styles, but all must be placed correctly into the frame in order to work most effectively. Contrary to intuition, a scarf joint located directly over a post is at risk of failure because of the high reaction lateral shear forces exerted there. Laps and scarfs whose lengths are not at least three times the vertical dimension of the joined members are also at risk of failure.

There are many variations of all the joints used to join timber, each variation dependent on its place and its job within the frame. A well-designed frame will provide a secure structural framework, using appropriate material and not requiring any "unnecessary" joinery.