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A timber truss is a cut or hewn log that is used for creating a truss roof structure. Timber trusses are usually exposed in a finished product and are selected for aesthetics as well as for strength. In an open ceiling design, timber trusses are stronger over wider lengths than any single beam of the same length can be. This is partly true because timber trusses are usually thicker. Sometimes that thickness is achieved in a single piece of timber; other times it's comprised glulam — multiple pieces of wood that are glued together.
The most common types of wood used for a timber truss are Douglas fir, western red cedar, and southern yellow pine, although in some areas of the world other types are utilized. Each piece is usually well-seasoned before use in both commercial and residential structures. Seasoing the wood helps minimize shrinkage and twist as the truss ages. Some builders recycle older timber trusses for their aesthetic appeal, but most usually are wary of using older timbers as they can weaken at the eave joint due to time fatigue.
Many different types of timber trusses can be found in barns, homes, businesses, and covered bridges. Common designs include king post, queen post, scissor, and parallel chord/bridge. Each type offers different load-bearing advantages and styles. A timber truss can also take on a rustic appearance through the different types of finishes. Most finishing options include smooth, rough sawn, scalloped or hand-hewn, and wire brushed for an aged appearance.
Methods of joining timbers together to form a timber truss vary by application. Some use mortise and tenon joinery that employs pegs of hardwood driven into tight-fitting holes. Others use heavy bolts with thick steel plates, called gussets, with an antiqued, blackened finish. Steel ties, or webbing, are sometimes used to help spread the load more evenly across the entire truss. This helps increase structural stability as well as offer a unique interior appearance. Plates and ties reduce the amount of timber necessary for the truss, therefore minimizing the heft, price, and feel of the finished timber truss.
Glulams differ from single timber board applications in that they are made from thicker boards glued together to form the final timber truss piece. For their size, they are stronger than solid timber due to their use of glue and the staggered arrangement of the boards in the laminate. The finished product looks much like a loaf of wood that has been glued together. Glulams typically are more expensive than sawn timbers of equivalent size, but offer a unique finished product that often is preferred over the single timber truss design.
Another advantage of a timber truss system, besides strength and beauty, is that timber trusses usually offer a better fire rating over steel. In case of a fire, a timber truss system can last for up to one hour before failure. Conventional steel trusses typically fail quickly in a fire, warping and twisting under the intense heat.
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