Timber-Framing 101
A TIMBER-FRAMING PRIMER
A timber frame is a load-bearing wooden structure held together by wood joinery. Variations of this construction method have been used for millennia by cultures around the world, and it was popular in 19th century America, as seen in surviving barns. Today, timber-framing is enjoying a resurgence, owing to superior strength, durability, and flexibility. Not to mention elegance. “Post-and-beam” is often used interchangeably with “timber-frame,” but the terms are not synonymous. Post-and-beam structures can be timber-framed, but they can also be held together with bolts and other steel connections.
Glossary of Timber-Frame Terms
Bent — the combination of posts, tie beam, rafters and other members that establishes the shape of the building.
Bay — the area between the bents, spaced as needed or engineered. Often placed 8′ to 20′ on center.
Hammer Beam Truss — a more ornate truss that can have either curved or straight timber members.
Hand Hewn — cut using a combination of a broad axe and adze.
Housed Joints — timbers held in place by mortise and tenon and secured in a housing to prevent rotation.
King Post — a vertical post that supports the center of a roof.
King Post Truss — often used when a clear span is desired. The timber member that connects at the peak of the building is the King Post. The other members of the truss are joined into the King Post.
Knee Brace — typically set into the frame at an angle to the posts and plates to strengthen it and hold it from racking.
Loft Joist — structural members that support loft flooring and are commonly joined to the frame at the loft level.
Mortise — the recessed cut in the timber that receives the tenon to connect the joints. A trunnel is then used to secure the joint.
Planed Timber — each side is smooth and even.
Post — vertical timber that forms a wall or an interior structural member.
Principle Rafter — the primary roof structure that forms the roof pitch and supports the lesser structural roof members, such as purlins.
Purlin — also called a sub-rafter, a lesser member connected to and supported by the principle rafters. The roof sheathing is commonly attached to the purlins.
Queen Post — a vertical post, placed off-center to support the roof structure.
Queen Post Truss — the combination of posts, tie beam, rafters and other members that provides a clear span area such as a drive through area of a barn.
Sill — the structural member that sits on the foundation and supports the floor framing.
Scissor Truss — allows various clear spans through diagonal bracing crossing the rafter members, attaching to the top of the rafters.
Strut — roof member that sits on the tie beam and gives additional support to the principle rafter, canted purlin, principle purlin or other roof bearing member.
Tenon — the protrusion on the timber, made for insertion into the mortise.
Trunnel — aka, tree nail, the peg that secures the tenon into a mortise.
Tie Beam — a horizontal timber that spans the width of a structure and ties the two opposing walls together
Wind Brace — similar to a knee brace although typically placed in the roof frame to resist racking and movement due to wind pressure on the building.