spur

A spur is a timber member running at an angle from a vertical post to a beam, rafter, or other structural element, used to prevent the frame from racking (twisting out of square). In roof construction, spurs commonly brace the junction where a rafter meets a post, absorbing the lateral thrust that would otherwise bend the post. They are essential in timber frames where high loads or long unsupported spans create side forces that vertical members alone cannot safely resist.

The angle of the spur matters. A steeper angle, closer to vertical, transfers more load efficiently to the post and foundation; a shallower angle spreads the load across a longer timber but provides less stiffness. Most spurs are cut and fitted at 45 degrees or similar, though the exact angle depends on the geometry of the frame and the direction of the expected thrust. The spur is typically half-lapped or mortised into both the post and the receiving member to create a rigid joint that will not slip under load.

In traditional carpentry, spurs appear in tied roof trusses, where the tie beam prevents the rafters from spreading apart, but intermediate posts still need lateral bracing. A spur connects each post to the tie beam or to a rafter above it, locking the frame together. In modern stick-frame carpentry, the term is less common because lateral bracing is often achieved through sheathing, diagonal bracing, or collar ties rather than individual timber struts.

The name spur comes from the resemblance to a spur on a boot or riding spur, a projecting point that provides purchase and stability. A spur braces a frame much as a spur digs into ground or a horse's flank, providing grip and preventing slip. In some regions and older texts, spurs are also called struts, braces, or props, though these terms are sometimes reserved for temporary or lighter-duty supports.

Failure of a spur usually occurs at the joints, especially if the lap or mortise is not cut square or if the wood has split. Movement at the joint, visible as gaps or vertical cracks running from the joint downward, signals that the spur is no longer carrying load effectively. In historic frames, spurs are inspected for rot and beetle damage in the same way as other load-bearing timbers, since once weakened, they cannot prevent racking of the frame they are meant to stabilize.