Collar Tie and Collar Beam

A collar is a horizontal timber fixed between a pair of opposing rafters somewhere above the wall plate. It is one of the oldest elements in roof carpentry, and in the language of builders it is usually called a collar tie, although historians and engineers draw a distinction between a collar acting as a beam and a collar acting as a tie.

How a collar works

• Collar beam: when the collar sits part-way up the rafters, it is mostly pushed on (in compression) and props the rafters against sagging inward.
• Collar tie: a collar is a true tie, pulled in tension and resisting the rafters spreading apart, mainly when it is positioned very high near the apex or low near the base of the rafters.

In modern codes and building practice, the member that resists the outward thrust of rafters near the bottom is often called a rafter tie or ceiling joist. Collar ties placed high in the roof are commonly used to help hold the roof together at the ridge against wind. Local codes specify the size, spacing and position of these members, so the details vary by jurisdiction.

Historic roofs

In the history of timber roofs, the simplest form is the common rafter roof: rafter pairs on a tie beam. Adding a collar produced the collar-beam roof, which carpentry historians describe as suitable for spans of up to about 4.5 m (15 ft). Crown-post roofs always include collars, which are carried by a central post rising from the tie beam. Arch-braced roofs add curved braces between the rafters and the collar.

Practical notes

Removing collars or ties during attic renovations can allow rafters to spread and the ridge to sag. Any change to these members should be checked by a qualified designer.