What is a Tie Beam?
A tie beam (ankerbalk in Dutch) is a horizontal structural beam that connects the lower ends (feet) of two opposing rafters in a pitched roof. Its primary function is to prevent the rafters from pushing the walls outward, a force known as thrust or spreading.
How a tie beam works
When a pitched roof carries loads — from its own weight, roof tiles, snow, or wind — the angled rafters transfer these loads downward and outward. Without a horizontal restraint, this outward thrust would push the tops of the walls apart, eventually causing structural failure.
The tie beam solves this by connecting the rafter feet across the full width of the building. It acts in tension, holding the base of the triangle together and converting the outward thrust into a purely downward vertical load on the walls. This is the same principle that makes a triangle the strongest shape in structural engineering.
Position and function in a roof truss
In a traditional roof truss, the tie beam forms the bottom chord — the horizontal base of the triangular truss. The two rafters form the top chords. Together, these three members create a rigid, self-supporting triangle.
The tie beam often doubles as a ceiling joist, supporting the ceiling of the room below. In larger spans, additional vertical and diagonal members (king posts, queen posts, or struts) may connect the tie beam to the ridge or rafters to prevent the beam from sagging under its own weight.
Materials and dimensions
Tie beams are traditionally made from solid timber, with the cross-section sized according to the span and the loads involved. For larger modern buildings, engineered timber products such as glulam (glued laminated timber) or steel beams may be used. The beam must be strong enough to resist both the tensile forces from the rafters and any bending caused by ceiling loads.
Tie beam vs. collar beam
A tie beam sits at the base of the rafters, at wall plate level. A collar beam, by contrast, connects the rafters at a higher point. While a collar beam reduces the free span of the rafters, it is less effective at resisting outward thrust than a full tie beam because it does not act at the point where the thrust is generated.
Related terms
- Collar beam
- Rafter
- Roof truss
- King post
- Wall plate
- Raised tie beam truss
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