A collar beam is a horizontal beam that connects two opposing rafters high up in a roof. The beam sits high in the roof, well below the ridge, and prevents the rafters from spreading apart under the roof load. The space above the collar beam thus remains open.
How it works
A pitched roof pushes sideways, outward. Without something to counter it, the rafters would tip outward. The collar beam acts as a tension connection:
- It keeps the two rafters at a fixed distance from each other
- It absorbs the tensile force created by the roof wanting to spread outward
- Because it sits high in the roof, usable headroom remains beneath the beam
Across a series of rafters, the collar beams together with the rafters form a system that stiffens the whole roof.
Applications
The collar beam occurs in many traditional and modern roof structures:
- Attics and loft conversions — the collar beam limits the clear height; during a conversion one sometimes wishes to raise or reposition it (only after a structural check)
- Rafter and purlin roofs — as a stabilising element between the rafters
- Historic and listed roofs — where the collar beam often remains in view
Point to watch when converting
Do you want to fit out a loft and remove or raise the collar beams for more headroom? This may never be done casually: the collar beam is a load-bearing, stabilising component. Have a structural engineer assess whether and how this can be resolved structurally.
Related terms
- Rafter
- Tie beam
- Roof structure
- Ridge
- Timber frame
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