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Collar ties

Diagram of a section of a roof showing rafters and underpurlin, with collar ties (horizontal timbers running between every second pair of common rafters) just above the underpurlins.

Collar ties must be provided in all coupled roof construction. They supplement the ceiling joists and prevent the rafter couples spreading.

Where the rafter span is such that it requires support from underpurlins, collar ties are fitted to opposing common rafters at a point immediately above the underpurlins.

Collar ties must be fitted to every second pair of common rafters.