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Collar timber roof

Short answer

A collar roof has a horizontal collar beam joining rafters at one-third to one-half the rise, reducing sag and allowing slightly larger spans (~5 m) while maintaining more headroom than a couple-close roof; walls still resist some thrust.

Describe collar timber roofs with necessary sketches.

A collar roof is a couple (or couple-close) roof in which a collar beam (horizontal tie) joins each pair of rafters at about one-third to one-half of the rise above the wall plate. The collar stops the rafters sagging and spreading, and allows slightly larger span (about 5 m) than couple-close roofs.

            ridge
             /\
            /  \
     collar/----\ collar beam
          /      \
         /        \
      __/__________\__
     wall plate   wall plate

Members: wall plate (75 x 100 mm), common rafters (50 x 100 mm at 450 mm c/c), collar beam (40 x 100 mm, bolted), ridge board (40 mm thick), battens and covering.

The collar beam does not take the full thrust like a tie beam; hence walls should still be strong enough to resist some outward thrust. It is used for small residential spans and cottages.

✓ Answer checked by Claude Opus 5.5 (high effort)

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