What is a couple roof?
A couple roof is a roof structure made of a series of rafter pairs that run from the ridge down to the eaves and, together with the floor or ceiling joists, form a triangle. The rafters carry the roof boarding or battens directly. Unlike a purlin roof, it needs no purlins spanning from wall to wall. The Dutch term is sporenkap.
How does it work?
- Rafter pairs at regular spacing. Because the pairs are closely spaced, the roof load is spread evenly over the supporting structure.
- Vertical loads pass through the wall plate or floor into the walls.
- Horizontal forces. Sloping rafters push outwards at their feet (horizontal thrust). The floor joists or a tie beam take this thrust by holding the rafter feet together; this version is often called a close couple roof.
- Collar. On larger spans a collar beam joins the two rafters part way up, reducing deflection and the thrust on the wall plate. This is known as a collar roof. At the ridge the rafters are joined to each other; modern prefabricated roofs often use nail plates.
- Knee wall. A light partition in the attic that only divides off the eaves space usually has no structural role in this type of roof, although its studs may carry a small part of the load. Check before removing one.
Application
- Leave collars and ties in place. If you want more headroom in the attic, have a structural engineer check it first.
- Adding a dormer or roof window? This usually means cutting rafters. Support the cut rafters with trimmers.
- Check the eaves. On old roofs, look for rot and movement at the wall plate; that is where all the forces meet.
- Insulating. You can insulate between the rafters or over the roof boarding; fit a vapour control layer on the warm side.
Common mistake: sawing out a collar or tie for extra headroom. The rafters then sag and push the walls outwards at the eaves.
Related terms
- Purlin roof
- Roof structure
- Rafter
- Collar beam
- Tie beam
- Horizontal thrust
- Wall plate
- Trimmer joist
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