What is a roof diaphragm?
A roof diaphragm is a roof plane that is stiff in its own plane and can only deflect at right angles to it. Forces acting parallel to the roof surface, for example wind on a gable, are then carried through the roof into the bracing walls. In Dutch this is called a stijf dakvlak, a “stiff roof plane”.
A pitched roof of rafters or purlins with only tile battens on top is floppy in its own plane: the rectangular roof slope can rack into a parallelogram. There are two ways to stiffen it:
- Sheathing. Boards or panels such as plywood or OSB, properly fixed to the rafters, make the roof slope act as one plate.
- Wind bracing. Diagonal braces in the roof plane form a triangulated frame with the rafters or purlins.
A roof diaphragm is therefore one specific form of the general structural diaphragm.
Application
- Do not strip sheathing without replacing its function. Swapping boarding for battens and membrane during a re-roof can weaken the bracing of the roof. Ask a structural engineer.
- Fixings matter. Panels only act as a diaphragm if they are fixed with enough nails or screws; the engineer sets the pattern.
- Check large openings. Dormers and big roof windows interrupt the diaphragm. New openings call for a stability check.
- Connect to the walls. The forces must actually reach the walls through the wall plate and its fixings.
Common mistake: assuming the tiles make the roof stiff. Tiles and battens add almost no in-plane stiffness; sheathing, panels or bracing do.
Related terms
- Structural diaphragm
- Roof sheathing
- Structural stability
- OSB board
- Plywood
- Rafter
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