What is a roll cap?
A roll cap is the shaped metal capping that covers the joint between two adjacent bays on a metal roof or wall cladding. It sits over a timber batten — the roll batten — against which the zinc, copper or aluminium bays have been turned up. Batten, upstands and cap together form a joint that is watertight and lets the metal move freely. In Dutch the component is called a roef.
Why the joint is made this way
Metal expands and contracts substantially with temperature. A ten-metre zinc bay is measurably longer on a summer afternoon than on a frosty night. If the bays were fixed rigidly to each other and to the decking, the metal would split or buckle. By joining them with a loose capping, each bay can move on its own. The joint still keeps water out because it sits well above the drainage plane.
The build-up from the bottom up
- Decking — usually planed softwood boarding or a board material suitable for metal, over a well-ventilated space.
- Separating layer — a structured matting or dimpled membrane, depending on the system, to drain condensation and separate metal from timber.
- Roll battens — rectangular or trapezoidal in section, running down the fall, typically at centres of about 500 to 600 mm.
- Bays — the metal sheets, turned up against the battens on both sides and held down by clips fixed to the decking. The clips hold the metal yet let it slide.
- Roll cap — folded over the batten and either clipped or welted to the upstands on both sides.
Material and practice
The cap is of the same metal as the bays: zinc, copper, lead, or nowadays prefabricated aluminium or coated steel. Common zinc thickness for roofing is 0.7 or 0.8 mm. Critically, do not combine metals that attack one another — zinc below copper is a classic mistake, because copper-bearing run-off corrodes the zinc rapidly.
Related terms
- Batten roll roof
- Roll batten
- Zinc
- Roof boarding
- Batten
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