Cap sheet
A cap sheet is the top, often mineral-surfaced or foil-faced, ply of a torch-on modified-bitumen roof, fused over the base sheet as the weathering surface.
Ask Chalkline about this →A cap sheet is the top ply, often mineral-surfaced or foil-faced, of a torch-on modified-bitumen roof. It is fused over the base sheet to form the weathering surface of a two- or three-ply built-up system.
In a built-up modified-bitumen roof, the base sheet (or sheets) goes down first, and the cap sheet is torched over it as the final layer that takes the weather. Cap sheets carry a protective surfacing because the bitumen itself degrades under UV:
- Mineral-surfaced: ceramic granules embedded in the top face protect the bitumen from UV and give a trafficable, coloured finish.
- Foil-faced: a metallic foil laminate reflects UV and gives a bright finish, common on exposed upstands and where a lighter surface is wanted.
The cap sheet is fused to the base by melting the underside with a torch as it is rolled out, so the two plies bond into one membrane with the laps staggered against the base sheet’s laps.
For a builder the practical points are to fully bond the cap sheet with properly lapped and sealed seams (a cold or starved lap is the classic torch-on leak), to protect the mineral surface during following trades (it is the UV protection, scuff it off and the bitumen below weathers), and to manage the hot-works risk that comes with torching: permit, extinguisher, and a fire watch after you finish, because smouldering can start in concealed timber.
Also known as: Capsheet, top sheet, weathering ply.
Category: Roofing / Membranes.
Related
See also
References
- Modified bitumen roofing (Chalkline) (verified 2026-06-01)
Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.