The final ply installed in built-up or modified bitumen roof systems. It is typically surfaced with mineral granules or a reflective foil to resist ultraviolet light, foot traffic, and physical damage. This layer acts as the primary defense against the elements, shielding the waterproofing base sheets beneath it.
Make sure you get full bleed-out at the seams when you torch down that cap sheet.
Granule loss on a cap sheet is often the first sign of premature aging or excessive foot traffic on a modified bitumen roof.
Related terms
Base sheet
The foundational first layer of felt or membrane in a multi-ply roof system.
Modified bitumen
A roofing membrane made of asphalt mixed with rubber or plastic polymers for flexibility and strength.
Torch-down
A roofing method where modified bitumen sheets are melted onto the substrate using an open flame.