Sheet membranes include self-adhered rubberized asphalt, modified bitumen, PVC, TPO, butyl and HDPE pre-applied sheets. Factory manufacture guarantees uniform thickness, but every lap, corner and penetration is a potential leak path. Laps are typically 2 to 6 inches, rolled or welded, and staggered so seams do not meet. Workmanship at laps decides most failures.
Sheet membrane laps need to be rolled with the hand roller, I can see fishmouths at every end lap.
Stagger end laps so no more than three layers meet at any point; four-way intersections are where leaks start.
Also heard as
- sheet waterproofing
- roll membrane
Related terms
Fluid-applied membrane
A liquid waterproofing applied by roller, squeegee or spray that cures into a continuous rubbery film with no laps.
Peel-and-stick
Field name for self-adhered membrane: rubberized asphalt or butyl sheet with a release liner on the back.
Fishmouth
An open wrinkle at the edge of a membrane lap that forms a channel water can run into.
Shingle lap
Overlapping layers so each upper piece lies over the one below, sending water down and out like roof shingles.