CSLB C-39 Roofing ContractorLow-Slope RoofingEasy
A contractor is re-roofing over an existing asphalt-saturated felt substrate and plans to install a new PVC single-ply membrane. To prevent asphalt bleed-through from staining or reacting with the new membrane over time, what should be installed as a separator layer between the old substrate and the new membrane?
- AA vapor retarder
- BA base flashing ply
- CA slip sheet
- DA cant strip
Show answer & explanationAnswer & explanation
Correct answer: C. A slip sheet
A slip sheet (separator sheet) is installed between incompatible or reactive layers to prevent chemical bleed-through and allow for differential movement, protecting the new membrane from staining or degradation.
Why the other options are wrong
- A. A vapor retarder controls moisture diffusion, not chemical bleed-through from an old substrate.
- B. Base flashing covers vertical transitions, not field separation of layers.
- D. A cant strip transitions vertical surfaces at parapets, unrelated to this separation issue.
Slip Sheet (Separator Sheet)
A slip sheet is a non-reactive layer installed between an old substrate and a new membrane to prevent chemical incompatibility, staining, and to allow differential movement.
- Commonly used over asphaltic substrates before single-ply installation
- Prevents bleed-through and adhesive reactions
- Also reduces friction allowing thermal movement
Memory trick: Slip a sheet in to keep old asphalt from ruining the new roof.