A roofing contractor is performing a reroof on a residential building with a relatively flat roof (pitch 1:12). The existing roof is a single-ply membrane. During the tear-off, the crew finds that the roof deck is structurally sound, but the interior of the building has experienced minor water intrusion from the old roof. What type of underlayment is most appropriate to install directly over the roof deck for a new single-ply membrane system to maximize waterproofing and resist future leaks?
- AA high-temperature self-adhering polymer modified bitumen underlayment.
- BA single layer of 15-lb asphalt saturated felt.
- CA mechanically fastened gypsum-based cover board.
- DA loose-laid fiberglass reinforced mat.
Show answer & explanationAnswer & explanation
Correct answer: C. A mechanically fastened gypsum-based cover board.
For single-ply membranes over a deck that has experienced water intrusion, a cover board is crucial. A mechanically fastened gypsum-based cover board provides a smooth, stable substrate, adds fire resistance, and protects the membrane from deck irregularities. It also allows for some vapor transmission unlike an impermeable self-adhering membrane that might trap moisture if not perfectly sealed.
Why the other options are wrong
- A. While self-adhering underlayments offer good waterproofing, they can trap residual moisture from prior leaks if the deck isn't perfectly dry, potentially causing issues. A cover board provides a more forgiving and robust solution for the scenario described.
- B. 15-lb felt is not suitable as a primary underlayment for single-ply membranes and offers minimal protection against future leaks.
- D. A loose-laid fiberglass mat offers minimal structural integrity or protection against future leaks for a single-ply system.
Single-Ply Cover Board
A cover board, often gypsum-based and mechanically fastened, is crucial under single-ply membranes, especially over decks with prior water intrusion, to provide a stable, protective, and fire-resistant substrate.
- Protects membrane from deck irregularities.
- Adds fire resistance.
- Provides smooth surface for membrane adhesion/attachment.
- Can manage minor residual moisture better than fully adhered impermeable layers.
Memory trick: After wet leaks, for single-ply's new start, a cover board protects the deck's heart.