Commercial Building Inspector B2Roof Ceiling ConstructionEasy
A contractor is installing a new roof on a commercial building with a roof slope of 1.5 units vertical in 12 units horizontal (1.5:12). The roof covering is a single-ply membrane. According to IBC Section 1507.12, what is the minimum required underlayment for this roof covering?
- ATwo layers of asphalt-saturated felt, 15 lb minimum.
- BNo underlayment required for single-ply membranes.
- COne layer of asphalt-saturated felt, 15 lb minimum.
- DOne layer of asphalt-saturated felt, 30 lb minimum.
Show answer & explanationAnswer & explanation
Correct answer: B. No underlayment required for single-ply membranes.
IBC Section 1507.12.3 for Single-Ply Membranes states that 'Underlayment shall not be required for single-ply membranes.' Single-ply membranes are designed to be waterproof on their own and do not typically rely on underlayment for water shedding or vapor control in the same way as shingle or tile roofs.
Why the other options are wrong
- A. Two layers of felt are for very low slopes or ice barrier applications with other roof coverings.
- C. Felt underlayment is typically for asphalt shingles, tiles, or metal roofs, not single-ply membranes.
- D. 30 lb felt is for steeper slopes or specific conditions for shingle/tile roofs.
Single-Ply Membrane Underlayment
The requirement for a protective layer beneath a single-ply roof membrane.
- IBC Section 1507.12.3 specifically addresses underlayment for single-ply membranes.
- Single-ply membranes are inherently waterproof.
- Underlayment is generally not required for these systems.
Memory trick: Underlayment is like an extra rain jacket; some roofs don't need it.