Residential Building Inspector B1Roof-Ceiling ConstructionMedium
An inspector is reviewing plans for a residential roof. The plans indicate the use of asphalt shingles on a roof with a pitch of 2:12. What is the minimum required underlayment for this roof pitch according to the IRC?
- AOne layer of No. 30 felt
- BTwo layers of No. 15 felt or one layer of No. 30 felt
- COne layer of No. 15 felt
- DSelf-adhering polymer modified bitumen sheet
Show answer & explanationAnswer & explanation
Correct answer: B. Two layers of No. 15 felt or one layer of No. 30 felt
For roof slopes between 2:12 and 4:12, the IRC requires a double-layer underlayment or an equivalent self-adhering membrane. Two layers of No. 15 felt or one layer of No. 30 felt satisfy this requirement.
Why the other options are wrong
- A. One layer of No. 30 felt is equivalent to two layers of No. 15 felt, but the question implies a minimum, and two layers of No. 15 felt is a common method.
- C. One layer of No. 15 felt is typically for slopes 4:12 and greater.
- D. While a self-adhering membrane is an acceptable alternative, it's not the minimum required felt underlayment.
Low-Slope Asphalt Shingle Underlayment
For asphalt shingle roofs with a slope between 2:12 and 4:12, a specific underlayment system is required to prevent water penetration due to reduced drainage.
- Applies to slopes 2:12 up to less than 4:12.
- Requires two layers of underlayment felt.
- Common options are two layers of No. 15 felt or one layer of No. 30 felt.
Memory trick: Slopes need protection, especially when they're low and slow.