Question 21
Domain 5: Roof-Ceiling Construction and Attic AssembliesWhat is the minimum required roof underlayment per the IRC for asphalt shingles?
Correct answer: B
Explanation
The IRC requires roof underlayment for asphalt shingles to be a single layer of underlayment meeting ASTM D226 Type I, commonly called "15-pound felt," or an equivalent material. This satisfies the code’s minimum underlayment requirement for asphalt shingle roofs.
Why each option is right or wrong
A. No underlayment required
B. One layer of ASTM D226 Type I felt (15-pound felt) or equivalent
IRC Section R905.2.3 requires a layer of underlayment for asphalt shingles, and the minimum specified material is ASTM D226 Type I felt, commonly known as 15-pound felt, or an equivalent underlayment. The code does not require multiple layers for this application unless a separate roof slope or manufacturer condition triggers a different requirement; for the standard asphalt-shingle installation, one compliant layer is the minimum.
C. Two layers of 30-pound felt
D. Self-adhering modified bitumen only