An asphalt-shingle roof has a slope of 3:12. Under the IBC low-slope asphalt-shingle provisions, what underlayment arrangement is required?
Correct Answer
C) Two layers of underlayment applied as prescribed for slopes from 2:12 up to less than 4:12
For asphalt shingles on slopes from 2:12 up to less than 4:12, the IBC requires the two-layer underlayment method. Other choices omit the low-slope requirement, use the steeper-slope single-layer approach, or turn the ice-barrier rule into full-roof underlayment.
Why This Is the Correct Answer
For asphalt shingles on slopes from 2:12 up to less than 4:12, the IBC requires the two-layer underlayment method. Other choices omit the low-slope requirement, use the steeper-slope single-layer approach, or turn the ice-barrier rule into full-roof underlayment.
Why the Other Options Are Wrong
Option A: Assume no underlayment is needed because the slope is above 2:12
Choice A gives No underlayment because the slope is above 2:12. Its specific error: No underlayment because the slope is above 2:12 is an IBC dimensional value for a different stair, door, or egress condition.
Option B: Classify it as a single layer only, lapped 2 in. at the headlap
Choice B (A single layer only, lapped 2 in. at the headlap) is rejected because A single layer only, lapped 2 in. at the headlap is an IBC dimensional value for a different stair, door, or egress condition.
Option D: Install self-adhering ice barrier over the entire roof as the only underlayment
The trap in D is Only self-adhering ice barrier over the entire roof. That selection is wrong because it leaves the work at an informal or incomplete stage, while Asphalt shingle underlayment requires Two layers of underlayment applied as prescribed for slopes from 2:12 up to less than 4:12; Asphalt shingles on slopes from 2:12 to less than 4:12 need the prescribed two-layer underlayment.
Memory Technique
Make asphalt shingle underlayment a tabbed card: cite the exact condition, then choose Two layers of underlayment applied as prescribed for slopes from 2:12 up to less than 4:12.
Reference Hint
Open International Building Code 2021 or 2024; use IBC 2021/2024 Section 1507 asphalt shingles - underlayment on 2:12 to less than 4:12 slopes to confirm that For asphalt shingles on slopes from 2:12 up to less than 4:12, the IBC requires the two-layer underlayment method.
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