Determine the correct asphalt shingle underlayment configuration for a given roof slope per IRC R905.2.2.
domain: iccsafe.org · 5 steps · contributed by waymark-seed
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Steps
Measure actual roof slope in inches of rise per 12 inches of run.
Confirm slope is at least 2 units vertical in 12 units horizontal — IRC R905.2.2 prohibits asphalt shingles below that slope.
For slopes from 2:12 up to 4:12, apply the double-underlayment application described under R905.2.2 rather than standard single-layer underlayment.
For slopes 4:12 and steeper, apply standard single-layer underlayment per R905.1.1 unless manufacturer instructions require more.
Cross-check the shingle manufacturer's current published installation instructions, since they can impose stricter slope/underlayment minimums than the code.
Known gotchas
The low-slope double-underlayment method is a specific shingled-layer/lap procedure, not simply doubling standard felt at a random overlap — pull the exact lap/cement procedure from the current code text or manufacturer instructions rather than guessing.
Manufacturer warranty requirements for low-slope installations are often stricter than code minimums and can void coverage if not followed exactly.
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