CSLB General Building (B)Finishes and Building EnvelopeMedium
A roofer is installing underlayment on a roof plane with a slope of 3:12. Per California Residential Code roofing provisions, what underlayment application is required for this slope?
- AA single layer of underlayment
- BA self-adhered membrane over the entire roof deck
- CNo underlayment is required
- DTwo layers of underlayment applied shingle-fashion
Show answer & explanationAnswer & explanation
Correct answer: D. Two layers of underlayment applied shingle-fashion
For slopes between 2:12 and 4:12, code requires two layers of underlayment applied in a shingle-fashion (double coverage) pattern because the shallow slope increases the risk of water backup and wind-driven rain intrusion.
Why the other options are wrong
- A. Single-layer underlayment is only permitted above 4:12 slope.
- B. Full self-adhered membrane is not the prescriptive requirement for this slope range.
- C. Underlayment is always required under composition shingles.
Double Underlayment Zone
Roof slopes between 2:12 and 4:12 require two overlapping layers of underlayment to provide extra water protection due to slower water shedding.
- 2:12–4:12 = double layer required
- Above 4:12 = single layer sufficient
- Below 2:12 = shingles not permitted at all
Memory trick: 'Two to four, double the floor.'