CSLB C-39 Roofing ContractorEstimating and PlanningMedium
Per California Building Code provisions governing asphalt shingle roof underlayment, a double layer of underlayment is required for roof slopes within which range?
- AGreater than 6:12
- B2:12 up to less than 4:12
- C4:12 up to 6:12
- D0:12 up to 2:12
Show answer & explanationAnswer & explanation
Correct answer: B. 2:12 up to less than 4:12
Asphalt shingles are only approved for slopes of 2:12 or greater, and slopes between 2:12 and less than 4:12 require a double layer of underlayment (or an approved cap sheet system) due to increased water infiltration risk on low slopes.
Why the other options are wrong
- A. Steeper slopes shed water quickly and do not require double underlayment.
- C. This range meets standard slope requirements without double underlayment.
- D. Slopes below 2:12 are not approved for standard asphalt shingles at all.
Double Underlayment Requirement
Building code requires a double layer of underlayment on low-slope asphalt shingle roofs (2:12 to under 4:12) to compensate for slower water runoff.
- Asphalt shingles require minimum 2:12 slope
- 2:12 to <4:12 requires double underlayment application
- 4:12 and above only requires standard single-layer underlayment
Memory trick: Low and slow slopes need two layers to go.