CSLB C-39 Roofing ContractorEstimating and PlanningMedium
A low-slope roof section measuring 36 squares requires double-layer #15 felt underlayment per code. Each roll of #15 felt provides 4 squares of net coverage after overlap. How many rolls of felt are required?
- A18 rolls
- B14 rolls
- C9 rolls
- D22 rolls
Show answer & explanationAnswer & explanation
Correct answer: A. 18 rolls
Double-layer coverage requires twice the roof area in felt: 36 squares × 2 = 72 squares of felt needed. Divide by roll coverage: 72 ÷ 4 = 18 rolls.
Why the other options are wrong
- B. Underestimates the doubled coverage requirement.
- C. This is the single-layer roll count without doubling.
- D. Overestimates beyond the actual doubled requirement.
Double-Layer Felt Coverage
Low-slope roofs requiring double-layer underlayment need twice the roof's square footage in felt material before dividing by roll coverage.
- Double-layer coverage doubles the effective area needed.
- Standard #15 felt rolls typically cover 4 squares net.
- Always round roll counts up to the next whole roll.
Memory trick: Double layer means double the felt, then divide by roll size.