CSLB C-39 Roofing ContractorEstimating and PlanningHard
A roof valley measures 80 linear feet. Self-adhered membrane rolls are 36 inches wide by 36 feet long, and code requires a minimum 6-inch (0.5-foot) end-lap overlap between consecutive rolls along the valley run. How many rolls are needed to cover this single valley?
- A2 rolls
- B4 rolls
- C5 rolls
- D3 rolls
Show answer & explanationAnswer & explanation
Correct answer: D. 3 rolls
Each roll provides 36 ft of length but loses 0.5 ft to the overlap with the next roll, giving 35.5 ft of effective coverage per roll. Rolls needed = 80 ÷ 35.5 = 2.25, rounded up to 3 full rolls.
Why the other options are wrong
- A. This underestimates coverage and would leave the valley short by roughly 9 feet.
- B. This overestimates beyond what the overlap math requires.
- C. This significantly overstates the material needed.
Membrane Roll Overlap Takeoff
When calculating linear roll coverage with end-lap overlaps, subtract the overlap distance from the roll length to find effective coverage, then divide the total run by that effective length.
- Effective coverage = roll length minus overlap
- Always round up partial rolls to full rolls
- Applies to valley membranes, ice barriers, and similar linear runs
Memory trick: Overlaps eat into every roll's reach.