CSLB C-39 Roofing ContractorEstimating and PlanningMedium
A roof slope length from eave to ridge measures 26 feet. Underlayment rolls are 36 inches wide and installed horizontally with a 4-inch overlap between courses, leaving 32 inches (2.67 feet) of effective exposure per course. How many courses are needed to cover the full slope length?
- A11 courses
- B9 courses
- C8 courses
- D10 courses
Show answer & explanationAnswer & explanation
Correct answer: D. 10 courses
Courses needed = slope length ÷ effective exposure = 26 ft ÷ 2.67 ft ≈ 9.75, which must be rounded up to 10 full courses to fully cover the slope.
Why the other options are wrong
- A. This overestimates beyond what the math requires.
- B. This rounds down rather than up, leaving a gap.
- C. This underestimates the courses and would leave the roof uncovered near the ridge.
Underlayment Course Count
The number of horizontal underlayment courses needed equals the slope length divided by the effective exposure per course (roll width minus overlap), always rounded up to ensure full coverage.
- Effective exposure = roll width minus overlap
- Always round up partial courses to ensure no gaps
- Applies to felt and synthetic underlayment alike
Memory trick: Always round up so the roof stays dry.