CSLB General Building (B)Finishes and Building EnvelopeHard
A roof plane measures 24 feet (288 inches) from eave to ridge and requires double-layer underlayment per code because its slope is 3:12. Using 36-inch-wide underlayment rolls applied with a 19-inch starter strip and 19-inch head lap (each subsequent course exposing 17 inches), how many total courses of underlayment are needed to cover the plane?
- A17
- B16
- C19
- D15
Show answer & explanationAnswer & explanation
Correct answer: A. 17
After the 19-inch starter strip, remaining coverage needed is 288 − 19 = 269 inches; dividing by the 17-inch exposure per course gives 269 ÷ 17 = 15.82, which rounds up to 16 additional courses; adding the starter course gives a total of 17 courses.
Why the other options are wrong
- B. This is the count of additional courses only, before adding the starter.
- C. Overestimates the number of courses required by the calculation.
- D. Undercounts the courses needed to fully cover the remaining 269 inches.
Double Underlayment Coverage Calculation
For low-slope roofs requiring double-layer underlayment, courses are calculated using a starter strip plus repeating exposure widths until the full roof plane length is covered.
- Starter strip is typically 19 inches wide
- Each subsequent course exposes 17 inches (36-19 overlap)
- Always round up partial courses to ensure full coverage
Memory trick: 'Starter plus stacks of seventeen gets you to the ridge clean.'