Residential Building Inspector B1Roof-Ceiling ConstructionMedium
An inspector is reviewing the installation of a new asphalt shingle roof on a dwelling with a roof pitch of 7:12. What is the minimum required side lap for the felt underlayment?
- A6 inches
- B4 inches
- C2 inches
- D3 inches
Show answer & explanationAnswer & explanation
Correct answer: D. 3 inches
For roof slopes of 4:12 or greater, the minimum side lap for felt underlayment is 3 inches, as specified in IRC R905.1.1.
Why the other options are wrong
- A. 6 inches is the minimum side lap for felt underlayment on lower slopes (less than 4:12).
- B. 4 inches is not a standard minimum side lap for felt underlayment on this slope.
- C. 2 inches is typically for self-adhering polymer modified bitumen underlayment, not felt.
Felt Underlayment Side Lap
The horizontal overlap required between adjacent rolls of felt underlayment, dependent on roof pitch.
- Side lap varies with roof pitch.
- Steeper pitches require less overlap.
- IRC R905.1.1 governs these requirements.
Memory trick: Underlayment Laps: 'Up' the 'Roof', 'Less' 'Overlap' for 'Side', 'More' for 'End'.