Roof Deck and Underlayment Preparation
Roof Deck and Underlayment Preparation
This domain tests whether you can assess deck condition, select and fasten sheathing, and install underlayment and ice barrier so the shingle system has a sound base.
Core rules
- Replace delaminated, soft, or water-stained OSB/plywood; drive existing fasteners flush; sweep the deck. Roof sheathing panels need edge gaps, commonly 1/8 in., to allow movement.
- Confirm sheathing thickness: many shingle manufacturers require at least 3/8-in. APA-rated panels at 16-in. o.c. and 7/16 in. at 24-in. o.c.; the code table and manufacturer govern. Fasten roof sheathing with 8d common or approved screws, typically 6 in. o.c. at edges and 12 in. o.c. in the field.
- Underlayment must meet ASTM D226 Type I (No. 15) or Type II (No. 30), or an approved synthetic. Start at the eave, lap horizontal seams 2 in. and end seams 4 in., and cover nails.
- In cold climates, IRC R905.1.2 requires an ice barrier at eaves where the average January temperature is 25°F or lower, extending at least 24 in. inside the exterior wall line. Install drip edge at eaves and rakes.
Worked example
An installer lays No. 30 felt with 4-in. side laps and 2-in. end laps on a 4:12 roof. Is that correct? No. Felt side laps are 2 in. and end laps 4 in.; reversing them creates a watershed path and can void the manufacturer’s warranty.
Traps
Covering a wet deck, using staples too short to hold, skipping ice barrier because it rarely snows, and omitting drip edge are frequent errors.