How do you attach roof underlayment?
Short answer. Synthetic roof underlayment is attached with plastic cap nails or cap staples in the pattern printed on the roll, usually closer together along the laps than in the middle of the sheet. Felt can take cap nails too, and self-adhered membranes bond to the deck when you peel off the release film, with extra fasteners only where the manufacturer allows.
Step by step
- Prep the deck. Sweep it clean, pull or drive down old nails, and replace soft sheets. Underlayment telegraphs every bump.
- Install eave drip edge first. Underlayment laps over the drip edge at the eaves and goes under the drip edge on the rakes.
- Put down self-adhered membrane in valleys and other leak-prone areas before the field underlayment.
- Roll the first course along the eave, keeping it straight and parallel. Fasten along the top edge first so the sheet cannot slide, then work down in the printed pattern.
- Lap each course over the one below using the printed lap lines, and stagger end laps between courses.
- Drive fasteners flush. Overdriven caps tear the sheet; raised ones snag feet and shingles.
Why cap fasteners
The 1 inch plastic cap spreads the load so wind cannot tear the sheet around the fastener. Bare staples or roofing nails rip through synthetic underlayment in a gust, and most synthetic manufacturers do not allow them. Felt has historically been tacked with staples or nails, but cap nails hold it better too.
Code and manufacturer rules
The International Residential Code requires underlayment fastened well enough to stay in place in ordinary wind zones, with specific cap fastener spacing in high wind areas. The manufacturer's printed pattern is the practical rule on the roof, and following it keeps the underlayment approved for the shingle warranty.
See synthetic underlayment vs felt and ice and water shield in Georgia.