What is the nail pattern for roof underlayment?
Printed patterns
Most synthetic underlayments are printed with fastener targets and lap lines. The marks show where cap nails or cap staples go along the top lap, through the middle of each course, and at end laps. Following them is the simplest way to meet the manufacturer's requirements.
Cap fasteners
Cap nails have a plastic or metal cap about 1 inch across that spreads the holding force, so the sheet cannot tear around the shank in wind. Many synthetic makers require them, and bare roofing nails or staples alone may not meet their instructions.
Code requirements
- Most of Georgia: fasten enough to hold the underlayment in place until the roof covering is installed, following the manufacturer.
- High-wind areas: where design wind speeds reach the code's high-wind threshold, the code calls for a grid of fasteners about 12 inches apart between side laps and 6 inches apart at laps, with caps at least 1 inch in diameter. That rarely applies inland in North Georgia, but check with your building department.
Laps
Single-layer felt is lapped at least 2 inches horizontally and 4 inches at the ends; synthetics often call for larger laps, marked on the sheet. On slopes from 2 in 12 to under 4 in 12, two layers are required, lapped 19 inches.
Self-adhered membranes
Ice and water barrier sticks to the deck and seals around shingle nails. It is often fastened only along the top edge, or not at all, per the maker, so avoid nailing through it in the field.
Mistakes
- Too few fasteners, so wind lifts the sheet before shingling.
- Fasteners in the lower part of a lap where water runs.
- Overdriven caps that cut the sheet.
See synthetic underlayment vs felt and ice and water shield in Georgia.