How do you install roof underlayment?
Short answer. Roll it out horizontally starting at the eave, over the eave drip edge and any leak barrier, with each higher course lapping the one below by the head lap printed on the sheet and end laps staggered well apart. Fasten it with the cap nails the manufacturer specifies in its printed pattern, then install the rake drip edge on top.
Order of layers
- Clean, dry deck with nails set and debris swept off.
- Drip edge along the eaves.
- Leak barrier where used: eaves, valleys, around chimneys and other penetrations, along roof-to-wall joints.
- Underlayment over everything else, lapping onto the leak barrier.
- Drip edge along the rakes, on top of the underlayment.
Rolling it out
- Start low. The first row runs along the eave, and each row above laps over the one below like shingles, so water always flows across a lap instead of into it.
- Use the printed lines. Most synthetics have lap lines printed on them. Head laps are often 2 to 4 inches and end laps around 4 to 6 inches, but follow the product label.
- Stagger end laps. Keep the ends of rolls from lining up in the same vertical row.
- Keep it flat. Wrinkles and bubbles telegraph through shingles and can tear.
- Fasten to the pattern. Synthetics generally require plastic cap nails or cap staples, closer together at laps and edges. Bare staples tear out in wind.
Special areas
In valleys, the underlayment laps over the valley leak barrier. At walls, it turns up a few inches behind the siding or step flashing. Roofs between 2:12 and 4:12 slope need double coverage or a full leak barrier under shingles. Most synthetic underlayments allow a limited exposure period before they must be covered, but in Georgia's pop-up thunderstorms, the best practice is to shingle the same day. Compare materials in synthetic underlayment vs felt and see where leak barrier belongs in ice and water shield in Georgia.