How do you install underlayment on a roof, felt or synthetic?
Short answer. On a roof, underlayment is laid in horizontal rows from the eave up, each row overlapping the one below shingle-style so water always runs over a lap, never into one. Felt and synthetic go down the same basic way but differ in fasteners, lap widths and handling, and roofs between 2:12 and 4:12 slope need double coverage.
This page covers roof underlayment. Flooring underlayment is a different product with a different installation.
Installing asphalt felt
- Laps: on slopes of 4:12 and up, the residential code calls for felt laid parallel to the eave with head laps of at least 2 inches and end laps of at least 4 inches, offset between rows.
- Fasteners: enough to hold it flat until it is covered. Cap nails resist tearing in wind better than bare nails.
- Handling: felt absorbs moisture and wrinkles, so lay it on a dry day and cover it quickly. Wrinkles under shingles show up as ridges.
Installing synthetic underlayment
- Laps: follow the lines printed on the sheet, since lap widths vary by product.
- Fasteners: most products require cap nails or cap staples in a printed pattern.
- Handling: synthetics are light and tear-resistant but can be slippery with dew or dust, so the walkable side must face up and roofers need proper footing.
- Exposure: many synthetics tolerate weeks or months of sun before covering, but check the label.
The low-slope method
For slopes from 2:12 up to 4:12, the code calls for double coverage: a 19 inch wide starter strip at the eave, then 36 inch wide sheets each lapping the one below by 19 inches, so every point on the deck has two layers. Many roofers use a full self-adhered leak barrier on these slopes instead.
Whichever you use, leak barrier goes in the valleys and around penetrations first, and the underlayment laps over it. The tradeoffs between the two materials are covered in synthetic underlayment vs felt, and the layer order is shown in roof anatomy explained.