What is an underlayment sheet used for on a roof?
Four jobs the sheet does
- Backup waterproofing. Shingles shed water, but wind-driven rain and blown-off tabs let some get underneath. The underlayment catches it and sends it down the slope to the eave.
- Protection during the install. Once the old roof is off, the sheet keeps a sudden Georgia afternoon storm from soaking the bare deck before shingles are nailed down.
- A separation layer. It keeps shingles from sticking to resin or sap in the wood and gives them a smooth, even bed.
- Code and rating compliance. Shingle fire ratings and manufacturer warranties assume an approved underlayment is under them.
How it is supposed to be laid
Underlayment runs in horizontal rows starting at the eave, each row lapping over the one below so water always flows over a seam rather than into it. At the eaves it goes over the drip edge; at the rakes it goes under the drip edge. On slopes from 2:12 up to 4:12, the International Residential Code calls for a double layer, because water moves slowly on low pitches. Synthetic sheets have printed lap lines and are fastened with plastic cap nails or cap staples, which hold far better in wind than bare staples.
Where the sheet alone is not enough
Valleys, chimneys, skylights, sidewalls and pipe penetrations carry the heaviest water flow, so careful crews add self-adhered membrane there. Read more in synthetic underlayment vs felt and ice and water shield in Georgia.
One thing the sheet cannot do is act as the roof for long. Felt wrinkles and tears, and even synthetics are rated for limited sun exposure, so a roof left in underlayment for weeks after a storm is a roof waiting to leak.