When should you use roof underlayment?
Short answer. Use underlayment under every asphalt shingle roof, since the residential code requires it, and plan for two layers on slopes from 2/12 up to 4/12. You also want self-adhered membrane in valleys, at eaves where called for, and around chimneys, skylights and walls, and underlayment belongs under metal, tile and slate as well.
Always
- Under shingles. Required by code and by every shingle manufacturer.
- Under metal panels. Use a high-temperature underlayment, since metal gets hotter and some ordinary products soften or stick.
- Under tile and slate. The underlayment is the true waterproofing layer under these roofs, and it usually wears out before the tile does.
Double it
On asphalt shingle slopes from 2/12 up to 4/12, the code requires two layers, lapped so each course overlaps half of the one below, or an approved alternative. Water drains slowly on these slopes and wind can push it uphill.
Upgrade to self-adhered membrane
- Valleys. They carry the most water on the roof.
- Eaves. Required where your building department has a history of ice dams. In much of Georgia it is optional but common, especially on low slopes and in the mountains.
- Around penetrations. Chimneys, skylights, pipe clusters and dormers.
- Roof to wall joints. Under the step flashing.
When underlayment is not enough
Underlayment is not a temporary roof. If shingles cannot go on soon, or a storm tears off shingles, it needs to be covered with a properly secured tarp. Products carry limited sun-exposure ratings, and fasteners left exposed eventually leak.
On repairs
When replacing a section of shingles, damaged underlayment underneath should be patched or replaced, lapped correctly into the surrounding material, not left torn.
Compare materials in synthetic underlayment vs felt and see where membrane belongs in ice and water shield in Georgia.