Is roof underlayment necessary?
Short answer. Yes. Underlayment is required under asphalt shingles by the International Residential Code and by every major shingle manufacturer's instructions, and it is a core part of the roof's water protection rather than an optional extra. It catches wind-driven rain that gets past the shingles, protects the deck during installation, and is part of how shingle fire ratings and warranties are tested.
What underlayment does
- Backup water barrier: shingles shed most water, but wind-driven rain, blown-back water at valleys and the occasional lifted tab all push water underneath. Underlayment sends it back out to the eave.
- Protects the deck during the job: once the old roof is off, underlayment is the only thing between the wood and an afternoon storm until shingles go on.
- Separates materials: it keeps shingle asphalt from sticking to the deck and wood resins from reacting with the shingles.
- Completes the tested system: shingle fire ratings and manufacturer warranties assume an approved underlayment.
- Helps after a blow-off: if wind strips shingles, underlayment can keep water out long enough to get a tarp on.
Where extra protection goes
Self-adhered leak barrier is commonly added in valleys, around chimneys and skylights, along low-slope sections, and at eaves in the North Georgia mountains where ice dams can form.
Old roofs without it
Some older Georgia homes were roofed with little or no underlayment. When those roofs are replaced, underlayment must be added to meet current code, and the deck often shows staining or rot that a backup barrier would have prevented.
Cost perspective
Underlayment is a small part of the total price of a roof, which makes skipping it a poor trade.
See synthetic underlayment vs felt and ice and water shield in Georgia.