What is the best roof underlayment?
Short answer. For a Georgia shingle roof, the best underlayment setup is a quality synthetic underlayment over the open field of the roof, with self-adhered ice and water membrane in valleys, around chimneys, skylights and pipes, and along any low-slope sections. Under metal roofing, a high temperature rated membrane or a synthetic approved by the panel maker is the better pick.
Why synthetic beats felt for most roofs
- Much higher tear strength, so it survives foot traffic and wind during installation.
- Does not wrinkle when wet, which keeps waves from showing through new shingles.
- Rated for longer sun exposure than felt, useful if a storm interrupts the job.
- Lighter rolls cover more area with fewer seams.
Felt still works and costs less, but it tears and absorbs water. Synthetic has become the standard on most Georgia reroofs.
Where membrane belongs
Self-adhered membrane bonds to the deck and seals around nails. Water runs heaviest in valleys, collects behind chimneys and drains slowly on low pitches, so those are the spots for membrane. Covering an entire roof in it is rarely needed in Georgia, and fully sealing the deck can trap moisture if the attic is poorly ventilated.
What to look for on the label
- A product listed for use with your shingle brand, since the maker's own underlayment can extend system warranty coverage.
- Code compliance: felt meets ASTM D226 or D4869, membrane meets ASTM D1970, and synthetics carry an evaluation report.
- A slip-resistant walking surface for crew safety on steep Georgia roofs.
- Fastening with cap nails or cap staples.
See synthetic underlayment vs felt and ice and water shield in Georgia.