Does a roof need underlayment?
Short answer. Yes. The International Residential Code requires underlayment beneath asphalt shingles and most other steep-slope roof coverings, and shingle manufacturers require it for their warranties. Underlayment is the backup water barrier: it sheds water that gets past the shingles, protects the deck during installation, and keeps shingles from bonding to the wood.
What underlayment does
- Second line of defense: wind-driven rain, ice and water from damaged shingles drain down the underlayment instead of into the deck.
- Installation protection: keeps the deck dry between tear-off and shingling.
- Separation: prevents asphalt shingles from sticking to the deck in summer heat.
- Fire rating: it is part of the tested assembly that earns a roof its fire classification.
What meets code
- Asphalt-saturated felt meeting ASTM D226 or ASTM D4869.
- Synthetic underlayments evaluated to code requirements, now the most common choice.
- Self-adhered polymer modified bitumen membranes meeting ASTM D1970, used as leak barrier.
Coverage rules
Underlayment must lap correctly, with upper courses over lower ones. Slopes from 2/12 up to 4/12 need two layers or the manufacturer's low-slope method. Leak barrier along eaves is required only where ice dams are historically a problem, which in Georgia mostly means some mountain areas, though manufacturers recommend it in valleys everywhere.
What happens if it is skipped
Leaks show up in the first hard, wind-driven rain, the manufacturer's warranty is void, the job fails code, and a home inspector will flag it at sale. With Georgia's heavy downpours and long summers, underlayment earns its keep.
Compare the options in synthetic underlayment vs felt, and see where leak barrier belongs in ice and water shield in Georgia.