What is underlayment in construction?
Underlayment in different trades
- Roofing. Felt, synthetic or self-adhered membrane over the roof deck. Its job is water resistance, backing up the shingles or metal.
- Flooring. Foam, cork or rubber pads under laminate and vinyl plank for cushion and sound, or thin plywood panels under vinyl and tile for a smooth base.
- Tile. Cement board or membranes that give tile a stable, water-resistant base.
Underlayment vs sheathing
The two get mixed up. Sheathing, also called decking, is the structural layer: plywood or OSB nailed to rafters or studs, carrying loads and bracing the frame. Underlayment is non-structural and goes on top of the sheathing. On a roof, the order from the inside out is rafters, sheathing, underlayment, then shingles.
What building code says about roof underlayment
The International Residential Code, which Georgia adopts with state amendments, requires an approved underlayment under asphalt shingles and sets how it is lapped and fastened, including a double layer on slopes from 2:12 up to 4:12. Shingle makers add their own requirements for warranty coverage. Local building departments enforce the code on permitted reroofs, though inspection practices vary by city and county.
Why the roof version matters most
Flooring underlayment affects comfort and sound. Roof underlayment decides whether water reaches your house when shingles fail, which during Georgia storm season is a real question, not a theoretical one. See synthetic underlayment vs felt and roof anatomy explained.