What is the difference between a roof with underlayment and one with no underlayment?
Short answer. With underlayment, water that gets past the shingles from wind-driven rain, ice or a lifted tab runs off on a waterproof layer to the eave; with no underlayment, that water goes straight into the wood deck, where it causes rot, stains and leaks. A roof without underlayment also fails code, manufacturer requirements and usually the shingle warranty.
Side by side in real conditions
- Wind-driven rain: with underlayment, water pushed under shingles drains harmlessly. Without it, the deck soaks up water every hard storm.
- A lifted or missing shingle: with underlayment, the roof often stays dry until a repair. Without it, the next rain leaks into the attic.
- Rain during installation: with underlayment, the deck is dried in. Without it, the house is exposed.
- Valleys and low slopes: with leak barrier, slow-moving water stays out. Without it, these spots leak first.
- Ice dams in the mountains: with membrane at the eaves, backed-up water stays out. Without it, ceilings stain.
Rules and warranties
The International Residential Code requires underlayment under asphalt shingles, shingle fire ratings assume it, and manufacturers require it for warranty coverage. A roof without it can fail inspection and leave you without manufacturer support.
Finding none at tear-off
Some older Georgia homes were roofed with little or no underlayment. At tear-off, those decks often show dark staining, soft spots and rot along valleys and eaves. The new roof gets underlayment as required, and damaged sheets are replaced first, which can add decking cost to the job.
Cost perspective
Underlayment is a small share of what a roof costs, while rotted decking and interior water damage cost far more.
See synthetic underlayment vs felt and common causes of roof leaks.