What is the purpose of roof underlayment?

The jobs it does

• Second line of defense. Shingles shed most water, but wind-driven rain gets pushed up under them, and a storm can tear shingles off. Underlayment keeps that water out of the deck and the house.
• Temporary dry-in. During a replacement, the deck is covered with underlayment as soon as it is exposed, so a pop-up Georgia thunderstorm does not soak it.
• Smooth, uniform base. It evens out small irregularities in the deck so shingles lie flat.
• Fire rating. Some roof assemblies rely on the specified underlayment to achieve their tested fire rating.
• Warranty compliance. Shingle manufacturers require underlayment, and their stronger warranties often specify their own.

What the code says

Underlayment is required under asphalt shingles. On slopes from 2/12 up to 4/12, the code requires two layers because water drains slowly and backs up more easily. Ice barrier membrane is required in areas with a history of ice forming at the eaves, which your local building department determines. In valleys and around chimneys and skylights, most roofers use self-adhered membrane regardless.

What it is not

Underlayment is not a roof. Most products are rated for limited sun exposure, and a deck left covered only in underlayment for weeks will start to leak at the fasteners and laps.

Georgia angle

Summer storms here deliver short bursts of very heavy rain with strong gusts. That is exactly when a good underlayment and properly lapped membrane at the eaves and valleys earn their keep.

Choose materials with synthetic underlayment vs felt, see ice and water shield in Georgia, and view the layers in roof anatomy explained.