How is ice and water shield different from other roof underlayment?
Short answer. Ice and water shield is one of three main kinds of roof underlayment, and the only one that is self-adhered and seals around nails. Synthetic and felt underlayments are fastened with nails or staples and shed water, so most roofs use ice and water shield at valleys, walls, penetrations and other trouble spots and synthetic or felt across the rest of the deck.
The three underlayment types
- Self-adhered membrane, or ice and water shield. Peel-and-stick rubberized asphalt. Waterproof, nail-sealing and costly, used where water concentrates or backs up.
- Synthetic underlayment. Woven or spun polymer sheets fastened with cap nails. Light, strong, resistant to tearing and wrinkling, and the most common field underlayment on reroofs today.
- Asphalt-saturated felt. The traditional choice. Inexpensive, but it tears in wind, wrinkles when wet and degrades in sun.
What underlayment does
Shingles shed most water, but wind-driven rain, ice and debris push some under them. Underlayment is the backup layer that catches it and protects the deck during installation, and the code requires it under shingles. Ice and water shield is the strongest version of that backup.
Where each goes on a typical Georgia roof
- Ice and water shield: valleys, around chimneys, skylights and pipes, at sidewalls and headwalls, low-slope sections, and eaves where required.
- Synthetic or felt: everywhere else.
- Order: membrane goes down first in its locations, and field underlayment laps over its upper edges so water always flows onto the next layer.
A note on terms
Leak barrier, ice barrier, self-adhered underlayment and peel and stick usually all mean this membrane. On an estimate, look for both items: the membrane by location and the field underlayment by the square. See synthetic underlayment vs felt and roof anatomy explained.