What is the difference between ice and water shield and felt underlayment?
Short answer. Ice and water shield is a self-adhered rubberized asphalt membrane that bonds to the deck and seals around nails, so it stops water that is standing or backing up; felt is asphalt-saturated paper nailed or stapled down that only sheds water running downhill. That is why roofs use the membrane where water concentrates, like valleys, walls and penetrations, and felt or synthetic underlayment over the rest of the deck.
Side by side
- Attachment. Membrane: adhesive backing, peel and stick. Felt: cap nails or staples.
- Nail holes. Membrane seals around roofing nails. Felt leaves an open hole at every fastener.
- Water resistance. Membrane stays watertight even under standing water from an ice dam or a slow valley. Felt sheds running water but wicks and wrinkles when it stays wet.
- Exposure. Membrane tolerates weeks of sun within its rated limit. Felt tears in wind and breaks down quickly in sun.
- Vapor. Membrane is nearly vapor-tight; felt breathes a little.
- Removal. Membrane is hard to remove at the next reroof and often takes the deck surface with it. Felt tears off easily.
- Cost. Membrane costs several times as much per square foot, which is why it is not used everywhere.
Where each belongs on a Georgia roof
- Membrane: valleys, chimneys, skylights, pipes, sidewalls and headwalls, low-slope sections, and eaves where required or where gutter backups are a concern.
- Felt or synthetic underlayment: the open field of the roof. Synthetic has largely replaced felt on reroofs because it is lighter, stronger and lies flatter; see synthetic underlayment vs felt.
Can you skip felt and use membrane everywhere?
Yes, and some low-slope and high-wind specifications call for it, but on most Georgia shingle roofs full coverage adds cost and makes the deck nearly vapor-tight, so attic ventilation has to be solid. See ice and water shield in Georgia.