What does roof underlayment look like?
Short answer. Modern synthetic underlayment looks like a tough woven plastic sheet, usually gray, white, blue or black, printed with the maker's logo and a grid of fastening marks. Older felt underlayment is black tar paper, and ice and water shield is a thicker, sticky rubberized membrane with a peel-off backing, usually at eaves and valleys.
The three kinds you may see
- Synthetic. Polypropylene or polyethylene woven or spun into a light, strong sheet. Comes in rolls several feet wide. Often has a textured walking surface, printed overlap lines, and nail marks. Held down with cap nails, so you see rows of small plastic disks.
- Felt. Asphalt-saturated paper, black or dark brown. Older roofs used it almost universally. It tears easily and wrinkles when wet, and you may find it brittle and crumbling during a tear-off.
- Self-adhered membrane. A thick rubberized asphalt sheet, often with a granular or film top, that sticks directly to the deck after the release liner is peeled off. Seen along the eaves, in valleys, and around chimneys, skylights and pipes.
When you would actually see it
Usually only during a replacement, when the old shingles are off and the crew dries in the deck, or at the edges where it laps over the drip edge. If you can see underlayment from the ground on a finished roof, shingles are missing and the underlayment is exposed to sun that will break it down.
What failing underlayment looks like
- Felt that has torn, rotted or turned to dust under old shingles.
- Wrinkles that telegraph through new shingles as ridges.
- Sun-faded, brittle synthetic left exposed too long on an unfinished roof.
Compare the materials in synthetic underlayment vs felt, see where membrane belongs in ice and water shield in Georgia, and follow the sequence in what to expect on roof replacement day.