Do I need underlayment for my roof, and how much?
Short answer. Yes. If you are installing shingles, underlayment is required by the International Residential Code and by every major shingle manufacturer. To figure how much, take the roof area in squares, add a little for laps and waste, and divide by the roll coverage: many synthetic rolls cover about 10 squares, a roll of 15-pound felt about 4, and a roll of 30-pound felt about 2.
When you need it
- Under every new asphalt shingle roof, over the whole deck.
- Under metal roofing on a solid deck, per code and the panel maker.
- On repairs where the old underlayment was torn or removed.
How much to order
- Start with the roof area in squares (100 square feet each), from a measurement report.
- Subtract any areas that will get self-adhered leak barrier instead, such as valleys.
- Add about 10 percent for laps and cuts.
- Divide by the coverage on the roll label and round up.
Example: a 28 square roof with 2 squares of valley leak barrier leaves 26 squares; add 10 percent for about 29, which is three 10-square synthetic rolls.
Low slopes need double
Sections between 2/12 and 4/12 need two layers of underlayment or a self-adhered membrane, so double the quantity for those areas.
Also on the list
- Leak barrier: self-adhered rolls, commonly around 2 squares each, for valleys and penetrations.
- Fasteners: cap nails or cap staples as the underlayment maker specifies.
- Drip edge: installed under the underlayment at eaves and over it at rakes.
Synthetic has largely replaced felt on Georgia reroofs because it is lighter, stronger and tolerates sun exposure longer. Compare them in synthetic underlayment vs felt, and see ice and water shield in Georgia for where leak barrier goes.