Roof Underlayment Cost Per Square
Underlayment is the least expensive component of a roof per square foot, and the one most likely to be quietly downgraded when a bid comes in low. It is also the layer that decides whether a single failed shingle becomes a small repair or a stained ceiling. Knowing what each type costs per square makes that line item easy to check.
What one square covers, and how many rolls that takes
A square is 100 square feet of roof surface, not of ground area. Because surface grows with pitch, the same footprint takes more material on a steep roof.
Coverage per roll is where homeowners get caught. Felt is sold in nominal roll sizes that assume no laps, but installation requires them. A nominal 400 square foot roll of #15 felt, laid with 2-inch side laps and 4-inch end laps, covers roughly 3.5 squares in practice. Synthetic comes in 10-square rolls that are much lighter, which matters when one person is carrying them up a ladder. Self-adhered membrane covers about 2 to 3 squares per roll.
Asphalt felt: #15 and #30
Felt is the traditional choice and still the cheapest. #15 felt is the lightest common grade at around $15 to $30 per square in material. #30 felt is roughly twice the weight, resists wind-driven rain better, and runs about $25 to $45 per square.
The limitations matter more than the price. Felt tears easily, wrinkles when it takes on moisture, and degrades under ultraviolet exposure in weeks, so it should not be left exposed if shingles are delayed. It is slippery when damp, a real safety consideration on a pitched roof. And it cannot be used under metal in most specifications, because metal surfaces can reach 150 to 170 degrees Fahrenheit in summer sun and cause the asphalt to bleed.
Synthetic underlayment
Synthetic underlayment is a woven or spun-bonded polypropylene or polyester fabric priced around $40 to $85 per square. It resists tearing far better than felt, does not absorb water, lays flatter, and tolerates UV exposure for several months. Rolls are lighter, so crews install it faster, and many shingle manufacturers now specify synthetic as a condition of their longer warranty terms.
Self-adhered membranes
Self-adhered, or peel-and-stick, membranes are thick rubberized asphalt sheets, costing roughly $90 to $160 per square in material. They seal around fasteners and penetrations, which is why cold-climate codes require them along eaves, in valleys, around vents, and wherever pitch drops below a threshold.
Felt vs synthetic vs self-adhered
| Factor | #15 felt | Synthetic | Self-adhered |
|---|---|---|---|
| Material cost per square | $15-$30 | $40-$85 | $90-$160 |
| Installed cost per square | $35-$75 | $60-$130 | $110-$210 |
| Tear resistance | Low | High | Very high |
| UV exposure tolerance | Weeks | 3-6 months | 6 months or more |
| Water absorption | Absorbs | None | None |
| Seals around fasteners | No | No | Yes |
| Use under metal roofing | Not recommended | High-temperature grade required | Commonly used |
| Typical role | Budget reroof over sound decking | Standard whole-roof underlayment | Eaves, valleys, penetrations, low slopes |
Which one to choose
Above 4/12 the common specification is synthetic across the whole deck with self-adhered membrane at the eaves, valleys, and penetrations. Between 2/12 and 4/12, expect a double layer of underlayment or a self-adhered membrane over the entire surface, and check the shingle manufacturer's minimum slope before assuming shingles are permitted at all. Under metal roofing, use high-temperature synthetic or self-adhered membrane rather than felt. Under tile, a heavy synthetic over battens is typical. If you are replacing sheathing too, size the deck with the Roof Sheathing Calculator so both orders arrive together.
The labor side of the number
Underlayment goes down quickly; a crew covers 20 to 40 squares a day on a gable, so incremental labor during a full reroof is modest, typically $20 to $45 per square. The drip edge belongs under the underlayment at the eaves and over it at the rakes; skipping that sequence causes edge rot. Laps matter too: at least 2 inches on the sides and 4 inches at the ends, always lapping up-slope over down-slope.
Mistakes that cost more than the underlayment
- Using standard felt under a metal roof instead of a high-temperature product.
- Rolling underlayment over a wet or frost-covered deck, which traps moisture against the sheathing.
- Under-fastening. Follow the manufacturer's pattern with cap nails or plastic-cap staples.
- Omitting self-adhered membrane in valleys and at penetrations to save a few hundred dollars.
- Leaving synthetic exposed through a full winter. It tolerates months of sun, not months of snow and ice.
Underlayment is not the place to find savings. The gap between the cheapest and best option on a 30-square roof is often under $1,500, while one interior water stain and the repair behind it can cost more. If a metal roof is on the table, price it with the Metal Roofing Calculator, or check your pitch with the Roof Pitch Calculator.