Roof Underlayment Calculator: How Many Rolls Do I Need?
Enter your roof area and pitch to get the rolls of synthetic or felt underlayment to buy, plus the ice barrier the code requires at the eaves in cold climates.
Underlayment is the layer between the roof deck and the shingles. It sheds water that gets under the shingles and keeps the deck dry before the shingles go on. Ice barrier is a self-adhering membrane that seals around nails, and it goes along the eaves in cold climates where ice dams push water back up the roof. This calculator turns your roof area and pitch into the number of rolls of each.
How it works
The calculator starts from the sloped roof area. If you only know the house dimensions, get it from the roof square footage calculator first.
layers = 2 below 4/12, otherwise 1
coverage = roof area × layers × (1 + waste %)
rolls = coverage ÷ net coverage per roll (round up)
ice width = (eave overhang + 2 ft) × pitch multiplier
courses = 36 in strips needed to cover that width
ice rolls = (courses × eave length + valleys) ÷ 66.7 ft (round up)Net coverage per roll is the roll’s width minus the side lap, times its length. We use common roll sizes: 48 in × 250 ft synthetic with 4-inch laps, and 36-inch felt with 2-inch laps. The waste factor covers end laps and the material lost cutting around hips, valleys and the roof edge.
Low slopes get two layers. For asphalt shingles from 2/12 up to 4/12, the International Residential Code requires double coverage, so the calculator doubles the area. Below 2/12 asphalt shingles aren’t allowed at all, and you’ll need a low-slope membrane instead.
Ice barrier runs from the roof edge up to a point at least 24 inches inside the exterior wall. Measured level, that’s the overhang plus 2 feet. On a sloped roof the membrane has to cover the longer distance along the slope, so we multiply by the pitch multiplier and count how many 36-inch courses, lapped 4 inches, fill that width. Each course runs the full length of the eaves. Valleys get one 36-inch strip down their length.
Assumptions: the ice barrier goes under the regular underlayment area rather than replacing it, which adds a little extra. Roll sizes vary by brand, so check the label and adjust if yours differs.
Worked example
Take the ranch from our roof area and shingle examples: a hip roof at 5/12 with 1,966 sq ft of roof, 176 ft of eaves and an 18-inch overhang. The house is in a snow region where the building department requires ice barrier.
- Coverage: 5/12 is above 4/12, so one layer. 1,966 × 1.10 = 2,163 sq ft
- Synthetic: 2,163 ÷ 917 = 2.36, so 3 rolls. With #15 felt it would be 2,163 ÷ 408 = 5.3, so 6 rolls
- Ice barrier width: (1.5 + 2) × 1.0833 = 3.79 ft up the slope
- That’s more than one 36-inch course, so 2 courses along 176 ft of eave = 352 ft
- With 10% waste: 387 ft ÷ 66.7 ft = 5.8, so 6 rolls of ice barrier
Compare a 3/12 porch roof of 300 sq ft on the same house. It needs two layers, so 300 × 2 × 1.10 = 660 sq ft of coverage, which fits on one synthetic roll or two rolls of #15 felt.
Ice barrier reference table
Courses of 36-inch ice barrier needed to reach 24 inches inside the wall. Most homes with a 1- to 2-foot overhang need two courses at any pitch, and a deep overhang on a steep roof needs three.
| Eave overhang | 4/12 | 6/12 | 8/12 | 10/12 | 12/12 |
|---|---|---|---|---|---|
| None | 1 | 1 | 1 | 1 | 1 |
| 1 ft | 2 | 2 | 2 | 2 | 2 |
| 1.5 ft | 2 | 2 | 2 | 2 | 2 |
| 2 ft | 2 | 2 | 2 | 2 | 2 |
| 3 ft | 2 | 2 | 3 | 3 | 3 |
Tips and common mistakes
Measure the overhang level, not along the rafter tail. The code’s 24-inch point is measured from the inside face of the exterior wall. A 2-foot overhang means the ice barrier has to cover 4 feet of level run, and more along the slope.
Start at the eave and lap shingle fashion. Each course overlaps the one below it, so water running down the roof always flows over a lap, never into one. Felt is typically lapped 2 inches on the sides and 4 inches at the ends, while many synthetics call for 4-inch side laps and 6-inch end laps. Follow the printed lap lines.
Fasten synthetics as the label says. Most synthetic underlayments need plastic-cap nails or cap staples on a tight pattern, and some limit how long they can stay exposed to sun. Felt wrinkles if it gets wet before the shingles go on, and the wrinkles telegraph through.
Check whether you’re in an ice-barrier area. Local jurisdictions fill in Table R301.2 of the IRC, including whether ice barrier is required. Even where it isn’t, valleys, low-slope sections and the area around chimneys and skylights benefit from it.
Watch the low-slope sections. Porches, shed dormers and additions often sit below 4/12 even when the main roof doesn’t. Run them separately so they get the double layer.
Don’t walk on fresh underlayment in the wet or the heat. Felt tears and some synthetics are slick when damp or dusty. Use fall protection on any roof, and stay off steep ones until roof jacks or toe boards are in place.
Frequently asked questions
How much does a roll of roof underlayment cover?
A typical synthetic roll is 48 inches by 250 feet, sold as 10 squares, and covers about 917 sq ft once you allow for 4-inch laps. A roll of
Is synthetic underlayment better than felt?
Synthetic is lighter, stronger, less slippery in most cases and can stay exposed longer before the shingles go on, and one roll covers more than twice what a roll of
Do I need two layers of underlayment?
Yes, on asphalt shingle roofs between 2/12 and 4/12. The IRC calls for a 19-inch starter strip along the eave followed by 36-inch sheets lapped 19 inches, which gives two layers everywhere. From 4/12 up, a single layer lapped shingle fashion is the code minimum.
Where is ice and water shield required?
In any area where the local jurisdiction says there's a history of ice forming along the eaves, the code requires an ice barrier from the roof edge to at least 24 inches inside the exterior wall. Your building department's answer to the ice barrier question in Table R301.2 tells you whether that's you.
Should ice and water shield go in the valleys?
The code doesn't require it in valleys, but many shingle manufacturers recommend a 36-inch strip centered in each valley under the valley lining, and most roofers do it on every roof. It's the place water concentrates, so it's cheap insurance.