Roof Underlayment Calculator
Find how many rolls of roof underlayment to buy, whether synthetic underlayment or roofing felt (felt paper), and how much ice and water shield to run along the eaves and valleys. Measure the roof from above, enter its pitch, and see the rolls for each common size, with the overlap between courses taken off each roll and extra for waste.
Add a section for each part of the roof, such as a porch with a different pitch.
- Choose US or Metric. You can still change the unit of any single field.
- Enter the Length and Width of the roof as seen from above, including the overhang at the eaves and gable ends, and its Pitch as x/12 or in degrees. Use Add section for each part of the roof with a different pitch.
- Check the Overlap. It starts at 3 in (150 mm in Metric). Use the lap your roll calls for.
- For ice and water shield, enter the length of each straight eave under Eave and each valley under Valley, and check Rows at the eaves. Leave them blank if you don't need shield.
- Optionally, enter a Custom roll width and Custom roll length for an underlayment roll not in the table, a Custom shield roll length, and prices to estimate the cost.
Dimension fields accept whole numbers, decimals, fractions, and mixed
numbers, such as 40, 12.5, 3/4,
or 30 1/2. If you measured the roof on the slope itself,
enter it with a pitch of 0.
How to calculate underlayment rolls
Underlayment covers the whole roof deck, so the calculator starts from the roof area. It multiplies the area seen from above by a pitch factor, then adds the waste allowance:
Roof area = area from above × √(1 + (rise ÷ 12)²)
Total area = roof area × (1 + waste % ÷ 100)
Underlayment is rolled out in courses parallel to the eave, starting at the bottom, and each course laps over the one below it. So each roll covers its width minus the overlap, times its length:
Coverage per roll = (roll width − overlap) × roll length
Rolls = total area ÷ coverage per roll, rounded up
For a pitch in degrees, the factor is 1 ÷ cos(angle). With several sections, the calculator adds the sections set to Add and subtracts those set to Subtract. The area from above of each shape is calculated as follows:
| Shape | Area |
|---|---|
| Rectangle | length × width |
| Circle | π × diameter² ÷ 4 (or π × radius²) |
| Triangle (base and height) | base × height ÷ 2 |
| Triangle (three sides) | Heron's formula: √(s(s − a)(s − b)(s − c)), where s = (a + b + c) ÷ 2 |
| Trapezoid | (parallel side A + parallel side B) ÷ 2 × height |
| Ring | π × (outer diameter² − inner diameter²) ÷ 4 |
| Rectangular border | outer length × outer width − (outer length − 2 × border width) × (outer width − 2 × border width) |
| Known area | the area you enter |
The waste allowance starts at 10%. Owens Corning suggests adding 10% to 15% for waste on underlayment, counting rolls by the squares on the label. Because the calculator already takes the overlap off each roll, it starts at the low end. The extra covers cuts at the ridge, hips, valleys, and rakes, and the end laps where one roll meets the next.
Roll sizes and the overlap
The table lists the rolls in the calculator and how much each covers with the default overlap of 3 in (150 mm in Metric).
| Roll | Coverage per roll |
|---|---|
| Synthetic 48 in × 250 ft | 937.50 ft² |
| #15 felt 36 in × 144 ft | 396.00 ft² |
| #30 felt 36 in × 72 ft | 198.00 ft² |
| Synthetic 1,219 mm × 76.2 m | 81.46 m² |
| Membrane 1.5 m × 50 m | 67.50 m² |
| Membrane 1 m × 50 m | 42.50 m² |
The synthetic roll is the 48 in × 250 ft size of GAF's FeltBuster and IKO's Stormtite, and the felt rolls are TAMKO's No. 15 and No. 30. In Metric, the synthetic roll is the same Stormtite roll given in meters, and the membranes are UK breather membranes such as those from Permavent and Cromar. Synthetic rolls come in other sizes too. Owens Corning's ProArmor, for example, is 42 in × 286 ft, so enter it as a custom roll.
Reading the coverage on the label
Labels don't all mean the same thing by coverage. GAF lists FeltBuster as 10 squares (1,000 ft²) and notes that this excludes laps, so with a 3 in overlap a roll covers 937.50 ft², or 93.75% of the label. TAMKO lists its No. 15 felt at 408 ft² per roll, which is 34 in × 144 ft, already net of a 2 in lap. Owens Corning lists ProArmor at 929 ft² with a 3 in overlap. The calculator always works from the roll width and length, so enter those from the label rather than the squares.
Which overlap to use
The 3 in default is the side lap that GAF and Owens Corning give for
synthetic underlayment on slopes of 4/12 and steeper. The IRC asks for
felt to be lapped 2 in on those slopes, so for felt you can enter
2. That matches TAMKO's 408 ft² per roll. Leaving 3 in for
felt counts about 3% more, which errs on the side of having enough. The
Metric default of 150 mm is the minimum lap in the UK's BS 5534 for the
common case of a membrane on a 15° to 34° roof that isn't fully supported
by boards. The same table asks for 100 mm at 35° and over, and 225 mm at
12.5° to 14°. The overlap can be 0 to 12 in (304.8 mm).
How much ice and water shield you need
Ice and water shield is a self-adhering membrane that goes along the eaves and in the valleys, where ice dams and running water are most likely to get under the shingles. The calculator counts it by length:
Shield length = total eave length × rows at the eaves + total valley length
Each row runs the full length of the eave. In a valley, one width of shield is centered on the valley line, so measure each valley along the valley itself, from the eave to the top, not from above. The rolls in the calculator are 36 in (914 mm) wide, and the number of rolls is the length, with the waste allowance added, divided by the roll length and rounded up.
How many rows to run at the eaves
Rows at the eaves starts at 2. Where an ice barrier is required, the International Residential Code (IRC R905.1.2) has it run from the lowest edge of the roof to at least 24 in inside the exterior wall line. That 24 in is measured level, so add the overhang and the wall thickness, then multiply by the pitch factor to get the distance up the slope. On roofs of 8/12 and steeper, the barrier must also reach at least 36 in up the slope from the eave, so use whichever is longer.
For example, with no overhang, a 4 in wall, and an 8/12 pitch, (24 + 4) × √(1 + (8 ÷ 12)²) = 33.65 in, so the 36 in rule is the longer one and a single 36 in row is enough. Fine Homebuilding works through a 6/12 roof with a 12 in overhang and a 2×6 wall that needs 47 in up the slope, more than one 36 in row. Two 36 in rows lapped 3.5 in, as Grace calls for, reach 68.5 in, or about 5 ft 8 in. These examples are only a guide. Check what your local code asks for, and set the rows to match.
Shield roll sizes
The US rolls are 36 in wide and 75 ft (Grace Ice & Water Shield), 66.7 ft (Owens Corning WeatherLock G), and 65 ft (IKO StormShield) long. In Metric, they are 914 mm × 22.9 m (Grace) and 914 mm × 19.8 m (IKO). For another roll, enter its length as a custom shield roll.
Low-slope roofs and full coverage
On slopes from 2/12 up to 4/12, the IRC calls for two layers of
underlayment, with each 36 in sheet lapping the one below by 19 in. GAF
asks for a 25.5 in lap on FeltBuster below 4/12. Either way, it takes
about twice the underlayment. The calculator doesn't decide this for you.
For a section that needs two layers, set its Quantity to
2 so its area counts twice.
Some roofers cover the whole deck with ice and water shield instead of underlayment. To count rolls for that, enter the shield roll's width and length as a custom underlayment roll, and set the overlap to the side lap on its instructions.
Examples
A gable roof
A roof 40 ft × 30 ft from above with a 6/12 pitch has an area of 1,200 × √1.25 = 1,341.64 ft², or 1,475.80 ft² (14.76 squares) with 10% added. That is 2 synthetic rolls at 937.50 ft² each, 4 rolls of #15 felt, or 8 rolls of #30 felt.
Adding ice and water shield
The same roof has two 60 ft eaves and one 20 ft valley. Two rows along both eaves and one width down the valley is 60 × 2 × 2 + 20 = 260 ft, or 286.00 ft with 10% added. That is 5 rolls of 66.7 ft, 4 rolls of 75 ft, or 5 rolls of 65 ft.
Metric dimensions
A roof 12 m × 8 m from above with a 30° pitch has an area of 96 ÷ cos 30° = 110.85 m², or 121.94 m² with 10% added. With a 150 mm overlap, that is 2 rolls of 1.5 m × 50 m membrane at 67.50 m² each, or 3 rolls of 1 m × 50 m. Two rows of shield along two 12 m eaves come to 52.80 m, or 3 rolls of 19.8 m.
FAQ
About 3 synthetic rolls or 6 rolls of #15 felt. With 10% extra, 2,000 ft² is 2,200 ft². A 48 in × 250 ft synthetic roll covers 937.50 ft² with a 3 in overlap, so 2,200 ÷ 937.50 = 2.35, rounded up to 3. A #15 felt roll covers 396 to 408 ft², so it takes 6.
About 937.50 ft² for a 48 in × 250 ft roll with a 3 in overlap. The label says 10 squares (1,000 ft²), but that doesn't take off the laps between courses.
Where it's required, the IRC has it reach at least 24 in inside the exterior wall line, and at least 36 in up the slope on roofs of 8/12 and steeper. That's often more than one 36 in row, so the calculator starts at two rows. Check your local code.
Measure along the valley itself, from the eave to where it meets the ridge or the wall. A valley runs on a diagonal down the slope, so it's longer than it looks from above.
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