How it works
The calculator finds the sloped roof area, adds your extra allowance and divides by the area one roll covers after the laps.
Roof area = (Length + 2 × Overhang) × (Width + 2 × Overhang) × √(1 + (Rise ÷ Run)²) or entered directly
Area incl. extra = Roof area × (1 + Extra ÷ 100)
Coverage per roll = (Roll width − Side lap) × (Roll length − End lap)
Rolls = ⌈ Area incl. extra ÷ Coverage per roll ⌉
⌈ ⌉ means rounding up to the next whole roll. The square-root term is the slope factor: how much longer the roof surface is than its horizontal run, about 1.118 for a 6:12 roof.
From footprint to roof area
You don’t need to climb onto the roof. Measure the building from outside wall to outside wall, estimate how far the eaves and rakes stick out, and read the pitch from the plans or with a level and a tape (the rise in inches over 12 in of run). The calculator adds the overhang to every side and multiplies the plan area by the slope factor, as in the roof area calculator. This is exact for a gable roof and for a hip roof whose planes all share one pitch. For roofs with several pitches, dormers or additions, work out each part and enter the total as a known roof area.
How laps change the count
Underlayment is laid in horizontal courses starting at the eaves. Each course overlaps the one below it – the side or horizontal lap – and where one roll ends and the next begins in the same course, the pieces overlap again – the end or vertical lap. Neither overlap adds to the area covered.
Take the Owens Corning ProArmor roll used here: 42 in wide and 286 ft long. With the 3 in course lap from its instructions, each course covers 39 in of roof, and Owens Corning gives the coverage as 929 sq ft instead of the roughly 1,000 sq ft in the roll. Taking the 4 in end lap off the length as well gives 3.25 ft × 285.67 ft = 928.4 sq ft. TAMKO’s felt works the same way: its brochure lists 408 sq ft for a 36 in × 144 ft roll of No. 15 felt, which is 34 in of width (36 in less a 2 in lap) × 144 ft.
Synthetic or felt
Asphalt-saturated felt is the traditional underlayment. TAMKO’s No. 15 felt meets ASTM D226 Type I and No. 30 felt Type II; the heavier No. 30 roll is half as long, so it covers half the area. Synthetic underlayments are plastic sheets – TAMKO describes them as lighter and less likely to tear than traditional felts – and they come in much longer rolls. Which one your roof needs is set by the roofing manufacturer’s instructions and local code, not by this calculator. The arithmetic is the same for both.
Low-slope roofs
The laps in the product options are for slopes of 4:12 and steeper. Owens Corning’s ProArmor instructions say to cover the deck with two layers on slopes from 2:12 to less than 4:12, laying a half-width starter course and lapping every course by half a roll plus a few inches. TAMKO’s shingle instructions do the same for felt. When the footprint mode gives a pitch below 4:12, the calculator warns you; the roll count shown is still for one layer, so double it for a two-layer application.
Extra allowance
The calculator doesn’t model hips, ridges and valleys. Owens Corning asks for ProArmor to lap at least 6 in over each side of hips, ridges and valleys and 4 in up any wall, and every rake edge leaves an offcut. A plain gable roof needs little extra; a cut-up roof with hips, valleys and dormers needs more. Start with 0 % to see the minimum, then add an allowance that fits your roof.
Worked example in words
A 40 × 30 ft house with a 1 ft overhang has a 42 × 32 ft plan, or 1,344 sq ft. At 6:12 the slope factor is 1.118, so the roof is 1,502.6 sq ft. One ProArmor roll covers 928.4 sq ft after laps, so 1,502.6 ÷ 928.4 = 1.62 – you need 2 rolls. The same roof in No. 15 felt needs 1,502.6 ÷ 407.1 = 3.7, so 4 rolls.
Worked example
Inputs
- Measure by
- Footprint and pitch
- Building length
- 40 ft
- Building width
- 30 ft
- Overhang
- 1 ft
- Roof pitch
- 6:12
- Underlayment
- Owens Corning ProArmor synthetic – 42 in × 286 ft, 3 in / 4 in laps
- Extra allowance
- 0 %
Result
- Roof area
- 1,503 sq ft
- Area incl. extra
- 1,503 sq ft
- Coverage per roll after laps
- 928.4 sq ft
- Rolls (one layer)
- 2
Common mistakes
- Using the footprint instead of the roof area. A sloped roof is larger than the ground it covers – a 6:12 roof is about 12 % larger.
- Dividing by the area printed on the roll. Every course overlaps the one below, so a roll covers less roof than its gross area – about 3 in of every 42 in of width for the synthetic used here.
- Counting one layer on a low-slope roof. Below 4:12, the instructions used here call for two layers, which doubles the rolls.
- Forgetting the laps over hips, ridges and valleys. Owens Corning asks for ProArmor to lap at least 6 in over each side of hips, ridges and valleys.
- Mixing up felt weights. A roll of No. 30 felt is half the length of a roll of No. 15 felt, so it covers about half the area.
Assumptions and limits
- In footprint mode, every roof plane has the same pitch; roof area = (length + 2 × overhang) × (width + 2 × overhang) × slope factor.
- The underlayment is laid in one layer, parallel to the eaves, with the side and end laps you choose (the product options use the makers’ minimums for 4:12 and steeper).
- Coverage per roll = (roll width − side lap) × (roll length − end lap). Rolls are counted from the total area and rounded up; individual courses are not planned.
- Hip, ridge and valley laps, ice barrier and starter strips are not counted separately – use the extra allowance.
Frequently asked questions
How much does a roll of roof underlayment cover?
It depends on the product. Owens Corning lists ProArmor at 42 in × 286 ft, 10 squares, and 929 sq ft with a 3 in overlap. TAMKO lists No. 15 felt at 36 in × 144 ft (4 squares, 408 sq ft when applied with its laps) and No. 30 felt at 36 in × 72 ft (2 squares, 204 sq ft).
How much should underlayment overlap?
Follow the instructions for your product. For slopes of 4:12 and up, Owens Corning asks for ProArmor courses to lap at least 3 in and the ends at least 4 in; TAMKO’s instructions lap felt at least 2 in over the lower course and 4 in where the ends join.
Do I need two layers of underlayment?
On low slopes, often yes. Owens Corning says to cover the deck with two layers of ProArmor on slopes from 2:12 to less than 4:12, and TAMKO’s shingle instructions also call for two layers on lower slopes. Check your roofing’s instructions and local code.
Synthetic or felt – which needs fewer rolls?
Synthetic rolls are much longer. On a 1,500 sq ft roof, the ProArmor roll used here covers about 928 sq ft after laps (2 rolls), while No. 15 felt covers about 407 sq ft per roll (4 rolls) and No. 30 felt about 203 sq ft (8 rolls).
Does this include ice and water shield?
No. Self-adhered ice barrier at the eaves and in valleys is a separate product with its own roll size. Subtract that area if it replaces the underlayment there, or leave it in if the underlayment goes on top.
Sources
- ProArmor Synthetic Roof Underlayment – product page and installation instructions — Owens Corning. Roll size "10sq.", length "286 ft.", width "42 in."; "Coverage (with 3-in. overlap) 929 sq. ft." Installation instructions, slopes 4:12 or greater: "lapping each course at least 3 inches over the underlying course"; "lap the ends at least 4 inches"; "a minimum of 6 inches from both sides over all hips, ridges and valleys"; "a minimum 4 inches up the wall"; slopes 2:12 to less than 4:12: "cover the deck with two layers".
- TAMKO No. 15 ASTM Asphalt Saturated Organic Felt – Product Data — TAMKO Building Products. ASTM D 226 Type I; "Roll Dimensions 144’ X 36”"; "Coverage per roll 4 Squares"; laying lines 2”, 4”, 17”.
- TAMKO No. 30 ASTM Asphalt Saturated Organic Felt – Product Data — TAMKO Building Products. ASTM D 226 Type II; "Roll Dimensions 72’ X 36”"; "Coverage per roll 2 Squares"; laying lines 2”, 4”, 17”.
- Underlayment Products brochure — TAMKO Building Products. Coverage per roll "when applied according to instructions": No. 15 felt 36" × 144′ → 408 sq ft, No. 30 felt 36" × 72′ → 204 sq ft (roll width less a 2 in lap × roll length).
- Heritage Vintage – Application Instructions — TAMKO Building Products. Felt on slopes 4" per foot and greater: "lapping each course of the felt over the lower course at least 2". Where ends join, lap the felt 4"". Lower slopes: two layers of underlayment.
- Estimating How Much Roofing Is Required — IKO. Actual area = horizontal area × area/rake factor; 6/12 → 1.118. The factor equals √(1 + (rise/run)²) (Pythagoras).
Method reviewed November 4, 2026