Solve backwards
How much roof will my rolls of underlayment cover?
- 3 rollsThe example worked below
- Roofing Underlayment CalculatorEvery figure is its own
You have
How much roofing underlayment?
Worked by the Roofing Underlayment Calculator at the settings below.
The settings it uses
- Overlap Allowance (%)
- 10
What it will do
2,727 sq ft of roof surface
253 m²
One roof surface. Enough for a roof surface about 52 × 52 ft.
Or several. About 1 roof surface of the size the Roofing Underlayment Calculator opens on: a roof surface about 46 × 46 ft.
Check it forwards
- Roof Surface Area
- 2,727 sq ft
- It asks for
- 3 rolls
- You have
- 3 rolls
The Roofing Underlayment Calculator asks for 3 rolls at that size, and 4 rolls one step larger.
If you had more, or less
Around your amount
Each worked by the Roofing Underlayment Calculator at the same settings.
Half
2 rolls
1,818 sq ft
a roof surface about 43 × 43 ft
Yours
3 rolls
2,727 sq ft
a roof surface about 52 × 52 ft
Half as much again
5 rolls
4,545 sq ft
a roof surface about 67 × 67 ft
Double
6 rolls
5,455 sq ft
a roof surface about 74 × 74 ft
Common amounts
What each amount of roofing underlayment covers
The sizes it is usually sold in, each worked by the Roofing Underlayment Calculator at the settings above.
| You have | It covers | As one roof surface | Average roof surfaces |
|---|---|---|---|
| 1 roll | 909 sq ft(84.5 m²) | a roof surface about 30 × 30 ft | under 1 |
| 2 rolls | 1,818 sq ft(169 m²) | a roof surface about 43 × 43 ft | under 1 |
| 3 rolls | 2,727 sq ft(253 m²) | a roof surface about 52 × 52 ft | 1 |
| 6 rolls | 5,455 sq ft(507 m²) | a roof surface about 74 × 74 ft | 2 |
Ten squares to the roll
The calculator divides by 1,000 square feet a roll, about 93 m² — ten roofing squares, the common synthetic roll its source gives. Felt comes in far smaller rolls and some synthetics in larger ones, so with a different product read the roof surface from the table and divide it by your own roll's stated coverage.
Measure the slope, not the plan
The area here is the sloped roof surface, and there is no pitch multiplier anywhere in the calculator. A plan measurement taken from the footprint comes out short at every pitch above flat, so compare what you have with the same sloped area the roofing shingle calculator was given.
Valleys run double, and eaves may be something else
Valleys and hips where the membrane is run double, and skylights or chimneys you would deduct, are all left to the same ten per cent lap allowance — a roof of many valleys spends more of each roll. Eave and valley strips laid in self-adhered ice and water membrane are not taken off the count either.
How the answer is worked
Nothing here is a second formula. The Roofing Underlayment Calculator runs forwards — a size in, a quantity out — and this page grows its own starting roof surface in proportion until the quantity it asks for is what you have. Where it rounds up to whole containers, the answer is the largest size before the next one would be needed.
Where the calculator adds an allowance for cuts or breakage when you order, the same allowance comes off here, because it is the same calculator: a stack you already own finishes less than its face value. Where it adds none, neither does this page — the settings beside the answer say which.
Roofing Underlayment Calculator
runs it forwardsEstimate how many rolls of synthetic roofing underlayment you need beneath your shingles.
Work out what to buy insteadSolve backwards
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Radiant barrier foil, worked by the Attic Radiant Barrier Calculator.
Solve backwards
Start from what is already in the yard
Paint, sheets, rolls, bags and pieces, each read the other way by the calculator that orders it.
The calculator's own figures
Nothing is estimated twice: the page runs the same calculator you would order with.
The largest size that fits
Where a calculator rounds up to whole containers, the answer stops before the next one.
Both unit systems
Gallons and litres, square feet and square metres, side by side.
Frequently asked questions
- How much roof surface does 3 rolls of roofing underlayment cover?
- At the Roofing Underlayment Calculator's own settings, 2,727 sq ft (253 m²) — a roof surface about 52 × 52 ft. That is about 1 roof surface of the size the calculator opens on: a roof surface about 46 × 46 ft. Every figure follows the settings, which you can change in the calculator.
- How many squares does a roll of synthetic underlayment cover?
- A common synthetic roll covers about 1,000 square feet, 93 m², which is ten roofing squares. With ten per cent spent on side and end laps it covers about 910 square feet (84 m²) of sloped roof — roughly nine squares. Felt rolls cover far less; check the product.
- Do I measure the roof footprint or the sloped area for underlayment?
- The sloped area. Underlayment lies on the roof deck, which is larger than the footprint on every roof steeper than flat, and the calculator applies no pitch factor — so enter or compare against the same sloped surface area used for the shingles.
- Does underlayment go under ice and water shield?
- Ice and water shield is usually laid first along the eaves and in valleys, directly on the deck, with the underlayment lapped over it shingle-fashion further up. Where it is used, those strips replace underlayment there, so the rolls you have cover the rest of the roof.