Roof Underlayment Calculator
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Underlayment goes on the roof deck before the shingles and is what keeps water out when wind drives rain under the covering. It is sold in rolls that cover very different areas depending on the product, and on shallow roofs it has to go on twice. This calculator handles both.
Underlayment rolls for your roof
Rolls of synthetic or felt underlayment for a roof, doubled automatically on low slopes where two layers are required.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Rolls to order
- Squares to cover Roof area plus waste, doubled on low slopes.
- Layers
- Coverage per roll
Below 2 in 12 this is a low-slope roof, and shingle underlayment is the wrong product. The roof needs a membrane system; the flat roof membrane calculator is the one to use.
Between 2 in 12 and 4 in 12, shingle manufacturers and the building code call for two layers of underlayment, each course lapping the one below by half its width plus a little. The count above is already doubled.
At 4 in 12 and steeper, a single layer with ordinary laps meets most shingle installation instructions. In cold climates the eaves still need an ice barrier; the ice barrier calculator covers that.
Underlayment is the second line of defence under the roof covering, and the laps are what make it work. Follow the lap widths printed on the roll and the shingle manufacturer installation instructions, because the warranty usually depends on them.
How the calculation works
Take the roof area, add the waste allowance and express it in squares. If the pitch is between 2 in 12 and 4 in 12, double it, because the roof needs two layers. Divide by what one roll covers after laps, and round up.
A worked example
A 1,250 sq ft roof with 10 per cent waste needs 13.75 squares covered. At 6 in 12 that is two 10-square rolls of synthetic underlayment. The same roof at 3 in 12 needs 27.5 squares of coverage because of the second layer, which in No. 15 felt at about 4 squares a roll comes to 7 rolls.
Why low slopes need two layers
Water runs off a shallow roof slowly, and wind can push it back up under the shingles. Shingle manufacturers and the International Residential Code both call for a doubled underlayment on roofs from 2 in 12 up to 4 in 12, laid so that each course overlaps the one below by a little more than half its width. Below 2 in 12, shingles are not an option at all and the roof needs a membrane.
Felt or synthetic
Asphalt-saturated felt is the traditional product. No. 15 is lighter and covers more per roll; No. 30 is heavier. Synthetic underlayments are lighter to carry, tear less easily and cover far more per roll. Either can be the right choice. What matters is that the product is approved by the shingle manufacturer and laid to the laps printed on it.
Frequently asked questions
Is underlayment required under shingles?
Yes. Building codes and every major shingle manufacturer require an underlayment beneath asphalt shingles, and the shingle warranty usually depends on it being installed as the instructions say.
Should I use synthetic underlayment or felt?
Either is acceptable where the shingle manufacturer approves it. Synthetic is lighter, harder to tear and covers far more per roll; felt costs less per roll. The laps and fastening on the label matter more than the choice.
When does a roof need two layers of underlayment?
On pitches from 2 in 12 up to less than 4 in 12. Water drains slowly at those pitches, so the code and shingle makers call for two layers, each course lapping the one below by just over half its width.