Roofing Underlayment Calculator
Works out rolls of roofing underlayment from the roof area, allowing for the laps. Also covers the ice and water barrier along the eaves where that applies.
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The formula
rolls = ceiling(area × (1 + lap allowance) ÷ roll coverage)
Underlayment laps both horizontally and vertically, so the material used always exceeds the area covered. 15% is a reasonable allowance for a typical pitch — shallower roofs need wider head laps and therefore more.
What to watch out for
Underlayment is the roof's second line of defence, and it is the one that works when the first fails:
- Lay from the eaves up. Each course laps over the one below so water runs across the joints rather than into them.
- Cap nails, not staples. Staples tear out in wind, and the underlayment is often exposed for days before the covering goes on.
- Synthetic versus felt. Synthetic is lighter, stronger, safer to walk on and tolerates longer exposure. Felt is cheaper and still specified, but it tears and it absorbs water.
In climates with freeze-thaw cycles, a self-adhesive ice and water barrier goes along the eaves first — usually enough courses to reach about 600 mm inside the warm wall line, which is where ice dams back water up.
Frequently Asked Questions
How much roofing underlayment do I need?
The roof area plus about 15% for laps. A 100 m² roof needs 115 m², which is two rolls of synthetic at 92.9 m² each, or four rolls of #15 felt.
How many square feet in a roll of roofing felt?
#15 felt covers 400 square feet (four squares), #30 covers 200, and synthetic underlayment typically covers 1,000 square feet per roll.
Do I need an ice and water barrier?
In climates where ice dams form at the eaves, yes — and it is often required. It is a self-adhesive membrane laid along the eaves before the main underlayment, extending past the warm wall line.