Roof Underlayment Calculator
Rolls of roofing underlayment needed.
Enter your values
Enter the roof area in sq ft.
Enter the roll coverage in sq ft.
Enter the overlap/waste allowance in %.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Rolls needed
- 7
- Effective area with overlap
- 2760 sq ft
Quick answer
Roll count is the roof area plus waste for overlaps, divided by the usable coverage of one roll.
rolls = ceil(roof area x (1+waste%) / usable roll coverage)
At a glance
| What it does | Rolls of roofing underlayment needed. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Roof area, Roll coverage, Overlap/waste allowance |
| Main output | Rolls needed |
| Formula | rolls = ceil(roof area x (1+waste%) / usable roll coverage) |
| Cost | Free — no sign-up, no download |
How to use the Roof Underlayment Calculator
- 1Get the roof area, ideally already adjusted for pitch.
- 2Check roll coverage — synthetic underlayment often covers 400 sq ft (10 squares) per roll.
- 3Add 10-15% for the required horizontal and vertical overlaps.
Inputs explained
- Roof area(sq ft)
- Enter the roof area in sq ft.
- Roll coverage(sq ft)
- Enter the roll coverage in sq ft.
- Overlap/waste allowance(%)
- Enter the overlap/waste allowance in %.
Worked example
Using the values the calculator loads with:
Inputs
- Roof area2400 sq ft
- Roll coverage400 sq ft
- Overlap/waste allowance15 %
Results
- Rolls needed7
- Effective area with overlap2760 sq ft
Frequently asked questions
How much overlap is needed?
Typically 4 inches horizontal and 6 inches vertical, more in ice-dam zones.
Synthetic or felt?
Synthetic underlayment is lighter, tears less and covers more area per roll than 15 lb felt.
What do I need to enter into the Roof Underlayment Calculator?
Just 3 values: roof area, roll coverage, overlap/waste allowance. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Roof Underlayment Calculator work out the answer?
Roll count is the roof area plus waste for overlaps, divided by the usable coverage of one roll. It applies the formula rolls = ceil(roof area x (1+waste%) / usable roll coverage) and shows the working so you can check each step by hand.
Is the Roof Underlayment Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.