Underlayment Calculator
Free underlayment calculator for roof underlayment planning. Estimate roofing underlayment rolls from roof area or footprint plus pitch before the order goes out.
Built for roofing estimates
Use this underlayment calculator when you need a fast roll count before the order goes out.
If you already know the roof area, or you only know the footprint and pitch, this underlayment calculator helps you get to a practical order number fast. It is built for contractors, estimators, and homeowners who want to know how many rolls of roof underlayment to buy without guessing, overordering, or coming up short on delivery day.
Avoid short orders
A bad underlayment count can slow down the whole job. This tool helps you order enough material the first time.
See the coverage basis
Synthetic and felt do not cover the same area per roll. The result shows the basis clearly so the number makes sense.
Keep the estimate moving
Once you have the roll count, you can move into the full RoofingBOM workflow for shingles, waste, starter, drip edge, and ridge materials.
What problem this underlayment calculator solves
Roof underlayment is one of those materials that gets missed when the estimate is moving fast. The roof size might still be rough, the shingle order may not be finished, and the felt versus synthetic choice can change the quantity more than expected. This underlayment calculator solves that problem by giving you a fast, readable roll count from the roof size you already have.
That matters for both contractors and homeowners. Contractors need a clean number they can trust before sending an order to the supplier. Homeowners and DIY users need to avoid buying too much or too little. A useful underlayment calculator should reduce second trips, avoid confusion about coverage, and make the next estimating step easier instead of harder.
How to use the underlayment calculator
- Start with actual roof area if you already have it. If you only know footprint, enter footprint and pitch first.
- Choose the underlayment type you plan to use. Synthetic and felt have different coverage per roll.
- Select a waste level so the count reflects real cutting, laps, and normal jobsite loss.
- Review rolls needed, roof area used, order area, and the ordering note before placing the order.
If you know roof area
Enter roof area directly when you already have a roof report, drone measurement, or field measurement.
If you only know footprint
Use footprint plus pitch when you still need to convert flat area into true roof surface area.
Why pitch matters in an underlayment estimate
Footprint area is flat. The roof you actually cover is sloped. As pitch increases, the roof surface gets larger than the building footprint. If pitch is skipped, the underlayment estimate can come out low before waste is even added. That is why this tool lets you start from footprint but still convert into real roof area before calculating rolls.
This is also why the output shows both roof area used and order area. The goal is not just to give you one final number. The goal is to make the number explainable. When the count changes, you can see whether it moved because of pitch, waste, or product coverage.
Synthetic vs felt coverage
One of the easiest ways to miscount underlayment is to assume every roll covers the same area. It does not. Synthetic underlayment usually covers much more roof area per roll, while felt covers much less. That is why this tool keeps the coverage basis visible in the result instead of hiding it behind one generic number.
Synthetic underlayment
Uses a 10-square-per-roll basis, or about 1,000 square feet per roll.
#15 felt
Uses a 4-square-per-roll basis, or about 400 square feet per roll.
What is included and what is not
This tool is focused on field underlayment coverage for the main roof surface. It estimates how many rolls of underlayment you need based on roof area, pitch, coverage, and waste. That narrower scope is intentional because it makes the number easier to trust.
It does not try to bundle every protection layer into one total. Ice-and-water shield at eaves and valleys should be counted separately. Local code requirements should be checked separately. Manufacturer-specific overlap and installation details should also be verified separately. A clean estimate is more useful than a messy estimate pretending to cover everything.
Common mistakes this tool helps you avoid
Using footprint as roof area
If footprint is used without pitch, the estimate can come out low because the real roof surface is larger than the flat plan view.
Mixing felt and synthetic assumptions
Coverage changes by product type. If that basis is hidden, the count is much harder to trust.
Forgetting waste
A practical estimate should not stop at raw area. Waste needs to be included before the final roll count.
Folding leak barrier into the same number
Field underlayment and ice-and-water protection are not the same thing. Counting them together creates confusion later.