Underlayment Roll Calculator

How to Use the Underlayment Roll Calculator

Enter your total roofing squares and the coverage per roll for your underlayment product (synthetic underlayment commonly covers around 10 squares per roll; traditional 15 lb felt covers closer to 4). The calculator returns the number of rolls to order.

How the Calculation Works

This is a straightforward coverage division: total roofing squares ÷ squares covered per roll, rounded up to a whole roll since partial rolls aren't sold separately.

Example Calculation

13.2 roofing squares with a synthetic underlayment rated at 10 squares per roll: 13.2 ÷ 10 = 1.32, rounded up to 2 rolls.

Frequently Asked Questions

Synthetic underlayment vs. felt — which covers more?

Synthetic underlayment rolls are typically lighter and larger, often covering around 10 squares per roll, compared to roughly 4 squares for a standard roll of 15 lb felt. Always check your specific product's coverage rating, since it varies by brand and weight.

Do I need to account for overlap?

Yes — underlayment courses overlap by a few inches at each seam per manufacturer instructions, which is generally already factored into the manufacturer's stated coverage-per-roll rating. If your roof has an unusually complex layout, consider rounding up an extra roll.

Is underlayment required by code?

In most jurisdictions, yes — underlayment is a standard part of a code-compliant roofing system, providing a secondary water barrier beneath the shingles. Requirements vary by climate and local code, so check with your building department.