Roof Underlayment Calculator
Estimate the felt or synthetic underlayment needed for a roof.
How It Works
Rolls = Roof area ÷ Roll coverage. #15 felt covers 432 sq ft; #30 felt covers 216 sq ft.
Example
For a 2,000 sq ft roof with #15 felt: 2000 ÷ 432 = 4.6 → 5 rolls.
Frequently Asked Questions
What is underlayment for?
Underlayment is the water-resistant layer between the roof deck and the shingles.
When should I use #30 instead of #15?
#30 is heavier and often used on low-slope roofs or where more protection is needed.
Do rolls cover exactly their label size?
There is overlap loss at laps. Add 10–15% waste for realistic roll counts.