What does this tool do?
A sloped roof has more surface area than its footprint, so pitch must be applied before rolls are counted. This calculator uses a gable-roof pitch factor, adds 10% for overlaps and divides by the roll coverage.
The Formula
Roof area = Length × Width × √(1 + (Pitch ÷ 12)²); Rolls = ⌈Area × 1.10 ÷ Roll coverage⌉Frequently Asked Questions
How many rolls of underlayment do I need?
Divide the sloped roof area plus 10% waste by the roll coverage. A 1,300 sq ft roof needs about four 400 sq ft rolls.
Does roof pitch change the area?
Yes. Steeper roofs have more surface area than their footprint; this calculator applies the pitch factor for a gable roof.