Roof Underlayment Calculator

Calculate roof underlayment (felt or synthetic) needed. Enter roof area, pitch, and overlap for rolls required.

📐 Roof Underlayment Calculator

Roof Length (ft)

Building Width (ft)

Roof Pitch (/12)

Underlayment Type

Results

Actual Roof Area804.98 sq ft

With 15% Overlap925.73 sq ft

Rolls Needed1 rolls

💡 How to Calculate Roof Underlayment

Roof underlayment goes under shingles as a secondary moisture barrier. Calculate total roof area using the pitch multiplier, then add 15% for horizontal and vertical overlaps. Standard overlap: 4 inches horizontal, 6 inches at end laps.

Types: #15 felt (400 sq ft/roll), #30 felt (200 sq ft/roll), synthetic (1,000 sq ft/roll), and self-adhered ice & water shield (75 sq ft/roll). Ice shield is required in the first 3 feet of eaves in cold climates.

A 30×24 ft roof at 6/12 pitch: actual area = 805 sq ft + 15% overlap = 926 sq ft. Synthetic: 1 roll. #15 felt: 3 rolls. #30 felt: 5 rolls.