Roof Underlayment Calculator
Calculates breathable synthetic roof underlayment rolls (1,000 sq ft / 10-square rolls), traditional asphalt felt rolls, and self-adhering ice & water shield rolls for eaves, valleys, and low-slope roof planes.
Formula
Underlayment Rolls = Ceil((Roof Sq Ft × (1 + Waste %)) ÷ Roll Coverage); Ice Shield Rolls = Ceil(Eave Ft ÷ Roll Length)
1. Total Roof Area with Overlap = Roof Area × (1 + Waste %) 2. Synthetic Rolls (1,000 sq ft) = Ceil(Total Area ÷ 1000) 3. Ice & Water Shield (36" × 66 ft = 200 sq ft/roll) = Ceil((Eaves × Courses) ÷ 66)
Inputs
- True Roof Surface Area
- Total Eave Length (Gutter edge perimeter)
- Field Underlayment Material
- Ice & Water Shield Along Eaves & Valleys?
Worked example
Protecting an 1,800 sq ft roof deck with 100 linear ft of eaves using 2 courses of ice shield and synthetic felt.
Outcome: Total underlayment area with 10% overlap is 1,980 sq ft = 2 synthetic rolls (1,000 sq ft each). Ice & water shield requires 200 linear ft of 36" membrane (100 ft × 2 courses) = 3 rolls (66 ft per roll).
Why is synthetic underlayment better than traditional #30 asphalt felt?
Synthetic underlayment is 10x stronger, tear-resistant under high foot traffic, 100% water-impervious, UV-resistant for up to 6 months of exposure, and significantly lighter to carry onto the roof.