Roofing Underlayment Calculator

Roofing underlayment is a water-resistant or waterproof barrier material installed directly on the roof deck before shingles or other roofing materials. It provides an additional layer of protection against water infiltration, ice dams, and wind-driven rain.

To calculate underlayment needed, multiply your roof's length by width to get base area, then multiply by the pitch factor to account for slope. Add 10-15% for waste, overlaps, and cuts. Divide the total area by the coverage per roll to determine the number of rolls needed.

A typical waste factor for roofing underlayment is 10-15%. This accounts for overlaps (typically 2-6 inches), cuts around vents and chimneys, starter strips, and general waste. More complex roofs with multiple valleys, dormers, or hips may require up to 20% waste factor.

Roof pitch significantly affects the actual surface area of your roof. A steeper pitch means more surface area than the base footprint. For example, a 6/12 pitch increases the area by about 12%, while a 12/12 pitch increases it by 42%. The calculator applies the appropriate pitch multiplier to ensure accurate material estimates.

Standard underlayment rolls typically cover 400 square feet (4 squares), though sizes vary by manufacturer and product type. Synthetic underlayment often comes in 1000 sq ft rolls, while traditional felt comes in 400 sq ft (15 lb felt) or 200 sq ft (30 lb felt) rolls.

Synthetic underlayment is more durable, lighter, less prone to tearing, and provides better water resistance than traditional felt. However, it costs more. Felt (15 lb or 30 lb) is more economical and has been used for decades. The choice depends on your budget, climate, and roofing material requirements.