Roof Underlayment Calculator

Felt Underlayment Types and Applications

Asphalt-saturated felt has protected roof decks for over a century. The base material is recycled paper or fiberglass mat saturated with asphalt for water resistance. 15-pound felt (now labeled #15 or ASTM D226 Type I) weighs approximately 8-12 pounds per square in modern formulations, lighter than historical products but still meeting code minimums for steep-slope roofs.

30-pound felt (ASTM D226 Type II) provides double the thickness and durability. Roofers use it on low-slope applications (2:12 to 4:12 pitch) where water sheds slowly and underlayment faces prolonged exposure. It also serves as a better walkable surface during installation, reducing slips and tears compared to thin felt. The extra cost—roughly $15-20 per roll versus $8-12 for 15-pound—matters less when labor safety and material durability are priorities.

Felt installation requires care to prevent wrinkles and tears. Roll it out horizontally starting at the eave, overlapping each course 6 inches over the previous one. Staple or nail every 12 inches along seams and every 24 inches in the field. Avoid installing felt in wet conditions—it absorbs water, swells, and wrinkles permanently. Once shingles cover it, those wrinkles telegraph through as visible lines.

Synthetic Underlayment Advantages and Characteristics

Synthetic underlayment uses woven or spun polyethylene or polypropylene fibers laminated with asphalt or polymer coatings. This creates a lightweight, tear-resistant membrane that doesn't absorb water. A typical synthetic roll weighs 25-30 pounds but covers 1000 square feet, compared to a 60-pound felt roll covering 400 square feet. Labor savings from lighter rolls offset some of the material cost premium.

UV resistance varies widely among synthetic products. Entry-level synthetics withstand 30-90 days of sun exposure before degrading. Premium formulations last 6-12 months, critical for projects with delayed shingle installation or phased construction. Check product data sheets for rated exposure time—guessing wrong leaves the deck vulnerable to leaks and requires costly underlayment replacement.

Installation technique differs slightly from felt. Synthetic membranes lie flatter without wrinkling, but they're slippery when wet. Many products include printed walking strips or textured surfaces to improve traction. Use plastic cap nails or staples with wide crowns to prevent tearing through the material. Some codes require specific fastening patterns—verify local requirements before starting.

Coverage Calculation and Waste Management

Standard felt rolls cover 2 or 4 roofing squares (200 or 400 sq ft) depending on weight and manufacturer. Synthetic rolls typically cover 10 squares (1000 sq ft). Divide your total roof area by the roll coverage, then add 10% for overlap and another 10% for waste. A 2400-square-foot roof needs 2400 ÷ 1000 = 2.4 base rolls, × 1.2 for overlap and waste = 2.88, round up to 3 rolls of synthetic.

Waste comes from cutting around roof features, trimming at ridges and hips, and discarding damaged sections. Simple gable roofs waste 5-10%. Complex roofs with dormers, valleys, and irregular shapes waste 15-20%. Synthetic underlayment produces less waste than felt because tears and damage occur less frequently, but budget conservatively—running short mid-job costs more in delivery fees than ordering an extra roll upfront.

Leftover underlayment stores poorly. Felt absorbs moisture and deteriorates within a year even when kept dry. Synthetic lasts longer but degrades in UV if stored outdoors. Use partial rolls on small projects like shed roofs or doghouses rather than letting them sit unused. Most suppliers don't accept returns on opened rolls, so calculate carefully to minimize excess.