Roof Underlayment Calculator

Underlayment is the second waterproof layer between your roof deck and your finish roofing (shingles, tile, metal). It catches the water that always gets past the finish layer through wind-driven rain, capillary action, and aged flashing. Most building codes — including the International Residential Code (IRC) — require at least one layer under any roof covering, and many regions require two layers under tile or low-slope shingles.

Synthetic underlayment costs more per roll but covers 5 times more area than 30 lb felt (1,000 vs 200 sq ft), weighs less, lasts longer in UV exposure, and does not absorb water. It is the modern default for shingle and metal roofs. 30 lb felt is still preferred under tile in many jurisdictions because the IRC and many manufacturers specifically call for it, and because it bonds well to the mortar bedding of tile installs.

Standard practice is 2 inches of sidelap and 4 inches of headlap (the lap between courses). On low-slope roofs (2/12 to 4/12) the IRC requires double those laps, so 19 inches of headlap on a 36-inch-wide roll. Manufacturer specs override this — high-wind synthetics often call for 6 to 12 inch headlap regardless of slope. Roll coverage figures sold at retailers already account for the standard 2"/4" lap.

If you live in a region with a 5°F or colder average January temperature, IRC R905.1.2 requires ice barrier (peel-and-stick) at the eaves from the edge up to 24 inches past the warm wall — typically 3 to 6 feet up the slope. That is most of the northern half of the US, including New England, the Midwest, and the mountain states. Many roofers also apply ice and water in valleys, around chimneys and skylights, and on low-slope sections regardless of climate.

A 2,000 sq ft simple gable roof at 6/12 pitch has an actual roof area of about 2,236 sq ft. With a 5 percent waste factor that is 2,348 sq ft. At 1,000 sq ft of net coverage per roll, you need 3 rolls of synthetic underlayment for a single-layer install. Add a couple of rolls of ice and water shield if you are in a cold climate.

15 lb and 30 lb felt have a 12 to 20 year service life inside a roof system. Synthetic underlayment is rated for 30 to 50 years and typically outlasts the shingles above it. Peel-and-stick ice and water shield lasts 30+ years. The catch: any underlayment exposed to UV (a roof that sits stripped for more than a week or two) ages much faster. Synthetics typically tolerate 30 to 180 days of UV exposure depending on the brand.

No. Every major shingle manufacturer voids the warranty if you install over bare sheathing, and building codes uniformly require at least one underlayment layer. The deck wood is not waterproof, and small amounts of water under the shingles will rot the OSB or plywood in a few seasons.

Underlayment is fastened with 1-inch plastic-cap roofing nails or staples. Felt requires about 8 cap nails per square (100 sq ft), and synthetic underlayment typically requires 16 per square in a pattern set by the manufacturer. High-wind zones (Florida, coastal Carolinas, Gulf Coast) often require 32 per square. Peel-and-stick is self-adhered and needs no fasteners on the field, only at the perimeter.