Roof underlayment types
Underlayment is the water-resistant layer between your roof deck and the shingles — the backup that keeps water out if anything gets past the surface. The type you choose changes how much you order and how the roof performs.
Felt versus synthetic
Traditional underlayment is asphalt-saturated felt, sold in 15 lb and 30 lb weights. Synthetic underlayment — woven polypropylene or polyester — is lighter, stronger, tear-resistant, and covers far more per roll, and it has become the default on most new work. Synthetic also lies flatter and resists wrinkling, which gives a cleaner shingle surface, though it costs more than felt up front.
Coverage per roll
Coverage drives your roll count. A roll of 15 lb felt covers about 400 sq ft — four squares. A roll of 30 lb felt covers about 200 sq ft — two squares, because it is thicker. Synthetic rolls vary widely by brand but typically cover much more, often around 1,000 sq ft, which is why fewer rolls are needed. Always use your product's stated coverage rather than a generic figure, since synthetics differ a lot between brands.
Ice and water shield
Separate from field underlayment, a self-adhering ice-and-water membrane is required in cold climates along the eaves to protect against ice dams, and is also used in valleys and around penetrations. In the coldest regions and on low-slope sections, codes can require it more extensively. It is a different product line from field underlayment and is counted by the specific lengths it protects, not by total roof area.
Low-slope roofs need more
On asphalt-shingle roofs between 2:12 and 4:12, code requires a doubled underlayment — effectively two layers — because the shallow slope holds water longer and needs the extra backup. That doubles your underlayment count on those sections, so it matters for both the estimate and the budget. Above 4:12, a single layer is generally permitted. See roof pitch explained for the slope thresholds.
Choosing for your roof
For most steep-slope residential roofs in a temperate climate, synthetic underlayment over the field with an ice-and-water membrane at the eaves and valleys is a solid, common specification. In severe climates or on low slopes, expect to use more membrane. Estimate field underlayment by roof area divided by roll coverage, and add membrane separately for the protected edges — the underlayment calculator handles the field rolls.