Roof Underlayment Calculator

Underlayment is the layer that goes down before the shingles — a moisture barrier over the deck. It comes in rolls, and sizing the job is a matter of finding the true roof area and dividing by how much one roll covers.

As with shingles, the roof area is the flat footprint times the slope factor for your pitch, because the roll has to cover the tilted surface, not the shadow it casts. A `6/12` roof multiplies the footprint by about `1.118`.

Coverage depends on the product. A roll of synthetic underlayment typically covers about `1,000` square feet — roughly `10` squares — which is why one roll often blankets a small roof. Traditional No. `15` asphalt felt covers about `400` square feet per roll, and heavier No. `30` felt about `200`. Those are net figures after the printed head and side laps, so the paper you actually lay is a little less than the roll's raw dimensions. The calculator adds a small waste allowance for the overlaps at ridges, valleys, and roll ends, then rounds up to whole rolls.

Synthetic has largely replaced felt on new work because it is lighter, tears less, and covers more than twice the area per roll, but felt is cheaper per square and still common. Pick the product that matches what your supplier stocks; the calculator handles the coverage math either way.