Synthetic underlayment usually covers 10 squares per roll, 15 lb felt covers 4 squares and 30 lb felt covers 2. A 2,236 square foot roof is 22.4 squares, so about 3 synthetic rolls or 6 rolls of 15 lb felt. Add roughly 10% for the laps.
The Underlayment Calculator returns the number of rolls of 15-lb felt, 30-lb felt, or synthetic roofing underlayment needed to cover your roof. A standard 4 sq roll of 15-lb felt covers about 400 sq ft after 2-inch overlaps; synthetic rolls cover 1,000 sq ft. Enter your roof area and select a material and the tool calculates roll count plus 10% waste. Underlayment goes on top of plywood and beneath shingles to provide secondary moisture protection — see our installation guide for fastening and overlap details.
Underlayment is sold by the roll and rated in squares. Synthetic rolls typically cover 10 squares, 15 lb felt covers 4 squares and 30 lb felt covers 2, before overlap is deducted.
| Roof area | Squares | Synthetic (10 sq) | 15 lb felt (4 sq) | 30 lb felt (2 sq) |
|---|---|---|---|---|
| 1,000 sq ft footprint | 11.2 | 2 | 3 | 6 |
| 1,500 sq ft footprint | 16.8 | 2 | 5 | 9 |
| 2,000 sq ft footprint | 22.4 | 3 | 6 | 12 |
| 2,500 sq ft footprint | 28 | 3 | 7 | 14 |
| 3,000 sq ft footprint | 33.5 | 4 | 9 | 17 |
Areas are the sloped area at 6/12 pitch. Add roughly 10% for the horizontal laps and more if your roof has many penetrations.
Roll coverage is stated before overlap. A 10-square synthetic roll covers 10 squares laid edge to edge, but you lose the horizontal lap on every course and 6 inches at every end joint, so plan on roughly 10% less usable coverage than the label figure.
Ice barrier is sized differently: code generally requires it to extend from the eave edge to at least 24 inches inside the exterior wall line, measured along the slope. On a low-pitch roof with a deep overhang that can mean two courses of 36-inch barrier at the eave rather than one.
Synthetic underlayment usually covers 10 squares per roll, 15 lb felt covers 4 and 30 lb felt covers 2. A 2,236 square foot roof is 22.4 squares, so about 3 synthetic rolls or 6 rolls of 15 lb felt.
It is lighter, tears far less, walks better when wet and can stay exposed for weeks rather than days. On any job that will not be dried in the same day it pays for itself.
Typically 2 to 4 inches horizontally and 6 inches at the end laps, but follow the lines printed on the roll — manufacturers differ and the warranty follows their figure.
Synthetic underlayment is generally rated for weeks of UV exposure and holds up to rain and foot traffic. Felt is rated in days and wrinkles once it gets wet, which then telegraphs through the finished shingles. Check the exposure limit printed on the roll before you leave a roof dried-in.