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For most roofs, yes: one 10-square synthetic roll covers what 2½ rolls of #15 or 5 of #30 felt cover, it resists tearing, lies flat when wet and can stay uncovered for the time its maker rates. Felt is cheaper per roll and fine when shingles go on the same day. A 2,000 sq ft roof needs 3 synthetic rolls, 6 of #15 or 11 of #30.
Synthetic underlayment is the better choice for most roofs and felt is the cheaper one per roll. Synthetic is a woven or spun polypropylene or polyethylene sheet sold in rolls of commonly 10 squares (1,000 sq ft); felt is an asphalt-saturated mat sold as #15 (ASTM D226 Type I, about 4 squares per roll) and #30 (Type II, about 2 squares). Synthetic is lighter per square, resists tearing in wind and under foot, lies flat when wet and is rated by its maker to stay uncovered for weeks or months; felt is familiar to every inspector and fine when the shingles go on the same day. A 2,000 sq ft roof plus 10% for laps takes 3 rolls of synthetic, 6 rolls of #15 or 11 rolls of #30. Below 4:12 the IRC (R905.1.1) wants two layers under shingles, and ice and water shield still goes at the eaves and in the valleys.
Short answer: synthetic underlayment is the better choice for most roofs — bigger, lighter rolls, stronger in wind and under foot, flat when wet and rated to stay uncovered for weeks or months. Felt (#15 or #30) is cheaper per roll and fine when the shingles go on the same day. Compare the price per square, because a 10-square synthetic roll covers 2½ rolls of #15 or 5 of #30.
| Synthetic | Felt (#15 / #30) | |
|---|---|---|
| What it is | Woven or spun polypropylene or polyethylene sheet | Organic or fibreglass mat saturated with asphalt |
| Standard on the label | ASTM D8257, or an ICC-ES evaluation report under AC188 | ASTM D226 (Type I = #15, Type II = #30) or ASTM D4869 |
| Coverage per roll | Commonly 10 squares (1,000 sq ft) | #15: about 4 squares (400 sq ft); #30: about 2 squares (200 sq ft) |
| Rolls for a 2,000 sq ft roof + 10% | 3 | #15: 6 · #30: 11 |
| Fasteners | Plastic-cap nails or cap staples, at the maker's pattern | Nails or staples; caps in high-wind areas |
| Left uncovered | Exposure rated by the maker, from weeks to months | Cover with shingles quickly; it wrinkles and tears |
| When it gets wet | Sheds water and lies flat | Absorbs water and can wrinkle, telegraphing through shingles |
| Underfoot | Varies: check the walking-surface description, some are slick when wet or dusty | Grippy when dry, can tear under foot traffic |
| Vapour | Most are close to vapour-tight; a few are sold as breathable | Lets more vapour through |
| Best for | Most re-roofs and new roofs, metal roofs, jobs that stay open for days | Tight budgets, small jobs covered the same day, warranties that name felt |
| Roof area | Synthetic (10-square roll) | #15 felt (ASTM D226 Type I) | #30 felt (ASTM D226 Type II) |
|---|---|---|---|
| 1,000 sq ft | 2 | 3 | 6 |
| 1,500 sq ft | 2 | 5 | 9 |
| 2,000 sq ft | 3 | 6 | 11 |
| 2,500 sq ft | 3 | 7 | 14 |
| 3,000 sq ft | 4 | 9 | 17 |
For most roofs, yes: a 10-square synthetic roll covers what two and a half rolls of #15 or five of #30 cover, it resists tearing in wind and under foot, it lies flat when it gets wet, and makers rate it to sit uncovered for weeks or months. Felt is cheaper per roll, familiar to every inspector and fine when the shingles go on the same day.
Most synthetic underlayment is sold in 10-square rolls, 1,000 sq ft of net coverage, usually about 4 ft wide. Some brands sell smaller 2- to 5-square rolls; divide your roof area plus 10% for laps by the coverage printed on the roll you buy.
A roll of #15 felt covers about 4 squares (400 sq ft) and #30 about 2 squares (200 sq ft) after laps. Add 10% to your roof area and divide: a 2,000 sq ft roof takes 2,200 ÷ 400 = 6 rolls of #15 or 2,200 ÷ 200 = 11 rolls of #30.
Yes. The IRC requires underlayment under asphalt shingles (R905.1.1) and accepts felt to ASTM D226 or D4869; synthetic products are approved through ASTM D8257 or an ICC-ES evaluation report. Below 4:12, down to the 2:12 minimum for shingles, the IRC wants two layers of underlayment.
Ice and water shield is a self-adhered membrane for the eaves, valleys and around chimneys and skylights, and where your code requires an ice barrier. Synthetic or felt goes on the rest of the field. Many roofs use all of it: ice and water shield at the eaves and valleys, synthetic everywhere else.
Comparing brands? See the best roof underlayment guide.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.