By The Roofing Brief Team
Last reviewed: September 2026
Synthetic underlayment is lighter, tears less, holds up longer under sun and foot traffic, and installs faster because each roll covers far more roof. Asphalt felt costs less per square and has decades of code familiarity behind it. For most steep-slope asphalt shingle roofs, synthetic is the stronger all-around choice; felt still earns its place on budget jobs, small repairs, and short exposure windows where the extra durability is not needed.
Felt vs synthetic underlayment at a glance
| Attribute | 15 lb felt | 30 lb felt | Synthetic |
|---|---|---|---|
| Material | Asphalt-saturated paper or fiberglass mat | Same, heavier saturation | Woven polypropylene or polyethylene polymer |
| Governing ASTM standard | D226 Type I / D4869 | D226 Type II / D4869 Type IV | D8257 |
| Typical roll coverage | ~432 sq ft (about 4 squares) | ~200 to 216 sq ft (about 2 squares) | ~1,000 sq ft (about 10 squares) |
| Weight per square | ~7.5 to 12.5 lb | ~16 to 27 lb | Much lighter, often a few pounds per square |
| Approx. material cost per square | ~$8 to $10 | ~$12 to $18 | ~$17 to $25 |
| UV exposure tolerance | About 2 weeks | About 30 days | ~60 to 180 days depending on product |
| Walking traction | Fair, slick when wet | Fair | Better wet and dry on most printed surfaces |
| Tear resistance in wind | Wrinkles and tears more easily | Better than 15 lb | High |
Prices above reflect publicly listed 2026 retail figures for common products such as ASTM D226 Type I 15 lb felt and premium synthetics, and vary by brand, region, and quantity. Treat them as planning ranges, not quotes.
What roofing felt is
Roofing felt is the black, asphalt-saturated paper (also called tar paper or felt paper) that goes between the roof deck and the shingles. It works as a secondary water barrier, sheds any rain that gets past the shingles, and protects the wood deck during installation. Felt comes in two common weights. Type I 15 lb felt is the lighter, cheaper grade; Type II 30 lb felt is roughly twice as thick, lies flatter, and tolerates weather exposure longer. Both are governed by ASTM D226 or ASTM D4869. For the full breakdown of the two grades and their costs, see our guide to roofing felt and 15 lb vs 30 lb.
Felt’s weakness is water and time. Because the mat is paper or fiberglass saturated with asphalt, it can absorb moisture, wrinkle, and buckle if it gets wet before the shingles go on, and it degrades under UV within a couple of weeks. In gusty conditions the lighter grade can tear at the fasteners during installation.
What synthetic underlayment is
Synthetic underlayment is made from woven polypropylene or polyethylene polymers rather than asphalt-soaked paper. The result is a thin, strong sheet that resists tearing, sheds water rather than absorbing it, and carries a printed walking surface for traction. Most synthetics are qualified under ASTM D8257, the standard specification for mechanically attached polymeric roof underlayment used in steep-slope roofing. Owens Corning Titanium UDL30 and GAF Tiger Paw are two widely sold examples, each rated for roughly 180 days of UV exposure. For a closer look at one product line, see our Titanium UDL underlayment review.
Weight and handling
A single roll of 15 lb felt covers about 4 squares and weighs on the order of 7.5 to 12.5 lb per square. A 30 lb roll covers about 2 squares and can run 16 to 27 lb per square. A typical synthetic roll covers about 10 squares yet is light enough for one person to carry up a ladder. Fewer, larger rolls mean fewer seams, fewer trips, and faster coverage, which is a real labor difference on a full re-roof.
Water resistance
Felt provides genuine water protection, and 30 lb resists damage better than 15 lb because it is thicker. But asphalt felt can still absorb moisture, and prolonged dampness before the shingles go on may cause it to wrinkle. Synthetic sheets do not absorb water the same way, which lowers the risk of buckling if the deck is exposed to weather mid-job. Neither product is a finished roof surface; both are secondary barriers meant to sit under shingles.
Cost
Felt is the cheaper material. A 432 sq ft roll of 15 lb felt has been listed around $35 at national home-improvement retailers in 2026, which works out to roughly $8 per square. Premium synthetic rolls covering 1,000 sq ft have listed near $195, or roughly $19 to $20 per square. On a material basis, synthetic often runs about two to three times the cost of 15 lb felt per square. That gap narrows once you factor in faster installation and the longer exposure window, but felt remains the lower up-front spend.
Installation and safety
Synthetic underlayment generally offers better footing than felt in both wet and dry conditions, which matters on steep pitches. It also resists tearing at the fasteners, so it holds better if a section is exposed to wind before shingles are installed. Cap nails or plastic-cap staples are commonly specified for synthetics; always follow the manufacturer’s fastening pattern, because attachment requirements differ by product and by wind zone. If your roof is exposed for an extended dry-in period, check the product’s UV exposure limit; our guide to underlayment UV exposure limits covers how long each type can stay bare.
Code requirements (IRC R905)
The International Residential Code, section R905.1.1, requires underlayment to bear a label showing compliance with a recognized ASTM standard: D226, D1970, D4869, D6757, or D8257. Felt has long qualified under D226 and D4869. Synthetic underlayment is qualified under the newer D8257 specification; the ICC has approved code changes so that D8257 synthetics are recognized in the 2024 code cycle, and many jurisdictions already accept them through manufacturer product-approval reports. Because acceptance can vary by jurisdiction, confirm your local code before you buy.
Two code situations change the math. In high-wind regions, the IRC may require two layers of underlayment complying with ASTM D226 Type II, D4869 Type IV, or D6757. On low slopes below 4:12, a double layer of underlayment is generally required regardless of material; our guide to underlayment slope requirements explains where the double-layer rule kicks in.
Which should you choose?
Synthetic tends to be the better fit when the roof is steep, the dry-in period may run long, the crew wants safer footing, or the job is a full re-roof where labor savings offset the higher material price. Felt can be the sensible pick on a tight budget, on small repairs and patches, on short exposure windows, or where a local inspector or contractor prefers a familiar D226 product. Neither choice is wrong on its own; the right answer depends on climate, code, slope, and how long the deck will sit exposed.
Frequently asked questions
Is synthetic underlayment worth the extra cost over felt?
For most full re-roofs it often is, because synthetic covers more area per roll, installs faster, resists tearing, and tolerates longer sun exposure. On small repairs or tight budgets, felt can still be the more economical choice. The value depends on the size of the job and how long the deck stays exposed.
Does code allow synthetic underlayment?
Synthetic underlayment is qualified under ASTM D8257 and is recognized in the 2024 IRC cycle, and many jurisdictions already accept it through manufacturer product-approval reports. Because acceptance can vary locally, confirm with your building department before you buy.
How long can felt or synthetic stay exposed before shingles go on?
Asphalt felt is generally rated for short exposure, often about two weeks for 15 lb and up to about 30 days for 30 lb. Synthetic products commonly list roughly 60 to 180 days of UV exposure depending on the brand. Always follow the specific product’s stated limit.
What is the difference between 15 lb and 30 lb felt?
The 30 lb grade is roughly twice as thick, lies flatter, tears less, and tolerates weather exposure longer, but it covers about half the area per roll and costs more. The 15 lb grade is lighter and cheaper. Some high-wind areas require the heavier grade.
Can you install synthetic underlayment over old felt?
Underlayment is normally installed on a clean, dry deck after the old roof is torn off, not layered over existing felt. Fastening and preparation should follow the manufacturer’s instructions and local code. When in doubt, ask your roofer or inspector.