Roofing felt is the black, asphalt-saturated paper that goes between your roof deck and the shingles. Roofers also call it felt paper or tar paper. It is the traditional underlayment under asphalt shingles: a secondary water barrier that sheds any rain getting past the shingles, protects the wood deck during installation, and helps the roof meet fire and building code rules. Felt is sold mainly in two weights, 15 pound and 30 pound, and in 2026 it competes with lighter synthetic underlayment that now outsells it on most new roofs.
What is roofing felt?
Roofing felt is a base mat of organic fibers (cellulose and recycled paper) or fiberglass, saturated with asphalt so it repels water. It ships in rolls 36 inches wide, unrolls across the roof deck, and is fastened down before the shingles go on. The asphalt saturation is what separates felt from plain building paper: it sheds liquid water while still letting a little vapor pass.
Two mat types exist. Organic felt uses a paper or cellulose base and is the classic “tar paper” most people picture. Fiberglass-reinforced felt uses a glass-fiber mat that resists tearing and wrinkling better in humidity. Both are made to ASTM D226, the standard spec for asphalt-saturated organic felt used in roofing.
The “tar paper” nickname is a holdover. Modern felt is saturated with asphalt, not coal tar, but the name persists on job sites and in supply houses. When a roofer says tar paper, felt paper, or underlayment felt, they mean the same product.
What does roofing felt do?
Roofing felt has four jobs on a shingle roof, and all of them matter more than its low cost suggests. It is a backup barrier, not the primary waterproofing, but a roof deck left bare between tear-off and new shingles is exposed to weather and to installation damage.
- Secondary water barrier. Wind-driven rain, ice-dam backup, and leaks around nails can drive water past shingles. Felt sheds that water off the deck instead of into the wood.
- Deck protection during the job. Once the old roof is off, felt weatherproofs the deck for the days before shingles go down, and gives crews a cleaner, less slippery surface to walk.
- Separation layer. Felt keeps shingles from bonding directly to the deck and reduces “telegraphing,” where deck seams and imperfections show through the finished shingles.
- Fire rating support. Underlayment is part of the tested assembly that lets asphalt shingles carry a Class A fire rating, the highest rating under ASTM E108 and UL 790.
15 lb vs 30 lb roofing felt
The numbers describe the historic weight of enough felt to cover 100 square feet, which roofers call one square. Modern felt often weighs less than its label, but the “15” and “30” names stuck. The practical split: 30 pound felt is thicker, tears less, lies flatter, and survives weather exposure longer, at roughly 40 to 60 percent more cost per square than 15 pound.
| Property | 15 lb felt (No. 15) | 30 lb felt (No. 30) |
|---|---|---|
| Roll coverage | About 400 sq ft (4 squares) after laps | About 200 sq ft (2 squares) after laps |
| Relative thickness | Thinner, more flexible | Thicker, stiffer, flatter |
| Tear and wrinkle resistance | Lower, wrinkles when it gets wet | Higher, holds up under foot traffic |
| Weather exposure before cover | Often up to about 2 weeks | Often up to about 30 days per manufacturer guidance |
| Typical use | Standard slopes, budget jobs, repairs | Steep slopes, high wind, longer exposure windows |
| Material cost per square (2026) | About $5 to $9 | About $13 to $22 |
On slopes of 6:12 and steeper, many roofers default to 30 pound because it resists tearing when crews walk it and stays put better on the pitch. On a straightforward shingle replacement in dry weather, 15 pound often meets code and the manufacturer specification at lower cost.
Roofing felt standards and building code
Underlayment is not optional under asphalt shingles: the International Residential Code requires it. Section IRC R905.1.1 requires underlayment for steep-slope coverings, and R905.2.7 sets how it is applied under asphalt shingles. Felt qualifies when it meets ASTM D226 Type I or Type II, or ASTM D4869. The catch most homeowners miss is the low-slope rule.
| Spec or rule | What it covers | Why it matters |
|---|---|---|
| ASTM D226 Type I | No. 15 asphalt-saturated organic felt | The classic 15 pound felt referenced by code |
| ASTM D226 Type II | No. 30 asphalt-saturated organic felt | Heavier, tougher grade for steep or exposed work |
| ASTM D4869 | Lighter asphalt-saturated felt underlayment | A “#15” sold to D4869 can weigh less than one to D226, so specs are not identical |
| IRC R905.2.7, slope 2:12 to 4:12 | Two layers of underlayment, applied shingle-fashion | Low-slope shingle roofs require double coverage, not one pass |
| IRC R905.2.7, slope 4:12 and steeper | One layer, side laps 2 inches, end laps 4 inches | The single-layer method used on most homes |
Local amendments can add requirements, such as a self-adhering ice barrier at the eaves in cold climates. That ice-and-water membrane is a separate product to ASTM D1970 and is not the same as felt. See our guide to ice and water shield and when code requires it for where that layer goes.
How much does roofing felt cost?
Felt is one of the cheapest parts of a roof. As material only in many markets in 2026, a roll of 15 pound felt runs about $20 to $35 and covers 4 squares, or roughly $5 to $9 per square. A roll of 30 pound runs about $25 to $45 and covers 2 squares, or roughly $13 to $22 per square. Installed as part of a reroof, underlayment adds a small labor line, often $0.10 to $0.30 per square foot.
Because felt is a minor line item, upgrading a whole roof from 15 pound to 30 pound usually adds only about $50 to $150 in material on an average house. Asphalt-based roofing products track petroleum and freight prices, which have moved sharply in recent years. The Roofing Brief tracks those swings in our roofing material price and inflation index, built from BLS producer price data.
Roofing felt vs synthetic underlayment
Synthetic underlayment is a woven or spunbond polymer sheet that has taken most of the new-roof market from felt. It is lighter, tears far less, resists UV for months, and one roll covers up to 10 squares, which speeds installation. Felt still wins on upfront material price and on short, simple jobs. The honest split:
| Factor | Roofing felt | Synthetic underlayment |
|---|---|---|
| Material cost per square | Lower ($5 to $22) | Higher (often $9 to $18) |
| Weight and roll coverage | Heavy, 2 to 4 squares per roll | Light, up to 10 squares per roll |
| UV and exposure tolerance | Weeks | Often up to 90 to 180 days |
| Tear resistance and walkability | Lower, wrinkles when wet | Higher, stays flat |
| Best fit | Repairs, budget jobs, code minimum, historic matching | Full reroofs, steep or high-wind roofs, long exposure |
For the full breakdown, see felt vs synthetic roof underlayment, and for product picks see our test of the best synthetic underlayment brands. Felt still has a place, but on a new roof most contractors now spec synthetic.
How roofing felt is installed
Felt goes on from the bottom of the roof upward, so every course overlaps the one below and water always runs onto the top surface, never behind it. The sequence on a standard 4:12-or-steeper shingle roof:
- Confirm the deck is dry and sound. Never lay felt over wet sheathing, which can trap moisture and grow mold under the roof.
- Install drip edge at the eave first, then run the first felt course along the eave, aligned straight.
- Work up the roof, lapping each course over the one below by at least 2 inches on horizontal laps and 4 inches on vertical end laps.
- Lap felt 6 inches over ridges and into valleys, and cut in around pipes, vents, and chimneys.
- Fasten with roofing nails or cap nails. Cap nails, or plastic wind strips in high-wind areas, hold better than staples and resist tearing.
- Cover with shingles promptly. Do not leave felt exposed past its rated window, since UV and wind degrade it quickly.
Is roofing felt still worth using?
Roofing felt is still a valid, code-compliant underlayment, and it can be the better call on the right job. It wins on small repairs, tight budgets, patch-ins that must match an older roof, and any situation where the deck will be covered within days. Its weaknesses are weight, tearing, wrinkling when wet, and a short exposure window.
For a full reroof, especially on a steep or high-wind roof, synthetic underlayment usually earns its higher price through faster installation, longer exposure tolerance, and fewer callbacks. Felt is not obsolete, but it has moved from the default to the value option. Match the product to the slope, the weather window, and the budget, and confirm the choice against your local code, which may require a specific type or a double layer on low slopes. For the full menu of options, see our guide to roof underlayment types and when each is required.
Frequently asked questions
Is roofing felt the same as tar paper? Yes. Tar paper, felt paper, and roofing felt all describe the same product: an asphalt-saturated mat used as roof underlayment. The “tar” name is historic. Today the mat is saturated with asphalt rather than coal tar, but roofers and supply houses still use the older term interchangeably.
Do you need felt under shingles? In almost all cases, yes. The International Residential Code (R905.1.1) requires underlayment beneath asphalt shingles, and most shingle manufacturers require it to keep the warranty valid. Felt is one accepted option. The alternatives are synthetic underlayment or, at eaves in cold climates, a self-adhering ice-and-water membrane.
What is the difference between 15 lb and 30 lb roofing felt? The 30 pound felt is roughly twice as thick, tears less, lies flatter, and tolerates weather exposure longer, often about 30 days versus about two weeks. It covers 2 squares per roll versus 4 for 15 pound and costs about 40 to 60 percent more per square. Steep or exposed roofs favor 30 pound.
How long can roofing felt be left exposed? Not long. 15 pound felt is usually rated for about two weeks of exposure, and 30 pound for around 30 days per manufacturer guidance, though sun, heat, and wind shorten that. Felt should be covered with shingles promptly. Synthetic underlayment, by contrast, often tolerates 90 to 180 days.
Is felt or synthetic underlayment better? Synthetic is lighter, tears less, and resists UV for months, which makes it the common pick for full reroofs and steep or high-wind roofs. Felt costs less per square upfront and is fine for repairs, budget jobs, and short exposure windows. Both meet code when installed correctly. Match the product to the job.
How much does roofing felt cost? As material only in 2026, 15 pound felt runs about $5 to $9 per square and 30 pound about $13 to $22 per square in many markets. Installed as part of a reroof, underlayment adds a small labor charge, often $0.10 to $0.30 per square foot. Upgrading a whole roof from 15 to 30 pound typically adds only $50 to $150 in material.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.