Asphalt Roofing Felt (Tar Paper): 15 lb vs 30 lb, Uses and Synthetic Alternatives

Asphalt roofing felt, often called tar paper or felt paper, is a heavy paper or fiber mat saturated with asphalt. It's rolled out over the roof deck under shingles as an underlayment, a secondary layer that sheds water that gets past the shingles and protects the deck during installation. It comes mainly in two weights, commonly called #15 and #30; #30 is thicker, tougher and more tear-resistant. Felt is still widely used and inexpensive, but synthetic underlayments have taken much of the market because they're lighter, stronger and less likely to wrinkle. Always check the shingle manufacturer's requirements and local code before choosing.

What an asphalt roof is →

What Roofing Felt Is Made Of

Traditional roofing felt started as a mat of organic fibers, such as recycled paper or rags, soaked in asphalt. Today's products are made from cellulose, fiberglass or blends, saturated or coated with asphalt to repel water while still allowing some vapor to escape. The asphalt is the same basic material used in shingles, which is why felt and shingles work well together. Our explainer on what an asphalt roof is covers how the pieces of a shingle roof fit together.

#15 vs #30 Felt

Feature	#15 felt	#30 felt
Thickness	Thinner	Roughly twice as thick
Tear resistance	Tears more easily when walked on	Stands up better to foot traffic and wind
Weight per roll	Lighter, easier to carry	Heavier
Typical uses	Underlayment on moderate slopes, under siding	Steeper roofs, longer exposure before shingling, under tile or slate in some systems
Cost	Lower	Higher

The names are historical. Decades ago, #15 felt weighed about 15 pounds per roofing square (100 square feet), and #30 weighed about 30. Modern versions usually weigh less than those numbers suggest. ASTM D226 is a common standard, with Type I roughly corresponding to #15 and Type II to #30; ASTM D4869 covers another class of felt. If a spec calls for a standard, check the roll label rather than relying on the nickname.

Where Felt Is Used

• Roof underlayment under asphalt shingles, one of its most common uses.
• Under siding and stucco as a water-resistive barrier in some wall assemblies, depending on code.
• Under some tile and slate roofs, usually in heavier weights or as part of a specific system.
• Temporary dry-in on roofs before shingles go on, for short periods.
• Old-school flooring underlayment, still used by some installers under hardwood.

Felt vs Synthetic Underlayment

Factor	Asphalt felt	Synthetic underlayment
Material	Asphalt-saturated paper or fiberglass	Woven or spun polypropylene or polyethylene
Strength	Can tear, especially #15	Very tear-resistant
Weight and roll size	Heavier per square covered	Lighter, larger rolls, fewer seams
UV exposure	Should be covered fairly quickly	Many are rated for longer exposure; check the label
Wrinkling	Can wrinkle if it gets wet	Stays flat
Walkability	Can be slick when wet or dusty	Many have textured, slip-resistant surfaces
Cost	Usually lower material cost	Usually higher, sometimes offset by faster install

Both can be code-compliant when installed to the manufacturer's instructions. Some shingle warranties specify certain underlayments, so ask your roofer what the shingle maker requires.

Ice Barrier Is a Separate Layer

In cold climates, codes often require a self-adhering ice barrier membrane along the eaves, and it's commonly used in valleys and around penetrations. That's a different product, usually a rubberized or modified-bitumen sheet. Felt or synthetic underlayment covers the rest of the deck. Our guide to modified bitumen roofing explains the rubbery asphalt membranes used on low-slope roofs, which are related to many ice barrier products.

Installation Basics

1. Start at the eave and roll felt horizontally across the roof, working upward.
2. Overlap each course by the amount the manufacturer and code require, commonly marked with guide lines on the roll.
3. Overlap end joints and stagger them between courses.
4. Fasten with cap nails or staples as specified; plastic cap nails hold better in wind.
5. Shingle soon. Felt isn't meant for long exposure; wind and sun degrade it.

Roofing is dangerous work. Use fall protection and a roof safety harness kit, and consider hiring a pro for steep or high roofs. A roofing cap nailer speeds up installation, and a hook-blade utility knife cuts felt cleanly.

Common Felt Problems

• Wrinkles and ripples: felt that gets rained on before shingling can absorb moisture and buckle, and those ridges may telegraph through thin shingles. Many roofers let it dry and flatten or cut and patch wrinkles before shingling.
• Wind tears: lightly fastened felt left exposed on a windy day can rip loose, especially #15.
• Slippery footing: felt can be slick in dew, frost or dust, a real hazard on steeper pitches.
• Sun damage: long exposure dries out the asphalt and makes felt brittle, which is why it should be covered promptly.
• Nail tears at the edges: fastening too close to the edge or with bare staples can let the sheet pull free; cap fasteners spread the load.

How Much Felt Do You Need?

Roof area is measured in squares (100 square feet each). Measure each roof plane, add them up, then add extra for overlaps and waste. Roll coverage is printed on the label and differs between #15, #30 and brands, so divide your roof area by the net coverage of the roll you're buying. Our roof runoff calculator also asks for roof area, so it's a handy check on your measurements.

Disposal

Old felt comes off with shingles during a tear-off. In some areas, tear-off debris including shingles can be recycled into paving; our article on asphalt shingle recycling explains what recyclers accept and why some limit felt, nails and other debris.

Bottom Line on Asphalt Roofing Felt

Asphalt roofing felt is an asphalt-saturated underlayment that sits under shingles and sheds water that gets past them. #30 is thicker and tougher than #15, and synthetic underlayments are lighter and stronger still but cost more. Choose based on the shingle maker's requirements, local code and how long the underlayment may be exposed before shingling.