What Felt Paper Does: The Critical Role of Roofing Underlayment

Felt paper — also known as roofing felt or tar paper — is one of the most humble yet essential components of a roofing system. Installed between the roof sheathing and the shingles, this seemingly simple layer performs multiple critical functions that protect the entire building envelope. Understanding what felt paper does, how it works, and when to use different types can mean the difference between a roof that lasts 30 years and one that fails prematurely.The most obvious role of felt paper is to serve as a secondary weather barrier. While asphalt shingles are the primary defense against rain and snow, wind-driven rain can penetrate beneath shingles, especially at the eaves, rakes, and valleys. Felt paper catches this moisture and directs it harmlessly down the roof slope and off the building. In the event of shingle damage fro

Perhaps the most important function that is less widely understood is the role felt paper plays as an expansion joint. Wood roof sheathing (plywood or OSB) and asphalt shingles expand and contract at significantly different rates as temperature changes. On a sunny day, dark-colored shingles can reach surface temperatures of 160°F or higher, while the underlying sheathing remain

This differential movement creates stress at the interface between shingles and sheathing. Felt paper provides a slip layer that allows these two materials to move independently. Without this separation, the differential expansion can cause shingles to buckle, crack, or lose their sealant strips, dramatically shortening roof life.

creates stress at the interface between shingles and sheathing. Felt paper provides a slip layer that allows these two materials to move independently. Without this separation, the differential expansion can cause shingles to buckle, crack, or lose their sealant strips, dramatically shortening roof life.

Moisture Management During Construction

Felt paper is installed early in the roofing process, often before the final shingles are applied. This provides immediate protection for the roof sheathing and the interior of the building during construction. Even in completed roofs, felt continues to manage moisture by preventing condensation from the underside of hot shingles from wetting the sheathing. This is particularly important in climates with significant temperature swings.

Types of Roofing Felt

Organic Felt (Traditional #15 and #30)

Traditional roofing felt is made from cellulose fibers (usually recycled wood or paper) saturated with asphalt. It comes in two standard weights: #15 (about 15 pounds per square — a “square” covers 100 square feet) and #30 (about 30 pounds per square). Despite the names, actual weights vary: #15 felt typically weighs about 7-8 pounds per square when dry, while #30 weighs about 12-15 pounds.

#15 felt is the standard underlayment for most asphalt shingle roofs. It provides adequate protection for the typical 20-30 year roof life. #30 felt is heavier, more tear-resistant, and provides better protection. It is recommended for roofs with lower slopes (2:12 to 4:12), for tile and slate roofs, and for high-wind areas.

Synthetic Underlayment

In recent decades, synthetic underlayments made from woven polypropylene or polyethylene have gained popularity. These materials are lighter, stronger, and more resistant to UV degradation than traditional felt. They also provide superior slip resistance for roofers walking on the roof during installation.

Felt Paper Under Different Roofing Materials

Under Wood Shakes and Shingles

For wood shake roofs, felt paper serves an especially critical role. Wood shakes — particularly cedar — are not fully watertight even when properly installed. The gaps between shakes allow significant water penetration during wind-driven rain. The felt paper beneath is effectively the primary water barrier. Many building codes require two layers of #30 felt under wood shakes for this reason.

Under Tile and Slate

Heavy roofing materials like concrete tile, clay tile, and natural slate also rely heavily on felt underlayment. These materials are installed with open gaps at the overlaps, and wind-driven rain can easily penetrate. Additionally, because tile and slate are expected to last 50-100 years, the underlayment may need to be replaced during the roof’s lifetime without replacing the tiles themselves — a difficult but possible operation.

Under Asphalt Shingles

Standard asphalt shingle installations typically use one layer of #15 felt, applied horizontally with 2-inch to 6-inch overlaps depending on roof slope. For low-slope applications (2:12 to 4:12), two layers or #30 felt is required. Many manufacturers now require synthetic underlayment to maintain warranty coverage.

Installation Best Practices

• Start at the eaves: Apply felt horizontally, starting at the bottom edge and working upward, so that upper courses overlap lower courses (like shingles).
• Proper overlap: Use 2-inch overlaps for roof slopes of 4:12 or steeper; 6-inch overlaps for slopes between 2:12 and 4:12.
• Drip edge first: Install drip edge at eaves before the felt, and at rakes after the felt, so water flows properly.
• Fastening: Use plastic cap nails or staples with plastic caps, spaced 12 inches on center along overlaps and 24 inches through the field.
• Valleys: Install an extra layer of felt (or a self-adhering membrane) in valleys, extending 18 inches on each side of the valley centerline.
• Penetrations: Properly flash all vent pipes, chimneys, and skylights, with felt applied over the flashing for a layered water-shedding sequence.

Common Felt Paper Mistakes

One of the most common errors is using felt that has been exposed to the elements too long. Organic felt is only rated for about 30 days of UV exposure. After that, it becomes brittle and loses much of its waterproofing capability. Synthetic underlayments typically last 6 months or more, but should still be shingled as soon as practical.

Another frequent mistake is inadequate fastening. Felt that flaps in the wind can tear, and wind uplift on exposed felt can actually lift shingles installed on top of it. Using the correct number of fasteners and proper overlap is essential.

For more roofing knowledge, see our comprehensive guide on asphalt shingles and cool roof systems.

Conclusion

Felt paper may be out of sight once the roof is finished, but it performs vital functions that directly affect roof longevity and building durability. As a secondary weather barrier, expansion joint, moisture management layer, and construction-phase protector, quality underlayment is an investment that pays dividends throughout the life of a roof. Whether you choose traditional organic felt or modern synthetic, proper installation is essential. The few extra dollars spent on heavier felt or premium synthetic underlayment are among the best investments you can make in your roofing system.

Historical Development of Roofing Felt

Roofing felt has been used in North America since the late 19th century, when builders recognized that a layer of tar-saturated fabric beneath wooden shingles dramatically improved weather resistance. The earliest felts were made from rag fibers (cotton and wool waste) saturated with coal tar. By the 1920s, the asphalt industry had developed the modern organic felt — cellulose fibers saturated with asphalt — which remains the most common type today.

The introduction of asphalt shingles in the early 20th century created even greater demand for felt underlayment. Early asphalt shingles were less durable than modern ones, and the felt beneath them was often the primary water barrier. As shingle technology improved, the role of felt evolved from primary barrier to secondary defense and expansion joint. Today, building codes in most jurisdictions require felt underlayment under all steep-slope roofing materials, recognizing its critical protective function.

Regional Code Requirements for Felt Paper

Building code requirements for roofing felt vary by region based on climate conditions. The International Residential Code (IRC) provides baseline requirements, but many states and local jurisdictions have adopted more stringent standards:

Ice Dam Protection and Felt Paper

In cold climates, ice dams at the eaves are a primary cause of roof leaks. When snow on the roof melts from heat escaping through the attic, water runs down to the cold eaves and refreezes, forming a dam. Water backs up behind this dam and can penetrate under the shingles. Standard felt paper is not designed to handle standing water. For this reason, building codes in snow regions require a self-adhering polymer-modified bitumen membrane (often called “ice and water shield”) at the eaves, extending at least 24 inches past the interior wall line. This membrane provides a watertight seal around nail penetrations that standard felt cannot match.

The self-adhering membrane is installed directly on the roof sheathing before the felt paper. It extends up the roof slope from the eaves to a point at least 2 feet past the interior wall line — typically 4 to 6 feet total from the eaves edge. In valleys, around chimneys, and at other critical penetrations, this membrane is often used in place of felt for enhanced protection.

Felt Paper and Roof Ventilation

Proper roof ventilation depends on air movement through the attic space, from soffit vents to ridge vents. Felt paper plays a role here as well: it must be installed in a way that does not block these ventilation pathways. At the ridge, felt from each side of the roof should stop several inches below the ridge opening to allow air to exit. At soffit vents, the felt should not extend into the vent opening, and vent baffles should be installed to maintain a clear air channel from the soffit to the attic interior.

Some synthetic underlayments are designed to be vapor-permeable (“breathable”), allowing water vapor to pass through from the sheathing to the attic space. This can be beneficial in certain climate zones where wintertime condensation is a concern. Traditional organic felt is also somewhat breathable, though less consistently so than premium synthetics.

Compatibility with Different Roof Systems

Not all felt products work well with all roofing systems. Some synthetic underlayments can react with asphalt shingles, causing staining or adhesive failure. The Asphalt Roofing Manufacturers Association (ARMA) recommends verifying compatibility between underlayment and shingle products, especially when using synthetic materials. For metal roofs, a slip sheet (typically #30 felt) is often required between the metal panels and the roof deck to prevent galvanic corrosion and to reduce noise from thermal expansion.

For tile roofs, the heavy weight of the roofing material requires underlayment with high tear strength. Most tile roof manufacturers specify #30 felt or a high-performance synthetic. The additional weight and rigidity of these underlayments prevents them from tearing under the concentrated loads of tile installation and foot traffic.

Felt Paper and Energy Efficiency

While felt paper’s primary role is moisture management, it also contributes to the thermal performance of the roof assembly. The air gap created between felt and shingles provides a small insulating value. More significantly, by keeping the roof sheathing dry, felt paper preserves the thermal performance of any insulation installed between the rafters below. Wet insulation loses much of its R-value — fiberglass batts can lose up to 50% of their insulating ability when wet — so keeping the sheathing dry helps maintain the overall thermal envelope.

For more roofing knowledge, see our comprehensive guide on asphalt shingles and cool roof systems.