Meta description: Learn what 15-pound asphalt-saturated felt is, where it is used as roofing underlayment, how to install it, and how it compares with synthetic options.
For decades, 15-pound asphalt-saturated felt paper served as a familiar layer beneath asphalt shingles and other roofing materials. Often called “15 lb felt” or “#15 roofing felt,” it helps protect a roof deck from moisture during construction and provides a secondary water-shedding layer. The name can be confusing: modern products may not weigh 15 pounds, and specifications vary. Understanding the material’s standards, uses, limitations, and installation requirements helps homeowners and contractors choose and apply roofing underlayment more effectively.
What Is 15-Pound Asphalt-Saturated Felt?
Asphalt-saturated felt is a flexible roofing underlayment made from a fibrous base sheet—traditionally organic felt, and in some products a fiberglass-reinforced base—that is saturated or coated with asphalt. The asphalt makes the sheet more resistant to water than untreated paper. It is supplied in rolls and installed over a roof deck, beneath the finished roofing material.
The term “paper” is common in everyday conversation, but roofing felt is not ordinary household paper. It is a manufactured building product designed for use in roofing assemblies. It is also not a permanent waterproof roof by itself. Shingles, metal panels, or another approved roof covering provide the primary weather protection.
Historically, “15-pound” referred to the approximate weight of a 100-square-foot quantity of felt. A roofing square is 100 square feet, so the name described a traditional product category rather than a precise measurement of every current roll. Manufacturing methods and product formulations have changed, and modern “15 lb” felt may not weigh exactly 15 pounds per square.
For that reason, the product’s technical data sheet and marked standard are more useful than the name alone. Packaging may identify the felt by a designation such as ASTM D226 Type I. Buyers should verify the exact product and intended application instead of assuming every roll labeled “15 lb” has identical properties.
What Asphalt-Saturated Felt Does Under a Roof
Roofing felt provides a secondary layer between the roof deck and the roof covering. If wind-driven rain or melting snow gets beneath shingles, the underlayment can help shed water toward the eaves, where it can drain off the roof. It also helps protect the deck from exposure during installation, though it should not be treated as a substitute for a finished roof.
Felt can help separate the roof covering from the wood deck and reduce direct contact between materials. Depending on the roof system, it may also provide a temporary surface for workers during installation. Its effectiveness depends on correct product selection, proper fastening, adequate laps, sound flashing details, and timely installation of the final roof covering.
Underlayment is only one part of roof water management. Valleys, eaves, chimneys, skylights, plumbing vents, and wall intersections need correctly designed flashing and other required components. A felt layer cannot make poorly detailed penetrations or an unsuitable roof covering watertight.
Where 15-Pound Felt Is Commonly Used
Traditional asphalt-saturated felt is most familiar beneath asphalt shingles on sloped roofs. It may also be specified in certain roof assemblies using other materials, but compatibility and installation requirements depend on the covering manufacturer and local building code. Some roofing products require a particular underlayment, slip sheet, or fastening method, so a general-purpose felt should not be substituted without checking the instructions.
Felt may be suitable for projects where the roof design, climate, code, and covering manufacturer permit it. It is often available through roofing suppliers and building-material retailers. Availability and product names can differ by region, so installers should compare specifications rather than rely on a label alone.
It is generally not appropriate to assume that 15-pound felt is adequate for every roof. Low-slope roofs, areas with special wind or snow conditions, and assemblies using tile, metal, or specialty shingles can have distinct requirements. Some installations require heavier underlayment, self-adhered membrane at vulnerable locations, or an entirely different roofing system.
15-Pound Felt, 30-Pound Felt, and Synthetic Underlayment
Roof underlayments differ in weight, construction, water resistance, durability, and installation characteristics. “15 lb” and “30 lb” are traditional product names; they do not provide a complete performance specification. Synthetic underlayments, usually made from woven or nonwoven polymers, are another common option. They are often lighter per roll and may offer greater tear resistance, but their performance varies by product.
| Feature | 15-Pound Asphalt-Saturated Felt | 30-Pound Asphalt-Saturated Felt | Synthetic Underlayment |
|---|---|---|---|
| Material | Asphalt-treated fibrous sheet | Heavier asphalt-treated felt category | Polymer-based sheet |
| Handling | Can wrinkle or tear, especially when wet | Generally heavier and more substantial to handle | Often light per roll and designed for improved tear resistance |
| Exposure limits | Product-specific; prolonged exposure may cause damage | Product-specific; check the label and technical sheet | Varies widely; UV and exposure limits must be checked |
| Typical selection factors | Code, covering instructions, climate, and roof design | Same factors, with product weight and performance considered | Same factors, plus compatibility and specified fasteners |
Heavier felt is not automatically the right choice, and synthetic underlayment is not automatically superior for every roof. The best option is the one that meets applicable code, works with the roof covering, suits the exposure conditions, and can be installed according to its instructions. Cost comparisons should include required accessories and labor, not just the roll price.
How to Read Product Standards and Labels
ASTM D226 is a widely recognized specification for asphalt-saturated organic felt used in roofing and waterproofing. It includes different types, commonly referred to as Type I and Type II. Product markings and current standards should be checked directly because specifications and available products can change. A “15 lb” description alone does not establish that a product meets a particular standard or is approved for a particular roof.
Labels and technical data sheets may list the product standard, roll dimensions, coverage, required fastening, permitted roof coverings, exposure limitations, and installation instructions. They may also identify whether the product is intended for use beneath shingles or another material. If the information is missing or unclear, the manufacturer or supplier can clarify the intended application.
Building codes are adopted and enforced by state and local jurisdictions, and amendments can differ. The International Residential Code (IRC) contains provisions for underlayment in residential roof assemblies, but the locally adopted edition and project conditions determine what applies. Contractors should check the local code official’s requirements and the roof-covering manufacturer’s installation instructions.
How to Install 15-Pound Roofing Felt
Installation begins with a clean, dry, structurally sound roof deck. Loose nails, debris, damaged sheathing, and protruding fasteners should be addressed before underlayment is applied. The roof’s eave protection, drip edge, valley treatment, and flashing sequence must follow the project documents, local code, and product instructions; these details can affect where the felt starts and how it overlaps other materials.
- Confirm the product and roof requirements. Check the covering manufacturer’s instructions, local code, roof slope, and the felt label. Confirm the permitted exposure period, fastening method, and required laps.
- Plan the layout. Felt is usually installed in horizontal courses, beginning near the eaves and working toward the ridge. The course overlap and end laps must meet the product instructions and applicable code. Marking guide lines can help keep the courses straight.
- Roll out the sheet carefully. Keep the felt flat and aligned. Avoid wrinkles, fishmouths, and loose areas that could interfere with shingles or allow water to collect. Cut around penetrations only as directed and integrate the sheet with flashing.
- Fasten as specified. Use the type and pattern of cap nails, roofing nails, or other fasteners permitted by the product instructions and code. Fasteners should hold the sheet securely without tearing it. Wind exposure and the roof covering may affect the required pattern.
- Protect completed work. Install the roof covering promptly and stay within the manufacturer’s stated exposure limit. Repair damaged areas using an approved method before the finished roofing goes on.
These steps are general guidance, not a substitute for project-specific instructions. Details at eaves, valleys, ridges, hips, and penetrations are especially important. For example, a valley may require a separate membrane or a specified underlayment arrangement; simply laying felt across it does not necessarily meet the roof-system requirements.
Advantages and Limitations
Asphalt-saturated felt has several practical advantages. It is a long-established roofing material, is widely available, and can provide a secondary water-shedding layer when correctly installed. Some crews are familiar with its handling and application. For suitable roof assemblies, it may be an economical option.
Its limitations matter just as much. Felt can wrinkle, tear, or become more difficult to handle when wet. Foot traffic and wind can damage exposed material, and prolonged sun exposure may degrade products that are not designed for extended exposure. Roll weight and dimensions can also make handling less convenient than some synthetic products. These characteristics make prompt, careful installation important.
Felt is not a permanent membrane and should not be left exposed beyond the product’s stated limits. It does not replace ice-barrier membrane where required, nor does it correct poor ventilation, inadequate flashing, or defective shingles. A leak can still occur if water is driven beneath the underlayment through an incorrectly detailed roof edge or penetration.
Cost, Coverage, and Storage
Roll coverage varies by manufacturer and by how much material is lost to overlaps, end laps, cuts, and roof geometry. The printed square-foot coverage is not always the area a roll will cover when installed. Contractors estimate the number of rolls from the roof’s actual surface area and the specified overlap, then allow for waste and details such as valleys and dormers.
Prices change by location, supplier, product type, and market conditions. A comparison should account for roll coverage, required fasteners, accessories, and labor. A lower-cost roll may not be the best value if it is incompatible with the roofing material or requires a different installation approach.
Store felt according to the manufacturer’s directions, typically protected from weather and kept from becoming damaged or excessively wet before installation. Rolls should be handled carefully to avoid crushing or tearing. On a roof, workers should use appropriate fall protection and follow job-site safety practices; roofing surfaces and underlayment can be slippery, particularly when wet or dusty.
Common Questions About 15-Pound Felt
Does 15-pound felt actually weigh 15 pounds?
Not necessarily. The designation comes from a traditional weight category, generally associated with the approximate weight of a 100-square-foot quantity. Current products may differ in actual weight because of manufacturing changes. The product label and technical data sheet provide more dependable information.
Can 15-pound felt be left exposed?
Only for the period allowed by the specific product manufacturer. Exposure limits depend on the product and conditions such as sunlight, wind, and weather. The finished roofing should be installed promptly, and any damaged or wet material should be handled according to the manufacturer’s instructions.
Is 15-pound felt waterproof?
It is designed to resist and shed water as part of a roofing assembly, but it should not be treated as a permanent, fully waterproof roof. Seams, fasteners, roof edges, and penetrations all affect performance. The primary roof covering and proper flashing remain essential.
Can 15-pound felt be used beneath metal roofing?
Possibly, but only when the metal-roof manufacturer, project specifications, and local code allow it. Metal systems may require a particular underlayment or separation layer, and some conditions call for high-temperature self-adhered membranes. The metal-panel instructions should govern material choice and installation.
Choosing the Right Roofing Underlayment
When comparing 15-pound asphalt-saturated felt with heavier felt or synthetic underlayment, start with the roof covering manufacturer’s requirements and the locally adopted building code. Then consider slope, climate, anticipated exposure before the roof is finished, deck condition, fastening details, and the installer’s ability to follow the product instructions.
The most reliable choice is not determined by the “15 lb” name alone. It depends on the product’s documented performance and the complete roof assembly. Confirming specifications before work begins helps prevent incompatible materials, avoidable installation problems, and confusion over what the underlayment can—and cannot—do.