Meta description: Compare the main types of roof felt, including No. 15, No. 30, fiberglass-reinforced felt, and synthetic alternatives. Learn where each fits and how to choose.
Roof felt is a protective layer installed between a roof deck and its shingles or other covering. It can help shield the deck from moisture during installation and provide a secondary water-shedding layer after the roof is complete. The right choice depends on the roofing material, roof slope, local building code, climate, and manufacturer instructions. Understanding the types of roof felt—and how traditional felt differs from modern underlayments—helps homeowners compare options and ask informed questions before a roofing project.
What Roof Felt Does
Roof felt, often called roofing underlayment, is installed over the roof deck and beneath the finished roofing material. Traditional felt is made by saturating a reinforcing base with asphalt. The base may be organic fiber or glass fiber. Other underlayments use synthetic materials or self-adhering modified asphalt rather than traditional felt.
Underlayment is not a substitute for shingles, flashing, or sound roof construction. Its main functions are to provide temporary weather protection during installation and add a secondary layer that helps shed water that gets beneath the roof covering. It can also separate the roof covering from the deck, depending on the product and roofing system.
Roof underlayment is not automatically a waterproof roof. Seams, fasteners, penetrations, valleys, and edges all affect performance. Correct installation and compatible materials matter as much as the product category.
Main Types of Roof Felt
No. 15 Asphalt-Saturated Felt
No. 15 felt is a lighter traditional asphalt-saturated underlayment. It is commonly used beneath asphalt shingles on qualifying roof slopes when permitted by the applicable code and shingle manufacturer. It is generally easier to handle than heavier felt, but it may be more prone to tearing or wrinkling during installation.
The “No. 15” label is a traditional product designation. It should not be treated as a guaranteed statement of a roll’s current weight or performance. Product construction and labeling can vary, so installers should check the manufacturer’s technical data rather than assume all No. 15 products are identical.
No. 30 Asphalt-Saturated Felt
No. 30 felt is a heavier traditional felt than No. 15. Its added material can make it more resistant to damage during handling and installation, although it is also heavier and may be more cumbersome to work with. It is often considered for projects where a more robust felt layer is desired or where the roofing system calls for it.
As with No. 15, the number is a conventional designation, not a complete specification. The product’s applicable ASTM standard, installation instructions, and approved use should guide selection. Heavier felt does not, by itself, make an improperly installed roof watertight or resolve ventilation and drainage problems.
Fiberglass-Reinforced Asphalt Felt
Fiberglass-reinforced felt uses a glass-fiber base instead of, or as part of, a traditional organic-fiber construction. Asphalt provides the water-resistant treatment. Depending on the product, fiberglass reinforcement may improve dimensional stability and resistance to tearing compared with some organic felts.
Fiberglass-reinforced products are still asphalt-based underlayments, but they are not necessarily interchangeable with every product sold as No. 15 or No. 30. Their material properties, approved applications, and installation methods depend on the specific product. The packaging and technical data should identify the relevant standard and intended roofing use.
Organic-Fiber Roofing Felt
Older or traditional organic felts use a base made from cellulose-based fibers saturated with asphalt. These products may be identified by their construction or by the standards they meet. Organic felt can provide a familiar, relatively straightforward underlayment option, but it can be affected by moisture exposure and installation conditions.
“Organic” describes the reinforcement material, not a claim that the finished product is untreated or environmentally benign. Asphalt-saturated felt should be handled and disposed of according to product guidance and local rules.
How Roofing Felt Standards Fit In
Product standards help distinguish underlayments by construction and performance. In the United States, roofing felt and underlayment products may be tested or identified under ASTM standards, including standards for asphalt-saturated organic felt, fiberglass-reinforced asphalt felt, and other underlayment types. The exact standard matters because similar-sounding product names do not guarantee identical performance.
For example, ASTM D226 covers asphalt-saturated organic felt used in roofing and related applications, while ASTM D4869 addresses asphalt-saturated organic felts used as steep-slope roofing underlayment. ASTM D6757 covers a non-asphalt-saturated felt underlayment made with a glass-fiber base. These references are useful starting points, but the current product label and manufacturer documentation should be checked for the specific application.
Building codes may specify underlayment requirements based on roof covering, slope, and location. The International Residential Code includes underlayment provisions for roof coverings, but local jurisdictions adopt and amend codes on their own schedules. A contractor or building department can confirm which edition and local rules apply.
Roof Felt Compared With Synthetic Underlayment
Synthetic underlayment is a broad category of products made with polymer-based materials rather than traditional asphalt-saturated felt. Many synthetic products are lighter per unit of coverage and designed to resist tearing during installation. Some include printed alignment guides or textured surfaces. Their performance varies, so “synthetic” alone does not define a particular level of durability or weather resistance.
Traditional felt may be familiar to installers and can be suitable for many asphalt-shingle projects. Synthetic underlayment may offer handling advantages, especially on larger roofs, but it may cost more and requires following the manufacturer’s fastening and exposure limits. Neither category is universally best. The roofing system, site conditions, and installation instructions determine which is appropriate.
| Underlayment type | Typical characteristics | Important considerations |
|---|---|---|
| No. 15 asphalt-saturated felt | Traditional, relatively light felt | Check product specification, slope requirements, and handling limits |
| No. 30 asphalt-saturated felt | Heavier traditional felt | Can be more cumbersome; designation alone does not establish performance |
| Fiberglass-reinforced felt | Asphalt-treated product with glass-fiber reinforcement | Confirm approved use and installation details for the exact product |
| Synthetic underlayment | Polymer-based sheet, not traditional felt | Compare tested performance, exposure limits, and fastening instructions |
| Self-adhered modified bitumen | Adhesive-backed membrane, often used at vulnerable areas | Substrate preparation, temperature, and placement are critical |
Self-Adhered Underlayment and Ice Barriers
Self-adhered underlayment is often made from polymer-modified bitumen with a release liner. It bonds to a prepared roof deck rather than relying only on mechanical fasteners. These products are commonly used as ice barriers at eaves in regions where ice dams are a concern, and they may also be specified in valleys or around other vulnerable details.
Self-adhered membranes are not simply a premium version of ordinary felt. Their installation requirements can be more demanding. The deck may need to be clean, dry, and within a specified temperature range. Laps, transitions, and compatibility with other materials must follow the product instructions. Local code may define where an ice barrier is required and how far it must extend.
Ice barriers help limit water intrusion from ice dams, but they do not prevent ice dams from forming. Adequate attic insulation, ventilation, air sealing, and drainage are also important parts of managing the underlying conditions.
Choosing the Right Type of Roof Felt
A suitable underlayment is selected as part of a complete roofing system. These factors can help narrow the options:
- Roof covering: Asphalt shingles, metal panels, tile, and other materials may have different underlayment requirements.
- Roof slope: Low-slope and steep-slope roofs can require different materials, overlaps, or application methods.
- Climate and exposure: Rain, snow, wind, heat, and the time the underlayment will remain exposed can affect product suitability.
- Local code: Requirements vary by jurisdiction and may include specific underlayment layers or ice-barrier placement.
- Manufacturer approval: The roof-covering manufacturer may specify compatible underlayment and installation details.
- Deck and installation conditions: Surface condition, fastener type, and installer access can influence which product is practical.
For a typical asphalt-shingle roof, a code-compliant felt or synthetic underlayment may be appropriate, but the answer is not universal. In regions with ice-dam requirements, a self-adhered ice barrier may be needed at eaves in addition to field underlayment. A roofing contractor should identify the proposed product, explain where each material will be used, and confirm that the complete assembly meets local requirements.
Installation Details That Affect Performance
Even a high-quality product can underperform if installed incorrectly. Common details include laying courses in the correct direction, maintaining specified side and end laps, fastening with approved cap nails or other specified fasteners, and protecting the material from prolonged exposure beyond its stated limit. The exact requirements vary by product and roof system.
Underlayment should be installed over a sound, properly prepared deck. Wrinkles or fish-mouths can interfere with the flat installation of shingles and may create uneven surfaces. In valleys, at eaves, and around chimneys, vents, and skylights, the underlayment must work with flashing and other water-management components rather than replace them.
Roofing work also involves fall hazards and weather exposure. Installation is generally best handled by trained professionals using appropriate safety equipment. For a roof replacement, homeowners can request the product name, installation instructions, and a clear description of how the contractor will treat edges, valleys, and penetrations.
Common Roof Felt Questions
Is roof felt required under shingles?
Requirements depend on the adopted building code, roof design, and shingle manufacturer. Many roofing assemblies use underlayment, but the required type and installation can differ. Local building officials and the product instructions are the appropriate sources for project-specific requirements.
Is No. 30 felt always better than No. 15?
Not automatically. No. 30 is a heavier traditional felt, but the number alone does not establish a product’s full performance or suitability. The roofing system’s specifications and the product’s technical information should determine the choice.
Can synthetic underlayment replace roof felt?
In some roofing systems, an approved synthetic underlayment can serve the role traditionally filled by felt. It should only be used when permitted by local code and compatible with the roof-covering manufacturer’s instructions. Synthetic products are not all identical.
Does roof felt stop leaks?
Underlayment provides a secondary water-shedding layer, but it is not a cure for damaged shingles, failed flashing, poor drainage, or faulty installation. Persistent leaks need an inspection to identify and repair the source.
Checking Product and Code Information
Before choosing a product, homeowners and contractors can review its label, technical data sheet, installation instructions, and applicable ASTM designation. The International Code Council’s residential code provisions and the local building department can clarify jurisdiction-specific requirements. Roofing-material manufacturers also publish installation guidance for compatible underlayments. The most reliable choice is the product approved for the specific roof assembly and installed according to both code and manufacturer instructions.