Roofing felt, also known as tar paper or underlayment, protects a roof deck from moisture and helps protect shingles during installation. In the United States, two common options are 15-pound and 30-pound roofing felt. This article explains the differences, ideal applications, installation tips, and how to weigh costs and performance when selecting between 15-lb and 30-lb felt for residential and light commercial roofs. It also covers modern alternatives and best practices for durability and code compliance.
What Is Roofing Felt And How Do 15 And 30 Lb Compare?
Roofing felt is a heavy-paper layer saturated with asphalt, laid over the roof deck before shingles or tiles. The number (15 or 30) refers to the weight per hundred square feet, indicating thickness and durability. 15-lb felt is lighter and more economical, offering basic moisture protection and a convenient option for short-term or low-slope projects. 30-lb felt is thicker and provides greater puncture resistance, tear strength, and a longer service potential in challenging conditions. Both act as a shield against wind-driven moisture during construction and early life of the roof.
Key Differences In Performance And Durability
- Puncture and tear resistance: 30-lb felt generally resists damage better during installation and in windy sites, compared with 15-lb felt.
- Moisture protection: Both types offer vapor protection, but the extra thickness of 30-lb can enhance protection in harsher weather or longer construction timelines.
- Handling and installation: 15-lb felt is lighter and easier to handle for DIY projects, while 30-lb may require more care and wind protection during setup.
- Code and climate considerations: Some regions with extreme weather or longer roof exposure environments may prefer 30-lb or opt for modern synthetic underlayments that meet or exceed code requirements.
When To Use 15-Lb Roofing Felt
15-lb roofing felt is well-suited for straightforward, short-duration projects on dry, moderate climates. It works well under asphalt shingles on standard residential roofs when installers need a cost-effective, temporary moisture barrier during construction or in areas with milder weather patterns. For reroofing where the existing deck is in good condition and the project has a tight schedule, 15-lb felt can be a practical choice if local codes permit it and if the roof will be covered promptly with shingles.
When To Use 30-Lb Roofing Felt
30-lb roofing felt is preferred in harsher climates, longer construction timelines, or where additional protection is desired. It offers enhanced durability against wind, foot traffic, and potential roofing debris. In regions with heavy rainfall, snow, or frequent temperature swings, 30-lb felt reduces the risk of wind-driven moisture reaching the decking during installation or delays. Some codes and manufacturers specifically require 30-lb felt for certain roof slopes or in areas with persistent moisture exposure.
Installation Best Practices For 15 And 30 Lb Felt
- Overlap and alignment: Align sectors with a standard 2- to 4-inch overlap along seams, following local code guidelines and shingle manufacturer recommendations.
- Anchoring: Use roofing nails or staples per product guidance to prevent creasing or tearing, especially on steeper slopes.
- Protection during work: Keep felt dry during installation; wet felt loses strength and may wrinkle or tear more easily.
- Ventilation and drainage: Ensure proper ventilation and sufficient deck drainage before applying roof coverings to maximize underlayment performance.
- Compatibility with shingles: Verify that the felt type is compatible with the chosen shingle system and local code requirements.
Alternatives To Traditional Felt
Modern roofing often uses synthetic underlayment or self-adhering membranes that provide superior tear resistance, better waterproofing, and easier handling. These options tend to outperform traditional asphalt-saturated felt in many scenarios, especially on low-slope roofs or installations requiring enhanced protection against moisture during long construction windows. When selecting alternatives, consider climate, roof pitch, roof covering, and local building codes to ensure full compliance and performance.
Code, Warranty, And Longevity Considerations
Code requirements vary by state and municipality. Some jurisdictions specify minimum underlayment standards or prefer synthetic products. Manufacturers’ warranties may hinge on the underlayment type, roof slope, and installation method. For long-term protection, select a product that meets or exceeds code standards and offers a robust warranty. Documentation of product specifications, installation guides, and adherence to best practices helps maintain warranty validity and reduces the risk of moisture-related damage over time.
Maintenance, Inspection, And Longevity
Regular roof inspections after severe weather help identify wrinkles, tears, or lifted edges in the underlayment. While felt itself is not typically a long-term, end-of-life material, ensuring correct installation and timely shingle replacement preserves the deck’s protection. If a roof is reroofed, evaluate whether the underlayment needs replacement or upgrading to a higher-grade felt or synthetic membrane to improve future performance and durability.