40 Pound Roofing Felt, also known as 40 lb felt or asphalt roofing felt, acts as a moisture barrier and a temporary weather shield during roof construction and repair. This article explains what 40-pound roofing felt is, how it’s used, installation best practices, advantages and drawbacks, and how to select the right roll for a project in the United States.
What Is 40 Pound Roofing Felt
40 Pound Roofing Felt is a heavy-duty asphalt-impregnated paper or fiberglass mat used as a secondary barrier beneath shingles or metal roofing. The “40-pound” designation refers to its weight per unit area, typically measured as a standard roll weight that helps distinguish it from lighter felts such as 15-lb or 30-lb options. In the United States, this weight class is common for low to mid-slope residential roofs and for temporary weather protection during construction.
Felt serves two primary roles: preventing wind-driven moisture from penetrating the roof deck and providing a smooth surface for shingle installation. It also helps improve the long-term performance of roof assemblies by reducing the risk of capillary moisture wicking into the decking. 40-pound felt is available in various compositions, including traditional paper-based and fiberglass-reinforced varieties, each with its own tear resistance and tear durability.
Types Of 40 Pound Roofing Felt
There are two main construction types to consider:
- Paper-based 40-lb felt: Historically common, affordable, and easy to handle. It’s less resistant to tearing when wet but provides good moisture protection when installed properly.
- Fiberglass-reinforced 40-lb felt: Offers higher tear resistance and durability, especially in humid climates or steeper slopes. It tends to lay flatter and resists tearing during installation.
Some products are treated to resist ultraviolet exposure temporarily, which can be beneficial during delays in shingle installation. Always check manufacturer specifications for exposure limits and warranty terms, as these factors influence how long the felt can remain exposed.
Typical Uses And Applications
40 Pound Roofing Felt is commonly used in:
- Underlayment for asphalt shingles on low to mid-slope roofs (roughly up to 6/12 pitch, depending on local codes).
- Temporary weather protection during roof replacements or repairs.
- Underlayment for tile, shake, or metal roofing systems when specified by the manufacturer.
Local building codes may dictate whether felt is required as underlayment, and some jurisdictions permit breathable synthetic underlayments as alternatives. Always verify code requirements and manufacturer recommendations for the specific roofing system.
Installation Best Practices
Proper installation of 40-pound roofing felt is essential to its effectiveness. The following best practices help ensure a durable, code-compliant result:
- Inspect and prepare the deck: Remove debris, fix nails or protrusions, and ensure a clean, dry surface.
- Roll direction and alignment: Layout the felt from the eave to the peak, with successive courses overlapping by at least 2 inches (typically 19 to 24 inches wide per roll). Start at the lowest edge to ensure proper drainage.
- Fastening method: Use the recommended nails or plastic cap nails at the top of each sheet, spaced per manufacturer guidelines. Avoid overtightening which can tear the felt.
- Overlap and seams: Overlaps should be uniform and allow for water shedding toward the outside. Do not create pockets where water could accumulate.
- Ventilation and exposure: If the roof will be exposed for an extended period before shingles are installed, follow product guidance on maximum exposure time to prevent deterioration.
- Compatibility with underlayment: Confirm that the felt type is compatible with the selected shingles or metal roofing system to prevent chemical interactions or adhesion issues.
When using underlayment in steeper or high-wind areas, consider securing all edges and seams with tape or fasteners as per the manufacturer’s instructions to prevent uplift during storms.
Advantages Of 40 Pound Roofing Felt
- Cost-effective protection: An economical option for moisture control during installation and as a secondary barrier afterward.
- Compatibility: Works with a wide range of roofing systems, including asphalt shingles and some metal roof configurations.
- Ease of handling: Lightweight and manageable for DIY homeowners, especially paper-based varieties.
- Tear resistance (with fiberglass variants): Fiberglass-reinforced felts offer better durability during handling and installation.
Limitations And Considerations
- Weather exposure: Prolonged exposure to sun and rain can degrade the felt. Some products have limited exposure ratings; check limits before prolonged delays.
- Not a guarantee against leaks: Underlayment is a secondary barrier; proper shingle installation and flashing are essential to prevent leaks.
- Code variations: Local codes may require synthetic underlayment or other products over traditional felt, especially in regions with heavy rainfall or snow.
- Environmental impact: Paper-based felts have different environmental footprints than fiberglass variants; evaluate local recommendations and contractor preferences.
Choosing The Right Roll For A Project
When selecting a 40-pound roofing felt, homeowners should consider:
- Climatic conditions: Humid or storm-prone areas may benefit from fiberglass-reinforced felt for durability.
- Roof pitch: Low to mid-slope roofs align well with 40-lb felt, while steeper slopes may demand additional fasteners and edge securing.
- Manufacturer specifications: Always adhere to the roofing system’s recommended underlayment; some shingles require specific underlayment types for warranty compliance.
- Local code requirements: Some jurisdictions prefer or require synthetic underlayment over traditional felt in new construction or reroofing projects.
Maintenance, Inspection, And Longevity
After installation, inspect roofing felt as part of routine maintenance. Look for areas where seams show wear, edges peel, or fasteners are exposed. While 40-pound roofing felt is designed to last for the duration of the roofing project, the long-term protection it provides depends on proper installation, correct shingle choice, and timely maintenance. If visible damage or water intrusion occurs, engage a licensed roofing professional to assess deck condition and potential repairs.
Frequently Asked Questions
- Is 40-pound felt required under all shingles? Requirements vary by code and manufacturer. In some regions, synthetic underlayment is preferred or required, while in others, 40-lb felt remains an acceptable option.
- Can I install 40-lb felt over an existing roof? Yes, for some reroofing projects, but follow the manufacturer’s guidelines and ensure proper nailing and flashing practices to avoid future leaks.
- How long can 40-pound felt be exposed before shingles are installed? Exposure limits depend on the product; always follow the manufacturer’s maximum exposure time to prevent deterioration.
- What is the difference between 40-lb felt and synthetic underlayment? Felt is asphalt-impregnated paper or fiberglass sheet; synthetic underlayment is a polymer-based material offering higher tear strength and longer exposure durability in some cases.