Roofing felt remains a common underlayment choice across the U.S., used to protect roofs during construction and extend shingle life. This article explains the main types of roofing felt, their characteristics, performance differences, and practical guidance for selection and installation to help homeowners, contractors, and property managers make informed choices.
| Felt Type | Weight/Thickness | Common Uses |
|---|---|---|
| Asphalt-Saturated Organic Felt | 15 lb, 30 lb | Traditional underlayment for shingles |
| Asphalt-Saturated Fiberglass (Glass) Felt | 15 lb, 30 lb, Synthetic Equivalents | Modern underlayment with improved tear resistance |
| Synthetic Underlayment | Lightweight (40–120 mil) | High-performance underlayment for steep-slope roofs |
What Is Roofing Felt?
Roofing felt is an underlayment material installed between the roof deck and exterior roofing materials like asphalt shingles, metal, or tiles. It provides temporary water resistance, secondary protection from leaks, and a smoother surface for roofing materials. Variations arise from base mat composition, saturation materials, and manufacturing methods.
Main Categories Of Roofing Felt
Asphalt-Saturated Organic Felt
Organic felt uses cellulose-based fiber mats saturated with asphalt. Historically common, it is available typically in 15-pound and 30-pound weights. The heavier 30-pound felt offers better durability and water resistance than 15-pound versions.
Advantages include good conformity to irregular roof decks and a proven track record with asphalt shingles. Drawbacks include greater tendency to wrinkle, tear, and absorb moisture, which can accelerate roof deck rot under prolonged exposure.
Asphalt-Saturated Fiberglass (Glass) Felt
Fiberglass-based felt replaces organic fiber with glass mat as the base and is saturated with asphalt or modified bitumen. It resists tearing and moisture absorption better than organic felt, giving improved storage life and handling on the roof deck.
Glass felt is lighter than organic felt of equal weight and often used in modern asphalt shingle installations because it pairs well with manufacturer warranties and provides a dimensionally stable underlayment.
Synthetic Underlayment
Synthetic underlayments are engineered polymers (polypropylene or polyethylene blends) designed specifically as roofing underlayment. They offer superior tear resistance, UV stability, and water-shedding properties compared with traditional felt.
Synthetics are available in varying thicknesses (measured in mils) and can be slip-resistant, lightweight, and longer-lasting. They are increasingly specified by contractors seeking longer exposure times during construction and improved performance in extreme weather.
Comparison Of Performance Characteristics
| Characteristic | Organic Felt | Glass Felt | Synthetic Underlayment |
|---|---|---|---|
| Tear Resistance | Low | Moderate | High |
| Water Absorption | Higher | Lower | Minimal |
| UV Exposure Time | Limited | Moderate | Extended |
| Slip Resistance | Varies | Better | Typically Highest |
Weight And Thickness Explained
Felt is commonly labeled by weight (15 lb, 30 lb) which historically estimated the weight of a 100-square roll. Higher weight generally indicates thicker material and improved durability, but modern synthetic options use mil thickness and offer higher performance at lower weights.
Understanding these measurements helps match underlayment to roof type and climate. For high-slope or steep roofs, a 30-pound felt or a heavy synthetic is usual; for low-slope or highly-exposed projects, premium synthetic underlayments are preferred.
Installation Best Practices
Proper installation maximizes protection regardless of the type. Install underlayment parallel to the eave with correct shingle-lapping, fastener spacing, and starter course alignment. Follow manufacturer instructions closely for exposures and overlaps.
Common recommendations include tying into leak-prone areas (valleys, dormers, chimneys) with ice-and-water shield or peel-and-stick membranes. Use compatible roofing nails or staples and ensure proper ventilation to limit moisture buildup beneath the underlayment.
Handling And Storage
Store rolls off the ground under cover and avoid extended UV exposure. Organic felts can degrade in wet conditions; synthetic rolls tolerate longer outdoor storage. Always inspect rolls pre-installation for damage, wrinkles, and proper packaging labels.
Code, Warranty, And Compatibility Considerations
Roofing codes vary by locality, but many building departments accept felt or approved synthetic underlayments when installed per manufacturer directions. Shingle manufacturers may require specific underlayments to keep roof warranties valid, so verify compatibility before selection.
Insurance and local code requirements may also influence the choice of underlayment, especially in high-wind or wildfire-prone regions where fire-rated or class-specific products are required.
When To Choose Each Type
- Organic Felt: Suitable for budget-conscious projects where traditional materials are preferred, and exposure times are short.
- Glass Felt: A balanced choice for durability and compatibility with modern shingles without the higher cost of synthetics.
- Synthetic Underlayment: Recommended for long exposures, harsh climates, frozen installations, and when seeking superior tear resistance and moisture management.
Cost And Lifespan Comparison
Organic felt is typically the lowest-cost option but offers the shortest lifespan under exposure. Synthetic underlayment costs more upfront but often extends the practical life of the underlayment and protects the roof deck during longer construction periods.
| Type | Approx. Cost Per Square | Expected Field Lifespan |
|---|---|---|
| Organic Felt | Low | Short (months if exposed) |
| Glass Felt | Moderate | Moderate (months to a year) |
| Synthetic | Higher | Long (1+ year exposure) |
Specialty Felts And Modified Products
Some products blend asphalt-saturated felts with polymer modifiers for improved flexibility and adhesion. Ice-and-water shields, peel-and-stick membranes, and laminated felts provide extra protection in critical areas such as eaves, valleys, and around penetrations.
These specialty materials are often part of high-performance roofing systems and are selected where codes or climate demand enhanced leak resistance.
Environmental And Disposal Considerations
Asphalt-saturated felts are not readily recyclable and may be disposed of in construction waste streams. Synthetic underlayments sometimes offer recyclable options or reduced landfill impact due to lower material mass, though recycling availability depends on local facilities.
When choosing underlayment, consider longevity and lifecycle impacts; a longer-lasting product may reduce replacement frequency and overall waste.
Common Installation Problems And How To Avoid Them
- Wrinkling And Buckling: Use proper fastening patterns, maintain tension during installation, and avoid excessive overlaps that trap air.
- Improper Overlaps: Follow manufacturer overlap guidelines—insufficient laps can permit water migration under shingles.
- Incompatible Fasteners: Use recommended nails or staples; corrosion-resistant fasteners extend underlayment life.
Key Takeaways For Selecting Roofing Felt
Select underlayment based on climate, expected exposure time, roofing material compatibility, and budget. Synthetic underlayments offer best-in-class performance for durability and exposure resistance, while glass felts deliver a cost-effective middle ground. Organic felt remains an economical choice for short-term needs and traditional builds.
Consult local building codes and shingle manufacturer requirements before final selection, and prioritize correct installation to achieve the intended protection.
Resources And Further Reading
For technical specifications, visit manufacturer product pages and local building code resources. Industry organizations like the National Roofing Contractors Association (NRCA) provide guidance on underlayments and installation standards.
Contractors should also refer to product data sheets and warranty documents to ensure compatibility with roofing materials and to preserve warranty coverage.