Best Felt Paper for Roofing: Choosing Durable Underlayment for Lasting Roofs

Selecting the best felt paper for roofing helps protect a roof deck from water intrusion, improves shingle performance, and provides a temporary barrier during construction. This guide compares types, weights, and installation tips to help homeowners, contractors, and roofers make informed decisions.

Felt Type	Weight/Rating	Best Use	Pros	Cons
Asphalt-Saturated Felt	15 lb / 30 lb	Standard shingle roofs	Affordable, traditional	Less durable than synthetics
Glass-Fiber Felt	30 lb equivalent	Higher durability needs	Stronger, tear-resistant	Higher cost
Synthetic Underlayment	N/A (lightweight)	Low slope, long-lasting roofs	Lightweight, UV-stable	Can slip if not installed properly

What Felt Paper Means For Roofing

Felt paper, commonly called roofing felt or tar paper, serves as a secondary waterproofing layer beneath shingles. It protects the roof deck from wind-driven rain and secondary leaks when shingles are compromised and provides a smooth, consistent surface for shingle adhesion.

Historically made of organic cellulose fibers saturated with asphalt, felt evolved to include glass-fiber mats and other reinforcements to improve strength and longevity.

Types Of Felt Paper And How They Differ

Choosing the right felt starts with understanding main types: traditional asphalt-saturated organic felt, glass-fiber reinforced felt, and modern synthetic underlayments. Each type differs by durability, weight, and resistance to tearing and UV exposure.

Asphalt-Saturated Organic Felt

Organic felt uses cellulose fibers soaked in asphalt. It comes in 15-pound and 30-pound ratings, representing density rather than literal weight. The 15 lb is lighter, suited for vertical applications and low-cost projects; the 30 lb offers better durability for steeper roofs and longer exposure.

Glass-Fiber Reinforced Felt

Glass-fiber felts replace cellulose with a noncombustible glass mat that resists tearing and is more dimensionally stable. They outperform organic felts in wind resistance and moisture tolerance, making them suitable for harsher climates or longer jobsite exposure.

Synthetic Underlayment

Synthetic underlayments are typically made from polypropylene or polyethylene fabrics coated for waterproofing. They are lightweight, UV-resistant, and often offer slip-resistant surfaces. Synthetics generally provide superior tear resistance and longer exposure windows but cost more than traditional felt.

Weight Ratings And What They Mean

The industry uses “15 lb” and “30 lb” designations to indicate felt density. These ratings are legacy terms from when rolls were heavier; now they act as a shorthand for durability. 30 lb felt is thicker and more durable than 15 lb, but heavier and harder to handle.

For most modern shingle installations, building codes and shingle manufacturers recommend using at least 15 lb felt, though 30 lb or glass-fiber options are preferred in high-wind or steep-slope applications.

Performance Criteria To Evaluate

When selecting felt paper, evaluate these core performance criteria: tensile strength, tear resistance, UV exposure rating, water holdout, compatibility with shingles, and slip resistance during installation. High tensile and tear strength reduce damage during handling and high winds.

• Tensile Strength: Higher values mean better resistance to stretching and tearing.
• UV Exposure Limit: Indicates how long the material can be left exposed on the roof before degradation.
• Slip Resistance: Important for worker safety during installation on sloped roofs.
• Water Holdout: Measures ability to prevent water penetration between felt and deck.

Best Felt Choices By Application

Suitability depends on roof slope, climate, and project budget. High-wind and rainy climates benefit from stronger, glass-fiber or synthetic underlayments.

Residential Asphalt Shingle Roofs

For typical U.S. homes, 15 lb asphalt-saturated felt meets many code minimums, but 30 lb or glass-fiber felt provides added protection. 30 lb felt is recommended for steeper roofs or where temporary exposure during construction is expected.

Low-Slope Roofs

Low-slope roofs are more vulnerable to water pooling. Building codes often require self-adhered or synthetic underlayments in these areas. Use a high-performance synthetic or self-adhering membrane for best water protection.

High-Wind Coastal Regions

Coastal and hurricane-prone regions demand superior uplift and tear resistance. Glass-fiber felts or synthetics with higher nail pull-out resistance are preferable.

Installation Best Practices

Proper installation maximizes felt performance. Start with a clean, dry deck, align starter courses, and fasten per manufacturer recommendations. Use roofing nails or staples as specified and overlap edges according to local code and product instructions.

• Install felt horizontally with the upper layer overlapping the lower to shed water.
• Stagger vertical seams and maintain manufacturer-recommended overlaps.
• Use cap nails or staples that won’t corrode in your climate.
• Avoid leaving felt exposed beyond recommended UV exposure limits.

Common Mistakes To Avoid

Common errors reduce felt effectiveness. Avoid underlapping instead of overlapping, inconsistent fastener patterns, and leaving felt exposed for weeks. Failure to follow shingle manufacturer attachment instructions can void warranties.

Another frequent problem is using the wrong type on low-slope roofs or not using starter strips and drip-edge details that prevent water from tracking under shingles.

Comparing Cost And Value

Cost varies widely: 15 lb organic felt is cheapest, 30 lb and glass-fiber cost more, and premium synthetic underlayments are the most expensive. Consider lifecycle value: synthetics often save labor and reduce re-roofing risk, offsetting higher upfront costs.

Material	Average Cost Per Square (materials only)	Estimated Lifespan
15 lb Asphalt Felt	$5–$10	Short (months if exposed)
30 lb Asphalt / Glass-Fiber Felt	$8–$15	1–3 Years Exposure
Synthetic Underlayment	$20–$40	Longer Exposure (months to years)

Compatibility With Shingles And Warranties

Shingle manufacturers often specify underlayment requirements. Installing noncompliant felt can void warranties. Always verify manufacturer instructions and local building codes before selecting an underlayment.

Some asphalt shingles require specific underlayment types in eaves, valleys, or high-wind zones. Using an approved synthetic can sometimes enhance shingle performance and warranty coverage.

Environmental And Safety Considerations

Organic felts contain asphalt derived from petroleum; disposal and VOCs are considerations. Synthetic products may be recyclable but sometimes mixed polymers complicate recycling. Select products with environmental data sheets and conform to local disposal rules.

On safety, use slip-resistant surfaces where possible and follow fall protection requirements during installation to reduce accidents related to slippery underlayments.

Top Brands And Product Recommendations

Reputable brands offer a range of felts and synthetics tailored to climates and code needs. Certain lines emphasize UV exposure time, slip resistance, and reinforced mats for easier handling.

• Traditional Felt Manufacturers: Known for cost-effective 15 lb and 30 lb products suitable for standard roofs.
• Glass-Fiber Options: Marketed for stronger tear resistance and longer exposure windows.
• Synthetic Brands: Focus on high tensile strength, lightweight rolls, and long UV exposure times.

Decision Checklist For Choosing The Best Felt

Use this checklist to finalize a selection: roof slope, climate risk (wind/rain/snow), expected exposure time, compatibility with shingles, local building codes, and budget.

1. Confirm shingle manufacturer underlayment requirements.
2. Assess local climate and wind zones.
3. Decide on allowable exposure time during construction.
4. Compare tensile strength and tear resistance values.
5. Factor in labor costs and handling preferences.

Maintenance And Inspection Tips

Felt underlayment is usually covered, but during re-roofs or inspections, look for signs of moisture staining, splits, or prolonged UV damage. Replace compromised underlayment to prevent decking rot and interior leaks.

For roofs undergoing phased work, avoid extended exposure and re-cover or protect felt with temporary roofing or tarps if weather threatens.