Roofing Felt 15 or 30: Choosing the Right Underlayment for Your Roof

The choice between 15-pound and 30-pound roofing felt is a common decision for homeowners and builders. This underlayment sits between the roof deck and shingles, offering a moisture barrier and added protection during installation. Understanding the differences, installation considerations, and local building codes helps ensure long-term performance and cost efficiency for American homes.

What Is Roofing Felt

Roofing felt, often called tar paper, is a fiberglass or organic mat saturated with asphalt. It serves as a secondary water barrier and provides a smooth surface for shingles to adhere to during installation. Roofing felt comes in two primary weights in the United States: 15-pound (15#) and 30-pound (30#). The weight indicates the overall strength and thickness of the felt, directly affecting tear resistance, durability, and installation ease.

Understanding 15# vs 30#

15# roofing felt is lighter, easier to handle, and often sufficient in regions with low rainfall and mild climates. It provides a basic moisture barrier and can be adequate on well-ventilated roofs with modern asphalt shingles. However, it is more prone to tearing during installation and may offer less protection in severe weather or in areas with heavy wind-driven rain.

30# roofing felt is thicker and stronger, delivering greater tear resistance and durability. It performs better in challenging conditions, including high winds, heavy rainfall, and colder climates where shingles require longer exposure to outdoor elements during installation. 30# felt tends to stay flatter on the deck, simplifying application and reducing wrinkling that can affect shingle adhesion.

In practice, many builders choose 30# felt for durability, while 15# felt remains common on smaller roofs, renovations, or when cost savings are prioritized. Local building codes and manufacturer recommendations often influence the final choice.

Key Differences And Implications

• <strongTear Resistance: 30# is harder to tear during handling and installation, reducing the risk of damage before shingles are installed.
• <strongWind Resistance: Heavier felt provides a more robust barrier during windy conditions, particularly on exposed roof edges.
• <strongMoisture Barrier: Both provide a moisture barrier, but 30# usually offers a longer lasting layer under harsher weather scenarios.
• <strongApplication: 30# is less prone to wrinkling, aiding quicker, cleaner installation.
• <strongCost: 30# costs more per square, impacting overall project budgets.

Understanding these factors helps determine the most appropriate option for a given roof assembly and regional climate.

Installation Tips

Proper installation maximizes the effectiveness of either underlayment. Begin with clean, dry decking and inspect for protrusions or damage. Roll out the felt with the longer eave overhang toward the drip edge to promote water runoff. Align seams with weather exposure guidelines, commonly two to four inches depending on manufacturer and climate. Fasten with corrosion-resistant staples or roofing nails per local code, typically through all layers at prescribed intervals. Avoid stretching the felt, which can cause wrinkles that compromise adhesion.

In windy locations, add extra fasteners at edges and corners and follow manufacturer-recommended lap directions to maintain a continuous barrier. When re-roofing, remove old felt to prevent layering issues unless the existing underlayment is in good condition and compatible with the new shingles.

Durability And Weather Performance

Roofing felt serves as a secondary layer of protection against wind-driven rain and minor moisture intrusion during shingles installation. 30# felt generally provides superior durability and resistance to tearing in the field, which is beneficial on larger roofs or structures with challenging geometry. In hot climates, both types may experience asphalt bleed or distortion if not installed with proper tension. In cold climates, 30# felt tends to maintain integrity better under freezing and thawing cycles. It is important to pair underlayment with appropriate shingles and ventilation to optimize long-term performance.

Code And Standards

Most U.S. codes reference asphalt-saturated roofing felt as a standard underlayment component, with ASTM D226 and D2527 among commonly cited specifications addressing asphalt-saturated felt properties. Local jurisdictions may require specific wind-load or exposure ratings, particularly in coastal regions or areas prone to severe weather. Manufacturers publish exposure and installation guidelines; following these ensures warranty validity. Builders should verify compatibility with the chosen shingles and any additional underlayment layers, such as self-adhesive ice and water shields in most cold-weather zones.

Choosing The Right Felt For Your Roof

When selecting between 15# and 30# roofing felt, consider climate, roof slope, and roof size. For mild climates, low-slope roofs, or cost-focused projects, 15# may suffice, especially when paired with sturdy shingles and proper ventilation. For roofs with higher exposure, steeper pitches, or locations with heavy rainfall or strong winds, 30# offers enhanced protection and ease of installation. For renovations or smaller projects, evaluate whether the time saved with 30# offsets its higher material cost. Always consult local codes and manufacturer guidelines.

Additionally, consider future maintenance and potential roof replacements. Heavier underlayment can contribute to a longer-lasting roof deck barrier, particularly on roofs with extended exposure during installation. If in doubt, consult a local roofer or building official to ensure compliance and best practice for a specific project.

Common Myths

Myth: Heavier felt guarantees a leak-free roof. Reality: Underlayment is a defensive layer, but proper installation, ventilation, and shingle quality determine performance. Myth: More felt always means better protection. Reality: Excess weight increases cost and handling complexity without proportional benefit in many scenarios. Myth: All roofs need ice-and-water shield everywhere. Reality: Shield placement depends on climate, slope, and anticipated water exposure; follow code and manufacturer guidance for critical areas.