No 15 Asphalt Felt: A Comprehensive Guide to Usage, Benefits, and Installation

No 15 asphalt felt is a lightweight roofing and underlayment material used in residential and some light commercial construction. This article explains what No 15 felt is, how it is manufactured, where it is best applied, installation steps, advantages and limitations, and how it compares with other roofing underlayments. Readers will gain practical insights for choosing the right underlayment for roof decks, attic ventilation needs, and weather protection strategies.

What Is No 15 Asphalt Felt

No 15 asphalt felt refers to a specific weight class of felt underlayment used in roofing. It is typically a weatherproof sheet made from asphalt-saturated organic felt or fiberglass-reinforced material, designed to provide a secondary barrier against moisture. The “No 15” designation historically relates to a weight in pounds per 100 square feet, indicating its relatively light texture compared with heavier felts. This product is commonly chosen for its affordability, ease of handling, and compatibility with various roofing systems.

Materials and Manufacturing

No 15 asphalt felt is produced by saturating a base felt with asphalt, then reinforcing it with nonwoven or woven fibers. The base can be organic felt (cellulose fibers) or a fiberglass substrate in modern formulations. The asphalt coating provides waterproofing capabilities, while the reinforcing layer adds tear resistance and dimensional stability. Some No 15 products include mineral granules or surface coatings to improve UV resistance and provide a surface texture for easier nailing and alignment. Quality varies by manufacturer, with typical standards focusing on tear strength, wet adhesion, and vapor permeability.

Applications and Roofing Scenarios

This lightweight underlayment serves as a secondary barrier beneath shingles, metal, or tile roofing systems. It is most commonly used on pitched roofs with adequate attic ventilation and a slope that minimizes prolonged moisture exposure. No 15 felt is suitable for non-warranted temporary moisture barriers during construction or for regions with moderate precipitation. It is less common in areas with heavy rain, snow load, or high wind where higher-weight felts or synthetic underlayments may be preferred.

Installation Guidelines

Proper installation is essential for performance. Begin by inspecting the roof deck and repairing any damage. Roll out the No 15 felt with the coarse side facing up and align parallel to the eaves. Overlap seams by is practical; common overlaps range from 19 to 19 inches depending on local codes and manufacturer recommendations. Use corrosion-resistant roofing nails or staples to secure the felt, typically through a 1/nailing pattern at the edges and field of the sheet. Do not stretch the felt during installation, which can cause wrinkles or gaps. Ensure proper ventilation and seal edges at the eaves and rakes to prevent wind-driven moisture intrusion.

Advantages of No 15 Asphalt Felt

• Cost-effectiveness: An economical option for basic moisture protection.
• Ease of handling: Lightweight and simple to install, reducing labor time.
• Compatibility: Works with a range of roofing systems, including asphalt shingles and some metal roofs.
• Temporary moisture barrier: Provides a wind-driven rain barrier during construction before final roofing material is installed.

Limitations and Considerations

• Durability: Lighter weight felts may be less resistant to tearing and UV exposure than heavier felts or synthetic underlayments.
• Weather sensitivity: Prolonged exposure to sunlight and moisture can degrade performance if not protected by final roofing material promptly.
• Code and warranty: Local building codes and manufacturer warranties may prefer heavier felts or synthetic alternatives for certain applications.
• Not a final roof: It is an underlayment, not a finished roof surface; inadequate protection can occur if used inappropriately on certain roofing assemblies.

Comparing No 15 With Other Underlayments

No 15 asphalt felt sits between heavy traditional felts and lightweight synthetic underlayments. When compared to high-grade organic felts, No 15 offers lower cost but reduced tear resistance. Compared with synthetic underlayments, No 15 typically provides less durability and UV stability but can be acceptable in milder climates or as a temporary barrier. For projects in regions with high rainfall, snow, or strong UV exposure, many builders prefer 30- or 40-pound felts or modern synthetic underlayments for enhanced performance and longer service life.

Maintenance, Inspection, and Replacement

Routine inspection after severe weather helps identify lifted edges, tears, or damp spots. If damage is found, repair or replace the affected area before final roofing is installed. No 15 is not designed for long-term water exposure; prompt installation of shingles or other primary roofing material is essential. In older homes, consider consulting local codes to determine if No 15 remains acceptable or if upgrading to a heavier felt or synthetic option is advisable for improved performance and warranty eligibility.

Best Practices for Selecting Underlayment

• Assess climate and weather: Choose materials with UV resistance and adequate moisture protection for the local climate.
• Consider roof pitch: Steeper roofs may benefit from certain overlap patterns and nailing requirements to prevent wind uplift.
• Check codes and warranties: Follow local building codes and ensure compatibility with the chosen roofing system and warranty terms.
• Balance cost and durability: Weigh the upfront savings of No 15 against potential long-term costs of repairs or replacements.

Practical Tips for Homeowners

• Plan timing: Install underlayment after the roof deck is dry and prior to final shingle installation to minimize exposure.
• Store properly: Keep rolls flat and protected from direct sunlight until use to prevent warping.
• Edge sealing: Ensure edges and eaves are sealed to resist moisture intrusion and wind-driven rain.