The choice of roof underlayment plays a critical role in protecting a home from moisture, wind-driven rain, and ice damming. Felt underlayment, a traditional option, sits between the roof deck and the shingles, providing an extra layer of defense and a pathway for ventilation. This guide explains what felt underlayment is, the different types, installation practices, and how to choose the right material for American roofing projects.
What Is Felt Underlayment
Felt underlayment is a sheet material installed over the roof deck before shingles are laid. It serves as a secondary barrier to moisture infiltration and helps seal the roof surface during installation. Made from asphalt-saturated felt or synthetic fibers, it can add temporary weather protection and improve the roof’s overall durability when used correctly. Homeowners should understand that underlayment does not replace proper flashing, ventilation, or shingle selection but complements them as part of a complete roofing system.
Types Of Felt Underlayment
There are two primary categories: asphalt-saturated felt and synthetic underlayment. Asphalt-saturated felt comes in 15-pound and 30-pound variants, with a feltback or cap sheet finish. It is affordable and historically common in many U.S. regions. Synthetic underlayment uses nonwoven polymer materials, offering higher tear resistance, lighter weight, and better performance in extreme weather. Synthetic options frequently come in 60 or 90 or higher mil equivalents, with advantageous slip resistance and UV stability. The right type depends on climate, roof slope, and local building codes.
Roofing Codes And Weather Resistance
Building codes often specify minimum underlayment requirements based on climate. For example, certain high-wind or heavy-rain zones may require synthetic underlayment with enhanced moisture resistance or additional layers in critical roof areas. In many regions, codes also dictate the use of underlayment in conjunction with ice-dam protection in cold-weather zones. When selecting underlayment, verify local code requirements and manufacturer installation instructions to ensure a compliant, durable roof system.
Installation Guidelines
Proper installation is essential to maximize underlayment performance. Begin by inspecting the roof deck for smoothness and dryness, then repair any damaged boards. Roll out the underlayment parallel to the eaves, allowing for proper overhang and alignment. Overlaps typically range from 2 to 4 inches for asphalt-saturated felt and 4 to 6 inches for synthetic underlayment. Use corrosion-resistant fasteners and avoid puncturing the underlayment with nails or staples. Seal the seams where required, especially along eaves, valleys, and around penetrations.
Ventilation, Drying, And Coverage
Effective roof performance relies on a dry roof deck. Underlayment should be installed only when the deck is dry and free of dust, grease, and moisture. Adequate ventilation in the attic helps prevent condensation that can compromise the underlayment’s integrity. For low-slope roofs, some manufacturers recommend additional layers or specialty underlayment products to maintain water resistance along the seams and edges.
Advantages And Disadvantages
Key advantages of felt underlayment include cost-effectiveness, proven track record, and a protective barrier that improves shingle lifespan in some climates. Asphalt-saturated felt is affordable and familiar to many contractors. Synthetic underlayment offers higher tear resistance, lighter weight, and better performance in ultraviolet exposure. Disadvantages include potential brittleness in extreme cold for some asphalt products and a higher initial cost for synthetic options. Proper installation is essential for both types to prevent moisture intrusion.
Maintenance And Replacement
Underlayment does not typically require routine maintenance beyond inspection during roof work. Inspect after severe weather for signs of wear, punctures, or blistering. If the underlayment shows significant damage or when the roof is already at the end of its service life, replacement is necessary. Professionals often replace the deck- and underlayment together when reroofing. Timely replacement reduces the risk of leaks, wood rot, and interior damage.
Choosing The Right Felt Underlayment
Factors to consider include climate, roof slope, and the presence of ice-damming risks. For cold climates with frequent snow or ice, synthetic underlayment with enhanced water resistance and longer wear life is often preferred. In milder regions, asphalt-saturated felt may provide a cost-effective option. Look for certifications, UV resistance ratings, tear strength, and compatibility with the chosen shingles. Always follow the manufacturer’s installation guidelines and local building codes to maximize performance.
Common Mistakes To Avoid
- Choosing the wrong type for climate or roof slope
- Installing on a damp or dirty deck
- Inadequate seam overlaps or improper fastener usage
- Failing to seal or flash around penetrations and joints
- Not coordinating underlayment with ice-dam protection where required
By avoiding these missteps and adhering to best practices, homeowners can ensure that felt underlayment contributes effectively to a durable, weather-resistant roof system.