Roof felt is a critical underlayment layer that protects structures from moisture and weather during installation and after. Thickness, typically described by weight in pounds per 100 square feet (per 100 sq ft), influences durability, tear resistance, and ease of installation. This article explains common felt thicknesses, the differences between traditional asphalt-saturated felt and modern synthetic underlayments, and how to choose the right option for a given roof.
What Roof Felt Is And Why Thickness Matters
Roof felt serves as a secondary moisture barrier beneath shingles or tiles. It fills gaps around fasteners and provides temporary protection during weather events or construction delays. The thickness of roof felt correlates with its weight class: a heavier felt generally offers greater tear resistance and a longer service life, but may be stiffer and harder to install in tight details. In the United States, two main categories are used: traditional asphalt-saturated felt and modern synthetic underlayments. Each category uses weight or thickness as a proxy for strength and durability.
Traditional Asphalt-Saturated Felt Thickness
Traditional felt, often called asphalt-saturated felt, is commonly available in 15‑pound and 30‑pound grades per 100 sq ft. The weight designation reflects how much asphalt and reinforcement the roll contains. In practice, 15‑lb felt is thinner and more economical, while 30‑lb felt provides greater tear resistance and durability, especially in high-wind or Utah-style dry climates where fastener pull-out can be a concern. Some regions also use 40‑lb or higher for specific roof designs, though 15‑lb and 30‑lb remain the standard choices for most asphalt shingle installations.
Thickness can vary by manufacturer and product line, and actual measurements are influenced by the asphalt content and the fiberglass or organic mat used as a base. As a rule of thumb, 30‑lb asphalt felt is noticeably stiffer and less prone to wrinkling than 15‑lb felt when applied over rough roof decks. For reroofing projects, compatibility with existing shingles, roof pitch, and local building codes determine the appropriate felt grade. Always follow the shingle manufacturer’s installation instructions and local code requirements when selecting thickness.
Synthetic Underlayment Thickness
Synthetic underlayments have grown in popularity due to superior tear resistance, UV stability, and water resistance. These products are often described by their weight, sometimes labeled as “denier” or simply by thickness in millimeters or inches. Typical synthetic underlayments might feel thinner to the touch than 30‑lb felt but offer equal or greater tear strength and puncture resistance. Common guidance suggests synthetic underlayments range from roughly 0.5 mm to 1.5 mm in thickness, depending on the product line.
Compared with traditional felt, synthetic underlayments generally provide better performance in high-wan–wind zones, faster installation, and easier handling on steep pitches. They also tend to have enhanced moisture resistance and can be more forgiving in temperature extremes. However, installation details, such as nailing patterns and overlap requirements, vary by brand, so it is important to consult the product data sheet and follow the roof assembly instructions.
Don’t Overpay for Roofing Services – Call 877-801-4315 Now to Compare Local Quotes!
How To Choose Based On Climate And Roofing System
Choosing the right felt thickness depends on climate, roof design, and expected load conditions. Consider the following factors:
- Climate: In areas with heavy rainfall, snow, or persistent humidity, a more robust underlayment (heavier felt or higher-grade synthetic) can reduce the risk of moisture intrusion during storms or construction delays.
- Roof Pitch: Low-slope roofs often require more durable underlayments due to increased exposure time to rain; higher-load felt or synthetic options may be preferred on these surfaces.
- Wind Exposure: Regions with high wind speeds while installing or during storms benefit from stronger, tear-resistant underlayments, minimizing the chance of wind uplift and wrinkles.
- Shingle or Tile Type: The roofing material paired with the underlayment affects compatibility. Some shingle manufacturers specify underlayment requirements that influence the choice of thickness.
- Code and Manufacturer Requirements: Local building codes and shingle manufacturers’ installation guidelines often dictate the acceptable underlayment type and thickness, especially for reroof projects or in historic districts.
In practice, many U.S. installations use 30‑lb asphalt felt for traditional roofs or a high-quality synthetic underlayment rated for exposure to UV and weather. Contractors increasingly favor synthetic products for their reliability in diverse conditions and faster installation times, especially on larger or steeper roofs. Always verify compatibility with the roofing system and local codes before selection.
Installation Considerations And Compliance
Application methods influence the effective performance of any roof felt. Key considerations include:
- Overlap and Fastening: Felt overlays typically require specific overlaps (for example, 2 to 4 inches on horizontal seams) and nail patterns that ensure uniform protection. Incorrect overlaps can create moisture pathways.
- Sag and Wrinkle Prevention: Heavier felt and stiff synthetic underlayments can help minimize wrinkles, but improper handling during installation can still cause wrinkles that trap moisture.
- Deck Preparation: A clean, dry deck is essential to maximize adhesion and performance. Debris or wet surfaces can impair the underlayment’s effectiveness, regardless of thickness.
- Ventilation and Drainage: Adequate attic ventilation reduces condensation risks beneath the underlayment, complementing the protective layer with a properly vented roof system.
- Code Compliance: Some jurisdictions require specific underlayment thickness or type for new construction or reroofing. Always confirm with local building departments and follow the International Residential Code (IRC) guidance and manufacturer instructions.
Maintenance and inspection after installation also matter. Inspect for lifted edges, pooled moisture, or tears after severe weather. Damaged sections should be repaired promptly to maintain a continuous barrier against moisture.
Practical Quick Reference
- <strongTraditional felt: 15‑lb to 30‑lb per 100 sq ft; thicker grades offer higher tear resistance and durability.
- <strongSynthetic underlayment: Often thinner in feel but stronger in performance; thickness expressed in millimeters or engineered weight; designed for UV resistance and long service life.
- <strongDecision factors: Climate, roof slope, wind exposure, shingle type, and local code requirements guide the choice between asphalt felt and synthetic underlayments and the specific thickness.
For homeowners planning a roof replacement or new roof installation, consult with a licensed roofing contractor who can recommend the appropriate felt thickness and product type for the specific climate and roof design. A properly chosen underlayment thickness and material type contributes to long-term protection, ease of maintenance, and overall roof performance.