Roofing felt, also known as underlayment, goes between the roof deck and the shingles. In the United States, thickness is typically described by weight per 100 square feet—often 15-lb, 30-lb, or 40-lb felt. Actual physical thickness varies by product and manufacturing tolerances, so two 30-lb sheets may feel slightly different even though both are labeled the same weight. This guide explains what thickness means, common ranges, and how to choose the right underlayment for different climates and roof types.
What Is Roofing Felt?
Roofing felt is a protective layer installed over the roof deck to provide temporary weather resistance and a stable surface for shingles. Traditional felt is asphalt-saturated paper or fiberglass mat, while many modern products are synthetic underlayment made from polymers. The term “thickness” is a shorthand for the product’s strength and durability, which tracks closely with weight per 100 sq ft. In practice, builders select felt by weight (15-, 30-, or 40-lb) since that correlates with how much asphalt and how robust the sheet feels during installation.
Common Thicknesses And Weights
Most residential roofs use one of three common underlayment weights. 15-lb felt is the lightest option and is widely used for standard installations in moderate climates. 30-lb felt offers greater tear resistance and puncture protection, making it a popular middle-ground choice for windy or challenging sites. 40-lb felt is the heaviest traditional option, providing the most durability for demanding conditions or heavy-duty applications. Actual thickness often ranges roughly from 0.012–0.020 inches for 15-lb, 0.018–0.028 inches for 30-lb, and 0.028–0.040 inches for 40-lb, though exact figures vary by brand.
Thickness is not the sole determinant of performance. Some manufacturers offer synthetic underlayment with different thickness metrics measured in mils (thousandths of an inch). These products can be lighter or heavier than felt while delivering comparable protection. Always check the product data sheet for exact thickness and performance specs.
How Thickness Affects Performance
Thickness influences several key performance aspects. Heavier underlayment generally provides better tear and puncture resistance, which helps on rooftops with fingernails, fasteners, or rough decking. This can reduce installation damage and extend protection during construction. Thicker or heavier felt also tends to resist wind uplift better on exposed edges, minimizing billowing during high winds. However, thicker felt does not automatically deliver superior water resistance; once installed, the weatherproofing primarily comes from the shingles and the underlayment’s condition during installation.
Thickness also matters for permanence. Thicker underlayment can withstand more traffic and punctures while workers move across the roof. It may offer more margin if the roof deck is uneven or if a builder anticipates extended deck exposure before shingle installation. Nevertheless, the optimal choice depends on climate, roof pitch, deck condition, and the shingle manufacturer’s requirements.
Choosing The Right Thickness For Your Roof
For many standard residential asphalt shingle roofs, 15-lb felt remains a common and cost-effective option. In areas with higher winds, heavier snowfall, or steeper slopes, builders often opt for 30-lb or even 40-lb felt to increase durability and reduce the risk of punctures during installation. In some climates or building codes, a heavier underlayment may be required by the shingle manufacturer or local regulations.
It’s important to align underlayment thickness with both the deck condition and the roof system. If the roof deck is uneven or damaged, thicker underlayment can help bridge minor irregularities. If a roof uses synthetic underlayment, check the manufacturer’s installation instructions, as some products are designed to work with specific thickness ranges or wind-driven rain requirements. When in doubt, consult a licensed contractor who understands local codes and climate considerations.
Installation Considerations For Different Thicknesses
Installation practices vary with thickness. Lapping guidelines typically require overlaps of 2 to 4 inches along seams, with courses staggered to ensure water shedding. Nails or fasteners should be placed on the panel surface away from overlaps to avoid loosening or leaks. In windy regions, more secure fastening and careful sealing around penetrations are essential, particularly for thicker felt that can resist deformation during installation.
Storage and handling matter as well. Felt stored in direct sunlight or at extreme temperatures can become brittle or overly pliable, affecting performance and ease of cutting. Keep rolls off the ground, avoid folding, and install promptly after unrolling to prevent moisture absorption before the roof deck is finished.
Durability, Lifespan, And Maintenance
Underlayment is not intended to last as long as the roof itself. Thickness contributes to durability during construction and the early life of the roof, but degradation from UV exposure, moisture, and temperature cycles is still a factor once the shingles are installed. Proper ventilation, correct overlap, and adherence to the shingle manufacturer’s specifications influence how long the underlayment performs as designed. Replacement is generally not anticipated within the life expectancy of the roofing materials unless damage occurs.
When maintenance or re-roofing is considered, assess the current underlayment’s condition rather than relying solely on its age or thickness. Damaged sections, persistent moisture, or signs of delamination should prompt replacement with a product that meets the latest code and manufacturer guidelines.
Common Misconceptions About Roofing Felt Thickness
One common myth is that heavier always means better regardless of conditions. While thicker underlayment provides more tear resistance, it may not be necessary for every roof type or climate. Another misconception is that all felt lasts the same; performance varies by material (felt vs. synthetic) and by how well the product is installed. Finally, many homeowners overlook that thickness alone cannot compensate for poor deck condition or improper ventilation. Choosing the right product requires considering climate, roof design, and shingle manufacturer specs.
Frequently Asked Questions
- Q: Is thicker underlayment always better for snow loads? A: Not always. It helps with puncture resistance and wind uplift, but installation quality and roof design are critical. Follow the shingle maker and local code requirements.
- Q: Can I mix underlayment types on one roof? A: Some projects allow combinations (felt with a synthetic layer), but always verify compatibility with shingles and weather conditions.
- Q: How does thickness relate to insulation or energy efficiency? A: Underlayment thickness has minimal impact on insulation or energy performance compared with roof color, attic ventilation, and insulation thickness.
- Q: Do new codes require a specific thickness? A: Codes vary by jurisdiction and roof type. Check local building codes and the manufacturer’s installation instructions.