Difference Between 15 Roofing Felt and 30 Roofing Felt

Roofing felt, also known as tar paper, serves as an underlayment between the roof deck and the outer roofing material. In the United States, felt is commonly described by weight, with 15-pound and 30-pound options indicating the amount of asphalt soaked into the paper per 100 square feet. This article explains how 15 roofing felt and 30 roofing felt differ in protection, installation, and performance, helping builders, homeowners, and contractors make informed choices for various climates and roof types.

What Is Roofing Felt And What Do 15 And 30 Mean?

Roofing felt is a layered sheet installed over the roof deck to provide a secondary barrier against moisture and wind. The numbers 15 and 30 refer to the weight of the felt per 100 square feet, measured in pounds. A 15-lb felt is lighter and thinner, while a 30-lb felt is heavier and thicker. Both types are typically asphalt-saturated, though there are synthetic and specialty felts available. The primary purpose is to offer temporary protection and a smoother surface for the final roofing material to seal against wind and rain.

Key Differences In Protection And Performance

Durability and tear resistance: 30-lb felt generally provides greater puncture resistance and tear strength than 15-lb felt. This makes it more resilient on rough framing, around irregular shapes, and in areas with foot traffic during installation.

Moisture management: Both felts act as moisture barriers, but the thicker 30-lb material can shed water more effectively during heavy rain or snow events before the final roof is installed. The extra thickness also helps resist damage from wind-driven rain at exposed edges.

Sag and wrinkle resistance: Heavier felt is less prone to wrinkling when nailed or stapled, leading to a flatter, more uniform surface for shingles or other roofing materials to lay on. This can reduce air pockets and improve the appearance of the finished roof.

Thermal performance and climate suitability: In colder climates or on steeper roofs, the higher tear resistance and added durability of 30-lb felt can offer better long-term protection against wind uplift and hail impact. 15-lb felt may be adequate in milder climates or on newer, well-ventilated roof decks with low risk of damage.

Installation Considerations And Code Implications

Both 15-lb and 30-lb felts are installed in overlapping layers to provide continuous coverage. Typical practice calls for a headlap and sidelap to ensure water shedding. Common guidelines suggest a headlap of about 4 inches and a sidelap of around 2 inches for acceptable performance, though these can vary by local codes and the specific roofing system. Always follow the manufacturer’s installation instructions and local building codes.

Underlayment choice can be influenced by roof type, slope, and climate. Some codes require a minimum underlayment for certain roof coverings, and the decision between 15-lb and 30-lb felts may hinge on whether the roof will receive asphalt shingles, wood shingles, or tile. In high-wind regions, some contractors prefer 30-lb felt for added resilience against wind uplift during storms.

When To Choose 15-Lb Vs 30-Lb Felt

• <strongBudget-conscious projects: 15-lb felt is typically cheaper and may be suitable for steady, moderate climates with low wind exposure, provided there is proper deck preparation and ventilation.
• Weather and climate considerations: For regions with heavy rain, snow, or strong winds, 30-lb felt offers extra protection due to greater thickness and tear resistance.
• Roof type and slope: On steeper or more complex roofs, 30-lb felt can be easier to manage during installation because it’s less prone to tearing and wrinkling.
• Durability and long-term performance: If the roof is expected to endure extreme weather or is part of a high-value installation, 30-lb felt is generally the more durable choice.

Cost, Availability, And Practical Considerations

Cost differences between 15-lb and 30-lb felts reflect their material thickness and durability. 30-lb felt tends to be more expensive per square foot, but the added protection might reduce risk of leaks and wind damage. Availability is broad for both types at major home improvement retailers and roofing specialty suppliers. Contractors often choose 30-lb felt in regions with harsh winters or where the roof is subject to higher wind uplift, while 15-lb felt remains common for standard residential applications in milder climates.

Practical Tips For Use

• Inspect the deck: Ensure a clean, dry, and smooth surface before installing any felt to maximize adhesion and performance.
• Follow overlapping guidelines: Maintain proper headlaps and sidelaps as per manufacturer instructions and local codes to minimize water intrusion risks.
• Ventilation matters: Adequate attic ventilation reduces condensation buildup under the roof deck, complementing the function of the underlayment.
• Combine with appropriate roof coverings: Choose roofing materials compatible with underlayment type to achieve optimal waterproofing and long-term performance.

In summary, the choice between 15 roofing felt and 30 roofing felt hinges on anticipated weather exposure, roof design, climate, and budget. For many U.S. homes, 30-lb felt offers improved durability and protection in challenging conditions, while 15-lb felt remains a cost-effective option for straightforward applications in milder environments. Consulting local building codes and a qualified roofing professional can help determine the best underlayment for a specific project.