Difference Between 15 Roofing Felt and 30 Roofing Felt

When selecting roofing felt for a project, understanding the distinction between 15 roofing felt and 30 roofing felt is essential. Both are asphalt-saturated felts used as underlayment in many shingle roofs, but they differ in weight, durability, and suitability for specific climates and building codes. This article explains how 15 roofing felt and 30 roofing felt compare, and how to choose the right option for lasting protection and compliance.

What Each Type Is And How It Is Measured

Roofing felt, also called tar paper or underlayment, is typically sold by weight per 100 square feet. The terms 15 and 30 refer to the felt’s weight rating in pounds per 100 square feet. In practice, 15 roofing felt is lighter, more pliable, and easier to handle on steeper or irregular roofs. 30 roofing felt is heavier, more robust, and better at resisting tearing and wind uplift in challenging conditions. Both are asphalt-saturated felts designed to provide a secondary weather barrier beneath the final roofing material.

Durability And Tear Resistance

The key functional difference is durability. 30 roofing felt offers superior tear resistance and puncture tolerance due to its heavier weight and thicker construction. This makes it less prone to damage during installation and more capable of withstanding foot traffic, ladder impact, and minor hail or debris contact. 15 roofing felt is more susceptible to tearing or stretching under stress, which can be a consideration on high-wall, low-slope, or complex roof lines where movement is more likely. For harsh climates or complex roofs, the added durability of 30 roofing felt can reduce the risk of exposing the roof deck to moisture during weather delays.

Exposure Time And Weather Resistance

Exposure time refers to how long the underlayment can remain exposed to the elements before the final roofing material is installed. In many regions, extensive sun exposure is common during installation. 30 roofing felt generally performs better for longer exposure periods due to its thicker material and rugged construction. However, properly installed and promptly covered 15 roofing felt can still provide adequate protection for short-to-moderate exposure windows. Builders must follow local building codes and manufacturer guidelines to ensure the underlayment remains effective while awaiting shingles or tiles.

Codes, Standards, And Regional Use

Local building codes often specify requirements for underlayment strength, wind uplift resistance, and exposure limits. In some areas with high wind risk, steeper slopes, or heavy precipitation, 30 roofing felt is favored for the added security against wind-driven moisture. In milder climates or retrofit work where weight constraints are a concern, 15 roofing felt may be acceptable if it meets the code and the installation plan is carefully managed. Always verify with local code authorities and the roof manufacturer to ensure compliance for a given project.

Installation Considerations And Handling

Handling and installation practices differ between the two products. 15 roofing felt is more flexible and easier to work with on tight roof geometry, making it quicker to lay out and staple or fasten. It tends to lay flatter on curved surfaces and reduces worker fatigue on ladders or steep pitches. 30 roofing felt requires more careful alignment and heavier fastening due to its weight, but it creates a sturdier base for the final roof covering. When using either type, installers should follow manufacturer guidelines for nailing patterns, overlaps, and seam sealing to prevent moisture intrusion.

Seaming, Overlaps, And Water Management

Both 15 roofing felt and 30 roofing felt rely on proper overlaps to shed water effectively. Typical overlaps are 2 inches for horizontal seams and 4 to 6 inches for vertical overlaps, but these clearances can vary by product and region. 30 roofing felt may require more careful seam sealing because its greater stiffness can make it harder to create uniform folds on complex skylight or vent details. Regardless of weight, ensuring clean, straight overlaps and secure fasteners minimizes wind uplift and water infiltration.

Ventilation, Insulation, And Long-Term Performance

Underlayment is one layer in a roof’s overall performance. While roofing felt primarily protects the deck from moisture before the final roof covering is installed, the chosen weight can influence long-term performance. Heavier 30 roofing felt can contribute to a more resilient baseline under heavy precipitation or freezing cycles, helping reduce failure risk in cold climates. Lighter 15 roofing felt can deliver adequate protection when the roofing system is installed promptly and the climate is less severe. Consider the broader roof assembly, including insulation and ventilation, to optimize performance.

Cost And Availability

Cost differences reflect material weight and manufacturing complexity. 15 roofing felt is typically cheaper per square foot and may be preferred on budget projects or where weight constraints exist. 30 roofing felt often costs more due to its thicker, tougher construction and higher tear resistance. Availability varies by supplier and region, so it is advisable to compare prices and confirm stock for planned installation timelines.

Choosing The Right Underlayment For A Project

To determine whether 15 roofing felt or 30 roofing felt is appropriate, consider the following factors: climate severity, roof pitch, roof deck condition, expected exposure duration before shingle installation, and local building codes. For new construction in harsh winter climates with potential delays, 30 roofing felt provides extra durability. For straightforward installations on milder climates, and where weight or cost is a concern, 15 roofing felt can be a practical choice, provided it aligns with code requirements and manufacturer recommendations.

Practical Quick Reference

• Weight: 15-lb vs 30-lb per 100 sq ft
• Durability: 30-lb offers higher tear and puncture resistance
• Exposure: 30-lb generally better for longer exposure
• Installation: 15-lb easier to handle; 30-lb more robust during work
• Code Compliance: Check local codes and manufacturer specs
• Cost: 15-lb typically cheaper

Summary Of Key Differences

Weight and Toughness: 30 roofing felt is heavier and more durable than 15 roofing felt. Handling: 15-lb is easier to handle; 30-lb requires more effort but offers sturdiness. Exposure And Climate: 30-lb can tolerate longer exposure and harsher climates. Code And Cost: Local codes and project budget influence the choice. In practice, the best option balances durability, installation practicality, climate, and code requirements for the specific project.