Roof felt, also known as roofing underlayment, plays a critical role in protecting a home from moisture and weather. The thickness of roof felt, typically measured in mils (thousandths of an inch) or gauge, affects durability, tear resistance, and how well it sheds water during storms. Homeowners should understand common thickness ranges, how to choose the right option for their climate and roof type, and what installation practices influence performance. This guide explains standard roof felt thickness, factors that drive selection, and practical tips for installation and maintenance.
Understanding Roof Felt Types
Roof felt comes in several types, mainly differentiated by material and construction, with thickness being a key performance indicator. The most common underlayment materials in the United States are:
- Traditional felt underlayment made from asphalt-impregnated felt, or tar paper, which provides basic moisture protection and greater tear resistance than paper alone.
- Synthetic underlayment made from polypropylene or polyethylene fibers, offering higher tear strength, better coverage, and improved walkability.
- Rubberized asphalt underlayment combines waterproofing benefits with sheet flexibility, often used in valleys and around roof penetrations.
The thickness level within each type influences durability and performance. In general, thicker materials resist tearing and punctures better and provide longer-lasting protection in harsh weather.
Typical Thicknesses For Roof Felt
Thickness is commonly specified in mils, with common ranges including:
- 15 to 30 mils (0.015 to 0.030 inches): Typical for traditional felt and many synthetic underlayments in standard residential applications. Provides adequate protection for many climates and roof pitches.
- 40 to 60 mils (0.040 to 0.060 inches): Heavier underlayments used in areas with severe weather, high wind uplift, or roofs with high wind exposure. Offers superior tear resistance and puncture protection.
- 60 to 90 mils (0.060 to 0.090 inches): Premium or specialty applications, including roofs with heavy foot traffic, exposed installations for longer periods before final roofing, or retrofit projects with extra protection.
In some products, manufacturers use gauge or weight-based metrics rather than mils, but mils remain the most consistent reference in the industry. Always check the product data sheet for exact thickness and performance specs.
Key Factors That Influence Felt Thickness Choice
Several conditions determine the appropriate thickness for a given roof:
- Climate and weather exposure: Regions with heavy rain, hail, snow, or strong winds benefit from thicker underlayment to resist punctures and provide longer moisture protection.
- Roof pitch and slope: Steeper roofs may require thicker, more tear-resistant underlayment to handle foot traffic and protect against wind-driven water.
- Roof covering and installation window: If shingles are installed soon after underlayment, standard thickness is usually sufficient. Delayed installations or temporary roofing may justify thicker options.
- Ventilation and moisture risk: Poor attic ventilation increases moisture stress on underlayment; higher thickness can add durability but not replace proper ventilation.
- Local building codes and manufacturer guidelines: Codes often specify minimum underlayment standards; always adhere to local requirements and follow roof system instructions.
Choosing The Right Thickness For Your Roof
Selecting the right thickness involves evaluating performance needs against cost. Consider the following:
- Residential flat or low-slope roofs: Thicker underlayment (40–60 mils) provides better protection against standing water and punctures from debris.
- Steep-slope residential roofs: Standard 15–30 mils is common, but in exposed climates or high-wind zones, a tougher synthetic option may be warranted.
- Extreme weather regions: In areas with frequent hail, sleet, or snow, a thicker synthetic or rubberized asphalt underlayment can offer enhanced durability.
- Budget considerations: Thicker underlayment costs more upfront but can reduce risk of leaks and roof damage, potentially saving repair costs over time.
For most American homes, a high-quality synthetic underlayment in the 15–40 mil range strikes a balance between protection and cost, while thicker options are reserved for harsh climates or special construction needs.
Installation Considerations And Best Practices
Proper installation is essential to realize the benefits of underlayment thickness. Key practices include:
- Overlaps: Ensure correct overlaps per product instructions, typically 4 to 6 inches along seams, to prevent water intrusion.
- Fastening: Use the recommended nails or staples and spacing to prevent sagging or wind uplift, especially on high-wind roofs.
- Seams and penetrations: Seal all seams and around penetrations with compatible tapes or sealants as specified by the manufacturer.
- Deck preparation: A clean, dry deck is essential. Remove debris to prevent punctures and ensure uniform adhesion or laying.
- Ventilation: Adequate attic ventilation remains critical. Underlayment cannot compensate for poor ventilation and may fail prematurely in overheated roofs.
Thicker underlayment can be less flexible and heavier to install. Workers should take care on steep pitches and use roof jacks or safety systems as required by local regulations.
Maintenance, Inspection, And Longevity
While underlayment is a temporary layer, its condition matters for the roof’s long-term performance. Maintenance tips include:
- Regular inspections: Check for tears, punctures, or loose edges after storms, especially on roofs with thicker underlayment that may hide underlying issues.
- Repair strategy: Promptly address any damage with compatible repair materials to maintain the barrier’s integrity.
- Weather considerations: In regions with severe weather seasons, inspect after major events to detect impact damage that could compromise waterproofing.
- Compatibility with roofing materials: Use underlayment that is compatible with the final roofing material to avoid chemical reactions or adhesion problems.
With proper installation and routine checks, underlayment thickness contributes to a roof’s resilience, energy efficiency, and moisture protection over decades.