Roofing felt, or tar paper, provides an important secondary barrier against moisture and helps protect roof decking during installation. The nailing pattern used to attach underlayment is essential for durability, wind resistance, and easy shingle installation. This article explains common patterns, how to adapt them for different roof pitches and wind zones, and practical tips for achieving a secure, code-compliant installation across the United States.
What Roofing Felt Is And Why It Matters
Roofing felt is a sheet material installed over roof decking before shingles. It acts as a moisture barrier, protects insulation and decking during wind-driven rain, and helps the shingles lay flat. Underlayment patterns influence wind uplift resistance and flashing integration. A correct nailing pattern minimizes gaps and reduces the risk of blow-off in high-wind areas. Builders should follow local codes and manufacturer guidelines to ensure compatibility with the chosen roofing system.
Materials And Fasteners Used
Two common underlayment materials are traditional asphalt-saturated felt and synthetic underlayment. Felt is usually 15-pound or 30-pound weight, while synthetics offer higher tear resistance and better performance in cold weather. Nails are typically galvanized, and sometimes stainless steel or aluminum are used for coastal or corrosive environments. Staples are rarely recommended for traditional felt due to potential tear-out in hot weather. Always verify fastener length, corrosion resistance, and compatibility with the underlayment and shingles.
General Nailing Patterns For Felt Underlayment
Most felt installation follows a staggered pattern with edges and field areas secured. A typical practice is to fasten along the lower edge of each roll, then place fasteners at regular intervals across the field, ensuring no more than a few inches of open decking between nails. Common patterns include:
- Edge Nailing: Place nails along the lower edge and along the sides of the felt sheet to create a secure Belgian edge where shingles overhang. This helps resist lifting at eaves and rakes.
- Field Nailing: Space nails evenly across the field in a grid, often 6 to 8 inches apart along the width, and 12 to 18 inches apart along the length, depending on roll width and local code requirements.
- Overlap And Seams: Overlap adjacent sheets by at least 4 inches in most US climates. Stagger seams to reduce leak paths and ensure continuous coverage under the shingles above.
When margins and end joints align with roof geometry, ensure nails penetrate the deck and do not pull out under load. Avoid over-nailing at corners, which can cause the felt to wrinkle or tear. Always follow manufacturer guidelines for nail type, length, and placement to preserve warranty terms.
Underlayment Nailing For Asphalt Shingle Roofs
Asphalt shingles are the most common roofing material in the United States. The underlayment nailing pattern for shingles typically follows the manufacturer’s installation instructions, but general principles apply. Install the felt starting from the eave edge and work upward. Ensure the nails are fully driven and flush without tearing the felt. Keep nails away from roof penetrations and flashing areas to avoid leaks. In high-wind zones, the nailing pattern may be adjusted to increase nail density near rakes and hips, as recommended by the shingle and underlayment manufacturers.
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Patterns By Climate And Wind Zone
Wind uplift requirements vary by climate and local codes. In hurricane-prone or high-wind regions, increased fastener density along edges and along rakes is common. Some jurisdictions require two nails per fastener along the edges or specific spacing rules to reduce the risk of uplift. In milder climates, standard patterns may suffice, but all underlayment must still be securely fastened to prevent sagging or curling. Always consult current IRC or local building codes and the underlayment manufacturer’s wind-resistance table for precise spacing.
Common Mistakes And How To Avoid Them
- Over- or under-nailing: Both can compromise the underlayment. Use the recommended nail length and avoid penetrating through to the roof deck beyond what is required.
- Improper overlaps: Inadequate overlap creates leaks. Ensure a minimum 4-inch seam overlap as a baseline unless the manufacturer specifies otherwise.
- Insufficient edge fasteners: Edges are more prone to lift. Nail along the outer edge consistently to anchor the sheet.
- Damaged sheets: Nail holes near damaged edges can fail. Inspect sheets before installation and replace damaged pieces promptly.
Tools, Fasteners, And Safety Tips
Essential tools include a hammer or pneumatic nail gun, roofing nails with corrosion-resistant heads, a tape measure, chalk line, and utility knife for trimming. Use nails long enough to penetrate the deck by at least 3/4 inch, depending on the material thickness. For synthetic felts, the faster’s holding power may require different lengths. Always wear eye protection and ensure proper ladder or scaffold safety. Work with a partner in high-wind or steep-slope projects to improve handling and reduce risk.
Inspection And Quality Assurance
Post-installation inspection should verify nail penetration, even coverage, and adequate overlaps. Check for loose edges, wrinkles, or gaps that could lead to leaks. Ensure seams align with the shingle installation alignment and flashing is properly integrated. Document any deviations from the manufacturer’s instructions and adjust future installations accordingly. Regular maintenance checks after severe weather can catch issues early and extend roof life.
Code References And Best Practices
Code references typically come from the International Residential Code (IRC) and local amendments. Best practices emphasize using the manufacturer’s recommended underlayment and fastening pattern for the specific shingle system, roof slope, and climate. In high-wind zones, wind-resistance charts provide exact nail spacing and additional fasteners. Keeping up-to-date with code changes ensures compliance and optimal performance of the roof system.