How To Attach Roofing Felt: The Complete Professional Installation Guide

Attaching roofing felt—also known as underlayment—requires rolling the material horizontally across the roof deck from the lowest point upward, maintaining strict overlap margins, and securing it temporarily with corrosion-resistant cap nails before applying the primary shingle layer. Following standardized installation protocols ensures long-term moisture resistance, protects the structural roof deck from wind-driven rain, and extends the overall service life of your roofing system.

Pre-Operation & Equipment Checklist

Proper preparation is the single most critical factor in achieving a watertight, wind-resistant underlayment installation. Before stepping onto the roof, assemble all necessary materials, verify weather conditions, and equip yourself with the correct personal protective equipment (PPE).

• Essential Gear, Tools, and Materials:
  
  
  • Heavy-duty roofing felt (ASTM D4869 or ASTM D2269 approved, typically 15 lb or 30 lb organic or synthetic).
  • Plastic cap nails (also known as cap staples or ring-shank roofing nails with plastic caps) to prevent tearing under wind loads.
  • Hammer tacker or pneumatic stapler for fast temporary positioning.
  • Utility knife with hook blades for clean, safe cuts.
  • Chalk line and a 100-foot tape measure.
  • Roofing ladder, safety harness, and OSHA-compliant anchor system.
  • Broom or leaf blower for surface debris removal.
• Mandatory Prerequisite Knowledge and Standards:
  
  
  • Understanding slope requirements: Standard felt can be applied on slopes of 4:12 or greater, while low-slope applications (2:12 to 4:12) require modified overlap techniques or self-adhering ice and water shield.
  • Reviewing local building codes regarding underlayment weight and fire-rating classifications.
• Estimated Budget and Duration Benchmarks:
  
  
  • Material costs average $20 to $50 per roll depending on whether you choose traditional organic felt or advanced synthetic underlayment.
  • A standard 2,000-square-foot roof typically requires 4 to 6 hours for complete, meticulous felt installation by a two-person crew.

Step-by-Step Underlayment Installation Workflow

Step 1: Surface Preparation and Eaves Drip Edge Installation

Clear the entire plywood or OSB roof deck of all protruding nails, staples, sap, sawdust, and grit using a stiff-bristle broom. Inspect the decking panels for delamination, soft spots, or warping, replacing any compromised wood before proceeding. Install metal drip edge flashing along the eaves directly against the wooden deck, securing it with roofing nails spaced every 8 to 10 inches.

Pro-Tip: Always install the drip edge along the eaves under the underlayment, but install the drip edge along the rakes (gables) over the underlayment to direct cascading water away from the fascia boards.

Step 2: Laying the First Course at the Eaves

Unroll the roofing felt horizontally along the bottom edge of the roof, starting at the lowest corner (the eave line). Align the bottom edge of the felt so it hangs approximately 1/4 inch to 3/8 inch past the metal drip edge, allowing water to shed cleanly into the gutters without pooling on the wood. Roll out a 10-to-15-foot section, pull it taut to eliminate wrinkles and buckles, and tack it temporarily with a hammer tacker.

Warning: Never leave felt loose or unfastened on a roof deck if high winds or rain are forecasted; secure every square foot immediately upon rolling.

Step 3: Fastening the Underlayment Correctly

Drive plastic cap nails or round-head roofing nails through the felt into the roof deck. Place fasteners approximately 12 inches apart along horizontal rows and every 6 inches along side-lap seams.

Pro-Tip: Avoid driving nails too deep, which can cut through the felt, or too shallow, which can cause the shingle surface above to bulge. The plastic cap should sit flush against the material without compressing or puncturing the felt matrix.

Step 4: Overlapping Subsequent Courses Upward

Unroll the second course of felt directly above the first, maintaining a strict horizontal alignment. Traditional 15 lb and 30 lb felt requires a minimum 2-inch horizontal side lap, while synthetic underlayments often feature printed overlap lines indicating a 3-inch or 4-inch margin.

Pro-Tip: Always overlap upper courses over lower courses (shingle-style) to ensure that gravity pulls moisture downward and over the seams rather than beneath them. For end laps where two rolls meet horizontally, overlap the material by a minimum of 6 inches.

Step 5: Detailing Valleys, Ridges, and Penetrations

Continue rolling courses upward toward the ridge line, maintaining proper overlaps until the entire deck is covered. In roof valleys, lay a vertical strip of underlayment at least 36 inches wide down the center of the valley before running horizontal courses into it, overlapping the valley liner by at least 6 inches. Seal around plumbing vents, chimneys, and exhaust fans using step flashing, vent boots, and compatible roofing cement or self-adhering flashing tape integrated over the surrounding felt.

Roofing Underlayment Material Comparison

Common Site Failures and Field Fixes

• Root Cause: Wrinkling and buckling of the felt due to moisture absorption before shingle installation.
  
  
  • Actionable Fix: Cut along the crest of the wrinkle with a utility knife to relieve tension, press the material flat against the deck, and secure the edges with additional roofing nails or dab a layer of roofing cement underneath.
• Root Cause: Fastener pull-through and wind damage during a storm before shingle application.
  
  
  • Actionable Fix: Replace traditional smooth-shank nails with approved plastic cap nails, which offer significantly greater holding power and surface area to resist wind uplift.
• Root Cause: Leaking around roof penetrations where felt meets vent boots or chimneys.
  
  
  • Actionable Fix: Strip away the damaged felt around the penetration, install a layer of self-adhesive waterproofing membrane extending at least 12 inches up the slope and sides, and re-integrate the perimeter with standard underlayment laps.

Frequently Asked Questions

Do I really need roofing felt under modern architectural shingles?

Yes, building codes universally require an approved underlayment beneath asphalt shingles. Felt acts as a temporary weather-resistant barrier during construction, provides a secondary layer of defense against wind-driven rain, and prevents harmful chemical reactions between asphalt shingles and bare wood decking.

Can I use staples instead of plastic cap nails to attach felt?

While pneumatic staples or hammer tacker staples are frequently used for temporary holding during installation, they should never be relied upon as the primary permanent fastener. Staples have small crowns that easily pull through felt under wind loads; always use roofing nails with plastic caps for long-term security.

How long can roofing felt be left exposed to the sun and rain?

Traditional organic asphalt felt (15 lb or 30 lb) should generally be covered with shingles within 30 days, as prolonged exposure to ultraviolet rays degrades the asphalt saturation. Advanced synthetic underlayments can withstand direct UV exposure for 6 to 12 months depending on the manufacturer specifications.

Should I install roofing felt vertically or horizontally?

Roofing felt must always be installed horizontally, starting from the lowest point at the eaves and working your way up toward the ridge. This horizontal orientation ensures that water runs smoothly over the overlapping seams rather than penetrating underneath them.

What is the difference between 15 lb and 30 lb roofing felt?

The designations 15 lb and 30 lb historically referred to the weight of one square (100 square feet) of felt paper, though modern manufacturing often yields lighter actual weights. 30 lb felt is significantly thicker, more resistant to foot traffic, less prone to wrinkling, and provides better protection on roofs exposed to harsh weather conditions.

Secure Your Roof with Professional Underlayment Practices Today

Mastering the precise techniques of overlapping, fastening, and detailing ensures your roofing underlayment provides decades of dependable protection against water intrusion. Take your time during the setup and execution phases, and always adhere to local building codes and manufacturer specifications for a resilient, weather-tight roofing system.