How To Put Felt On A Roof: Step-by-Step Professional Installation Guide

To successfully install roofing felt, lay the underlayment starting from the lowest point of the eaves working upward, ensuring a minimum horizontal overlap of 4 inches (100 mm) and a vertical headlap of at least 2 inches (50 mm) on steep slopes. Secure the material using hot-dip galvanized clout nails spaced every 8 to 12 inches along the underlying rafters or roof deck, or apply a dedicated cold-applied bitumen adhesive for flat systems. This systematic overlapping and fastening method creates an impermeable secondary weather barrier conforming to ASTM D226 and BS 5534 standards.

Pre-Installation Planning & Material Specifications

Before beginning any roofing project, proper preparation of the deck and selecting the correct grade of underlayment are critical to preventing long-term structural failure. Traditional asphalt-saturated organic felt remains a highly reliable choice for moisture management due to its unique ability to "breathe," allowing interior water vapor to escape while blocking exterior liquid water.

For residential pitched roofs, the standard choice is either 15-pound (Type I) or 30-pound (Type II) felt. Flat roofs or outbuildings (like garden sheds) typically utilize thicker, mineral-surfaced cap sheets or polyester-reinforced torch-on felts. Ensure that the roof deck is completely dry, structural panels are securely fastened to the rafters, and all protruding nail heads are driven flush to prevent puncturing the new membrane.

• Essential Safety & Installation Gear: Hot-dip galvanized clout nails (minimum 20mm length with a 3/8-inch head), utility knife with hook blades, hammer or heavy-duty hammer tacker, tape measure, chalk line reel, cold roofing adhesive (for flat/low-slope applications), and fall-protection harness.
• Structural Material Specifications: 15-lb or 30-lb organic asphalt-saturated felt conforming to ASTM D226 / ASTM D4869 standards, or BS 8217-compliant polyester-reinforced felt for flat structures.
• Permissible Environmental Conditions: Ensure the ambient air and roof deck temperature is at least 40 degrees Fahrenheit (4 degrees Celsius) to prevent the felt from cracking during unrolling, and ensure there is zero surface moisture on the decking.
• Project Benchmarks: A standard 200-square-foot shed or domestic roof section requires approximately 2 to 3 hours of preparation and installation time with a two-person crew.

Sequential Execution Guide for Pitched and Flat Roof Felting

Step 1: Substrate Preparation and Deck Inspection

Inspect the entire roof deck for signs of rot, delamination, or structural sagging. Ensure all plywood or Oriented Strand Board (OSB) panels are spaced with a 1/8-inch gap at the joints to allow for natural thermal expansion. Clean the entire surface using a stiff-bristle broom to remove sawdust, dirt, and construction debris.

Check that the wood deck moisture content is below 15% using a pin-type moisture meter; trapping moisture beneath asphalt felt will lead to dry rot in the deck and premature wrinkling of the membrane.

Warning: Never lay roofing felt over wet or damp decking. The heat of the sun will vaporize the trapped water, causing the felt to blister, bubble, and tear, rendering your primary water barrier useless.

Step 2: Installing the Eaves Guard and Metal Drip Edges

Install a corrosion-resistant metal drip edge along the lower eaves of the roof. Secure the drip edge directly to the wood deck using galvanized nails spaced every 10 to 12 inches. Along the eaves, the drip edge must be installed beneath the roofing felt.

Conversely, along the rakes (the sloping gable edges), the drip edge should be installed over the roofing felt. This specific configuration prevents wind-driven rain from penetrating the wood-to-metal joints.

Step 3: Rolling and Aligning the Starter Course

Snap a horizontal chalk line exactly 36 inches (or the width of your roll minus the required eave overhang) up from the lower edge of the roof deck. Align the top edge of the first roll of felt with this chalk line. Unroll the felt horizontally, allowing the bottom edge to overhang the metal drip edge at the eaves by 1/4 to 3/8 of an inch. This overhang directs rainwater directly into the gutter system rather than allowing it to run down the fascia board.

Keep the roll taut as you deploy it across the deck to eliminate any waves, ripples, or air pockets.

Pro-Tip: If you are working in cold weather, unroll the felt and let it sit in the sun for 15 to 30 minutes. This softens the asphalt matrix, allowing the membrane to lie completely flat and resisting curl-back during fastening.

Step 4: Fastening Schedule and Nail Placement

Secure the starter course of felt using hot-dip galvanized clout nails or plastic-capped roofing nails. Drive fasteners straight and flush with the surface of the felt; do not over-drive them, as this can slice through the membrane. Follow a strict fastening pattern: space nails every 12 inches along the top and bottom edges (approximately 1 inch inside the margin) and space them every 24 inches along the horizontal center of the roll (the "field").

If you are working in high-wind zones, reduce the edge spacing to 6 inches and the center spacing to 12 inches to prevent wind uplift before the primary shingles or tiles are installed.

Step 5: Managing Laps and Overlaps

For pitched roofs with a slope of 4:12 (approximately 18 degrees) or greater, roll out subsequent courses of felt above the starter course, ensuring a minimum horizontal headlap of 2 inches (50 mm). If the roof slope is between 2:12 and 4:12, a double-coverage system is mandatory. For double-coverage, overlap each consecutive sheet by 19 inches (half the width of a standard 36-inch roll plus 1 inch), effectively creating a two-ply protective layer across the entire deck.

When joining the ends of two rolls along a horizontal run, ensure a minimum vertical sidelap of 6 inches (150 mm). Stagger these vertical end joints by at least 6 feet between adjacent courses to prevent concentrated water channels.

Step 6: Detailing Hips, Ridges, and Valleys

Valleys and ridges require special reinforcement because they are high-flow and high-stress areas of the roof. In valleys, lay a continuous 36-inch-wide strip of self-adhering ice and water protector or a heavy-duty 30-lb felt liner centered directly down the valley trough before running your horizontal courses over it. Run your horizontal felt courses across this valley liner, trim them back 2 inches from the center of the valley, and seal the edges with a continuous bead of cold plastic roof cement.

For hips and ridges, run the horizontal courses up to the top from both sides of the slope, overlap them across the peak, and then cap the entire ridge with a continuous 12-inch-wide strip of felt centered over the ridge line. Nail this cap layer securely along both sloping sides.

Structural Tolerances and Roofing Felt Material Standards

The choice of roofing felt depends entirely on the pitch of the roof, local climate challenges, and the final outer cladding material. The following table highlights the critical material thresholds and installation parameters required to ensure compliance with modern building codes.

Common Installation Failures and Corrective Field Actions

• Failure Scenario: Buckling, Wrinkling, or "Telegraphing" through Shingles
  
  
  • Root Cause: The felt was installed too tightly over a damp wood deck, or it absorbed atmospheric moisture before the shingles were applied, causing the organic fibers to expand and lift.
  • Actionable Fix: Use a utility knife to slit the center of the buckle parallel to the roll direction. Gently lift the edges, apply a thin layer of cold-applied roof cement beneath the cut, press the felt flat, and nail the edges securely. Apply a patch of new felt over the cut area, extending at least 6 inches in all directions, and seal its borders with asphalt mastic.
• Failure Scenario: Tear-Outs at Fastener Points Under Wind Load
  
  
  • Root Cause: Fasteners were driven using excessive pneumatic pressure, cutting the felt, or standard construction staples were used instead of large-head clout nails.
  • Actionable Fix: Replace all compromised fasteners. Install 1-inch-diameter plastic cap nails or metal cap nails 2 inches away from the torn holes. Avoid using standard hammer-tacker staples on roofs exposed to wind for more than 24 hours before final shingling.
• Failure Scenario: Water Leaking at the Lower Eaves Margin
  
  
  • Root Cause: The felt was tucked beneath the drip edge instead of overhanging it, allowing water to run behind the gutter and rot the wooden fascia board and rafter tails.
  • Actionable Fix: Carefully lift the lower course of felt, slide a new 12-inch-wide starter strip of felt under the existing course, and ensure this new strip lies over the top flange of the metal drip edge. Seal the horizontal seam with cold lap adhesive.

Frequently Asked Questions

Which side of the roofing felt should face upward during installation?

The printed side of the roofing felt—which typically features alignment lines, grid patterns, and safety warnings—must face upward. This textured side is specifically engineered to provide better traction for installers walking on the roof, while the smooth, oil-rich underside is optimized to sit flat against the wood deck and resist water migration.

Can I put new roofing felt directly over old roofing felt?

No, laying new felt over old, deteriorated felt is not recommended. The old layer often harbors hidden moisture, rot, or structural degradation in the decking underneath, and leaving it in place prevents you from performing a thorough structural inspection. Always strip the roof down to the bare wooden decking, make necessary repairs, and install a clean, single layer of high-quality underlayment.

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

Standard organic asphalt felt is not designed for prolonged UV exposure and should be covered with the final roof cladding (such as shingles or metal panels) within 14 days of installation. Prolonged exposure to sunlight dries out the asphalt saturation, making the felt brittle and highly susceptible to wind tear-offs and water degradation.

Is it necessary to use adhesive when putting felt on a pitched roof?

For pitched residential roofs with a slope greater than 4:12, mechanical fastening with galvanized clout nails is sufficient, and overall adhesive application is not required. However, you must use asphalt plastic cement or cold adhesive to seal laps around valleys, chimneys, dormer flashings, and low-slope intersections to prevent wind-driven rain from bypassing the mechanical joints.

What is the difference between 15-pound and 30-pound roofing felt?

The primary difference lies in thickness, tear resistance, and weight. 15-pound felt is thinner, lighter, and more economical, making it ideal for standard residential asphalt shingle roofs. 30-pound felt is significantly thicker and more durable, offering superior resistance to tearing during installation and serving as the preferred underlayment for heavy concrete tiles, slate, or wood shakes.

Safeguarding Your Structural Investment

Achieving a completely water-tight building envelope requires strict adherence to material standards and overlapping rules. If your roof slope features unique geometries or falls below standard pitch minimums, consulting a licensed roofing professional ensures your installation complies with local building codes.