How To Attach Roofing Felt: A Professional Guide To Underlayment Installation

Attaching roofing felt correctly requires laying asphalt-saturated underlayment flat across a dry structural deck, utilizing a 2-inch horizontal overlap and a 4-inch vertical overlap. Secure the material using 1-inch plastic cap nails or pneumatic cap staples spaced 12 inches on center along the seams and 24 inches on center through the center field. Adapting these fastening patterns and overlap dimensions to your specific roof pitch is vital to ensuring long-term moisture resistance.

Pre-Roofing Preparation, Structural Inspections, and Material Logistics

Before unrolling any underlayment, the roof deck must be systematically prepared. Applying roofing felt over a damaged, damp, or uneven deck leads to telegraphing wrinkles through the finished shingles, wood rot, and structural failure. Asphalt-saturated organic felt (commonly referred to as tar paper) is designed to shed water that bypasses the primary shingles, but it cannot perform this function if the underlying deck is structurally compromised.

An inspection must verify that the wood substrate (typically 15/32-inch oriented strand board or CDX plywood) is dry, with a moisture content below 15% when tested with a pin-type moisture meter. All fastener heads from the previous roof must be pulled or driven flush, and any rotted wood panels must be replaced.

Tool, Material, and Project Requirements Checklist

• Essential Gear and Tools:

  
  
  
  • Pneumatic cap nailer or manual hammer tracker with 1-inch plastic cap nails.
  • Heavy-duty utility knife with hook blades (hook blades cut felt cleaner without dulling against the deck).
  • Chalk line box with high-visibility blue or red chalk.
  • Standard tape measure and flat pry bar.
  • Personal Protective Equipment (PPE): Slip-resistant roofing boots, safety glasses, and a fall protection harness rated for the roof slope.

• Prerequisite Standards and Building Codes:

  
  
  
  • Compliance with International Residential Code (IRC) Section R905.1.1 for underlayment.
  • Use of ASTM D226 Type I (#15 felt) or Type II (#30 felt), or ASTM D4869 underlayment.
  • Local municipal wind zone requirements, which may dictate enhanced fastening patterns or the mandatory use of plastic cap nails over wire staples.

• Project Benchmarks:

  
  
  
  • Estimated Budget: $45 to $110 per square (100 square feet) for materials, including fasteners and drip edge.
  • Duration: 2 to 4 hours per 1,000 square feet of roof area for a two-person crew.

Step-by-Step Protocol for Installing and Fastening Roofing Felt

Step 1: Substrate Remediation and Surface Sweeping

Clear the roof deck of all debris, sawdust, old shingles, and stray nails. Sweep the entire surface with a stiff-bristled push broom. Any dirt or gravel left on the deck will puncture the felt underlayment when walked upon. Inspect the gaps between the plywood or OSB sheets; there should be a 1/8-inch expansion gap between panel joints to allow for seasonal thermal movement. If panels are buckled or tight, run a circular saw blade through the joints to relieve the tension before proceeding.

Step 2: Installing the Eave Drip Edge

Install the metal drip edge along the lower eaves of the roof deck before laying down the roofing felt. Align the drip edge flush against the fascia board and secure it with 1-1/4 inch galvanized roofing nails spaced every 10 to 12 inches.

Warning: The sequence of the drip edge is critical. Along the horizontal eaves, the drip edge must go under the roofing felt to prevent water from backing up behind the gutter. Along the sloped rakes (gables), the drip edge must be installed over the roofing felt to shed wind-driven rain away from the bargeboard.

Step 3: Rolling and Aligning the Starter Course

Position your first roll of roofing felt at the lowest corner of the roof deck. Unroll the first 8 to 10 feet of felt parallel to the eave line. Align the bottom edge of the felt roll so that it overhangs the eave drip edge by approximately 1/4 inch to 1/2 inch, or sits flush with the outer flange of the drip edge depending on local wind codes.

To ensure the roll stays perfectly straight across long spans, snap a reference chalk line parallel to the eave at a distance equal to the width of the roll minus 1/2 inch. Keep the felt taut as you roll it out to eliminate humps, sags, and diagonal wrinkles.

Step 4: Executing Proper Overlaps and Joint Seams

For roofs with a standard slope of 4:12 (approx. 18.4 degrees) or steeper, overlap each subsequent horizontal course of roofing felt by a minimum of 2 inches. Most modern felt rolls feature printed alignment lines on their face to guide this overlap.

When you reach the end of a roll and need to start a new one, create an end lap (vertical seam) of at least 4 inches. Offset these vertical end laps between adjacent courses by at least 6 feet to avoid creating bulky stack-ups that could disrupt the lay of the shingles.

Pro-Tip: If the roof pitch is between 2:12 and 4:12, you must apply a double-coverage underlayment system. This is achieved by overlapping each 36-inch wide sheet of felt by 19 inches, leaving only 17 inches exposed to the weather. Secure each layer thoroughly to prevent moisture capillary draw between the plies.

Step 5: Fastening with High-Performance Caps

Secure the underlayment using 1-inch ring-shank nails fitted with plastic caps, or pneumatic cap staples. Avoid using raw, uncapped wire staples (such as those from a hand-held hammer tacker) except as temporary hold-downs in areas that will be immediately covered or in low-wind zones. Uncapped staples easily tear through felt paper during high winds or when walked on by installers.

Drive the fasteners straight and flush against the felt surface. Do not overdrive them, as this will rupture the plastic cap and puncture the underlayment, creating a direct pathway for water leaks. Space the fasteners 12 inches on center along the overlapping seams. In the center field of the sheet, drive a row of fasteners spaced 24 inches on center to prevent the middle of the roll from billowing or lifting.

Step 6: Integrating Valleys, Ridges, and Penetrations

For valleys, roll out a continuous 36-inch wide sheet of self-adhering ice and water membrane or heavy-duty #30 felt centered directly down the valley line before installing the horizontal field felt. Lap the horizontal courses over this valley sheet by at least 6 inches.

At the roof ridge (peak), run the felt from one side of the roof over the peak, extending it down the opposite slope by at least 6 inches. Repeat this step with the felt from the other side, creating a double-layer cap over the ridge. Around chimneys, plumbing vents, and skylights, cut the felt tight to the penetration and seal the edges with a thin layer of SBS-modified asphalt plastic roof cement (ASTM D4586) to prevent water backup.

Technical Parameter Analysis: Underlayment Material and Performance Metrics

The selection of your roofing felt grade directly dictates its resistance to wind tear-off, moisture permeability, and UV exposure limits. The following table contrasts the physical properties and fastening requirements of standard underlayment classes under typical field conditions.

Field Failures, Wrinkling Redirection, and Corrective Action

Scenario 1: Wrinkles and Buckles in the Felt Sheet

• Root Cause: Organic asphalt felt is hygroscopic; it absorbs environmental moisture and humidity from the air, causing the wood fibers within the paper matrix to expand. When pinned down tightly by fasteners, this expansion results in significant buckling and wrinkling across the roof field.
• Actionable Fix: Use a sharp utility knife to slice horizontally through the center of the buckle. Flatten the overlapping paper flaps down against the deck. Secure the flattened flaps with additional plastic cap nails, then apply a thin layer of ASTM D4586 asphalt plastic roof cement over the slit to restore water-shedding integrity.

Scenario 2: Underlayment Tearing and Blow-Offs at Fastener Points

• Root Cause: The underlayment was attached using standard T50 wire staples or simple roofing nails without plastic caps. High wind pressures create uplift forces that pull the thin felt material directly over the small heads of uncapped fasteners.
• Actionable Fix: Remove any torn felt sections. Re-lay a fresh patch of underlayment over the exposed area, ensuring it extends at least 6 inches beyond the damaged perimeter in all directions. Secure this patch and all surrounding loose areas using 1-inch ring-shank plastic cap nails driven at 12-inch intervals.

Scenario 3: Water Backing Up and Rotting the Fascia Board at the Eave

• Root Cause: The roofing felt was installed underneath the eave drip edge, or the felt does not extend far enough to cover the top edge of the metal flashing, allowing water to run behind the gutter.
• Actionable Fix: Carefully pry up the lowermost edge of the felt along the eave. Slide the eave drip edge under the felt layer, ensuring the felt overlaps the top metal flange of the drip edge by at least 1/2 inch. Fasten the felt down over the metal flange using plastic cap nails spaced 12 inches on center.

Frequently Asked Questions

Can you use a standard staple gun to attach roofing felt?

Standard T50 or hammer tacker wire staples should not be used as the primary fastener for roofing felt. Standard staples lack the surface area required to hold paper underlayment down during wind events or when stepped on, which often leads to tear-offs and unsafe slipping hazards. Always use plastic cap nails or pneumatic cap staples to distribute the holding pressure over a wider surface area.

How much should roofing felt overlap on a low-slope roof?

For low-slope roofs with a pitch between 2:12 and 4:12, you must employ a double-coverage application. This requires overlapping each consecutive 36-inch wide sheet of roofing felt by 19 inches, which leaves a 17-inch exposure. If the roof slope is below 2:12, standard asphalt felt is not permitted; instead, a fully adhered, self-sealing ice and water barrier membrane must be used.

Do you install drip edge before or after the roofing felt?

Drip edge must be installed before the roofing felt along the horizontal eaves to ensure that any water running off the edge of the felt flows over the metal flashing and into the gutters. However, along the sloped rakes (gables), the drip edge must be installed after the roofing felt is down so that wind-driven rain is directed on top of the metal flashing rather than underneath the underlayment.

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

Standard #15 asphalt felt should not be left exposed to rain or UV rays for more than 24 hours, as it will absorb water, swell, and wrinkle severely. Heavy-duty #30 felt can withstand exposure for up to 72 hours under mild conditions. If your project schedule requires the underlayment to remain exposed for weeks or months, you must utilize a UV-stabilized synthetic underlayment instead of traditional organic felt.

Professional Underlayment Execution

Selecting the right grade of underlayment and attaching it with high-quality plastic cap fasteners ensures your roof deck remains protected from wind-driven rain and ice dams. If you are ready to secure your roof with professional-grade materials, consult with local building suppliers to source the correct ASTM-compliant asphalt felt for your climate zone.