How To Apply Roofing Felt: A Professional Step-by-Step Underlayment Guide

Preparing your roof deck with high-quality asphalt-saturated underlayment is the single most critical step in establishing a secondary weather-resistant barrier for your home. Installing roofing felt correctly requires laying the sheets parallel to the eaves, securing them with corrosion-resistant plastic cap nails spaced 12 inches on center along the edges, and maintaining a minimum 2-inch horizontal overlap on standard slopes. Adhering to these strict ASTM standards prevents wind-driven rain from penetrating the wood deck, ensures building code compliance, and extends the lifespan of your shingles.

Pre-Installation Planning and Material Selection for Roofing Underlayment

Before climbing onto the roof deck, you must evaluate the structural conditions of the roof, understand your local building codes, and gather the exact tools required for a safe, waterproof installation. Roofing felt, historically referred to as tar paper, serves as a vital secondary barrier beneath asphalt shingles, metal panels, or slate. It cushions the roofing material, provides a fire rating, and keeps the roof deck dry during construction and throughout the life of the roof.

Selecting the correct grade of underlayment is your first decision. Traditional felt is categorised into two primary types under ASTM standards: Type I (commonly called 15-pound felt) and Type II (commonly called 30-pound felt). While 15-pound felt is lighter and more economical for shallow slopes, 30-pound felt offers significantly higher tear resistance, handles foot traffic better during installation, and provides a thicker barrier against moisture.

Essential Gear, Tools, and Project Metrics

• Essential Tools and Equipment: ASTM D226 Type I or Type II asphalt-saturated felt, 1-inch plastic cap nails (or plastic cap staples), a utility knife with hook blades, a heavy-duty chalk line, a standard tape measure, a hammer or pneumatic cap nailer, and an aluminum drip edge.
• Mandatory Safety and Compliance Gear: OSHA-compliant fall protection harness, roof anchors, safety glasses, slip-resistant work boots, and heavy-duty utility gloves.
• Prerequisite Standards: Structural wood roof deck must have a moisture content below 15 percent verified by a pin-type moisture meter. The local slope must be at least 2:12; slopes below this threshold require specialized low-slope waterproofing membranes rather than traditional felt underlayment.
• Project Estimates: Expect a budget of $0.15 to $0.35 per square foot for materials depending on whether you choose Type I or Type II felt. A standard 1,500-square-foot residential roof typically requires 3 to 5 hours of installation time for a two-person crew.

The Professional Step-by-Step Underlayment Application Process

Step 1: Preparing and Inspecting the Roof Deck

The roof deck must be structurally sound, completely dry, and entirely free of debris before any felt is rolled out. Any moisture trapped underneath the felt will eventually heat up under the sun, turn into water vapor, and cause the wood deck to rot or the felt and shingles above to buckle and blister.

1. Inspect the entire wooden deck (OSB or plywood) for signs of water damage, delamination, or rot. Replace any soft, sagging, or compromised wood panels immediately.
2. Ensure that all deck panels are securely fastened to the rafters. Drive any protruding nails or screws flush with the wood surface to prevent them from puncturing the felt from below.
3. Sweep the entire roof surface using a stiff-bristle broom to remove dirt, sawdust, loose nails, and shingle granules. A clean deck ensures proper adhesion and flat laying of the underlayment.
4. Test the moisture levels of the deck. If the roof has recently experienced rain, allow the decking to dry fully until a moisture meter registers under 15 percent.

Warning: Never lay roofing felt over a wet or damp roof deck. Trapped moisture will expand and contract during thermal cycles, resulting in severe wrinkling of the felt and premature failure of the finished roof.

Step 2: Installing the Eave Drip Edge

The drip edge is a non-corrosive metal flashing designed to direct water away from the fascia and into the gutter system. The installation sequence of the drip edge relative to the roofing felt is critical to prevent water from wicking backward onto the roof deck.

1. Along the eaves (the horizontal bottom edges of the roof), install the metal drip edge directly onto the bare wood deck.
2. Fasten the drip edge with galvanized roofing nails spaced every 10 to 12 inches. Drive the nails close to the top edge of the metal flashing.
3. Ensure that adjacent pieces of drip edge overlap by at least 2 inches at the joints.
4. Keep the drip edge along the rakes (the sloping side edges of the roof) uninstalled for now. Rake drip edges must be installed over the top of the roofing felt later in the process to prevent wind-driven rain from sneaking under the sides.

Step 3: Laying the Starter Course (Bottom Row)

The first layer of roofing felt must be installed straight and true along the lowest edge of the roof. Any alignment errors made on this first course will compound as you work your way up to the ridge.

1. Measure the width of your felt roll (typically 36 inches). Subtract 1/2 inch from this measurement.
2. Go to both ends of the eave, measure up from the bottom edge of the metal drip edge, and make a pencil mark.
3. Stretch a chalk line tightly between these two marks and snap it. This creates a highly visible, perfectly straight reference line across the entire length of the roof.
4. Unroll the first 10 feet of felt along the eave. Align the top edge of the felt roll precisely with your snapped chalk line. The bottom edge of the felt should overhang the metal drip edge by roughly 1/4 to 3/8 of an inch to ensure water sheds directly into the gutters.

Pro-Tip: If you are working in high-wind conditions, unroll only 6 to 8 feet of felt at a time. Secure each section as you go to prevent the wind from catching the underlayment and tearing it from your hands.

Step 4: Fastening the Underlayment Safely

Fastener selection and spacing dictate the wind-uplift resistance of your underlayment. While manual staples are fast, they offer poor holding power and can tear through the felt under foot traffic or high winds. Professional standards require plastic cap nails.

1. Drive a plastic cap nail into the center of the sheet near the starting point to hold the roll in place.
2. Pull the felt roll taut to eliminate any sags, wrinkles, or bubbles.
3. Fasten the top and bottom edges of the felt sheet using 1-inch plastic cap nails spaced exactly 12 inches on center.
4. Drive a secondary row of fasteners down the center of the sheet (the field of the roll) spaced 24 inches on center. This staggered fastening pattern prevents the center of the sheet from billowing in the wind.
5. Always drive fasteners perpendicular to the roof surface. Ensure the plastic cap lies perfectly flat against the felt without cutting or tearing into the asphalt-saturated fibers.

Step 5: Executing Horizontal and Vertical Overlaps

Water naturally flows downward, which means every layer of roofing felt must overlap the layer below it in shingle fashion to shed water successfully. The amount of overlap required depends on the pitch of your roof.

1. For standard roofs with a slope of 4:12 or greater, overlap the upper course of felt over the lower course by a minimum of 2 inches. Most manufacturers print convenient alignment lines on the face of the felt to guide this overlap.
2. For low-slope roofs between 2:12 and 4:12, you must apply a double-coverage system. This is achieved by overlapping each successive course by 19 inches (half the width of a standard 36-inch roll plus 1 inch). This creates a continuous two-ply protective layer across the entire roof surface.
3. When you reach the end of a roll and need to start a new one along the same horizontal row, create a vertical end lap of at least 6 inches. Fasten this joint every 6 inches on center.
4. Stagger all vertical end laps between adjacent courses by at least 6 feet to prevent a continuous vertical seam, which is a common point of water infiltration.

Step 6: Flashing Valleys, Hips, Ridges, and Rakes

Valleys, hips, ridges, and vertical penetrations are the most vulnerable areas on any roof. They require extra layers of felt protection to handle the concentrated water flow.

1. Valleys: Before installing any horizontal courses of felt, lay a full-width (36-inch) sheet of felt vertically down the center of the valley. Secure the edges of this valley liner with fasteners spaced 12 inches on center. Run your horizontal courses of felt all the way across the valley, overlapping the vertical liner by at least 12 inches on both sides.
2. Rakes: Once all horizontal felt courses are secured, install the rake drip edge over the top of the felt along the sloping sides of the roof. This seals the edge of the felt and prevents wind-driven rain from getting underneath it.
3. Hips and Ridges: When you reach the top peak of the roof, wrap the felt over the ridge by at least 6 inches. Bring the felt from the opposite side of the roof up and over the ridge as well, overlapping the first side by at least 6 inches. This creates a double layer of felt over the entire ridge or hip line.

ASTM Underlayment Specifications and Performance Metrics

The choice between Type I and Type II felt dictates the durability of your roof during the construction phase and its overall resistance to physical damage. Below is a detailed technical comparison of the two primary asphalt-saturated organic felt specifications under ASTM standards.

Common Underlayment Installation Failures and Remedial Actions

Scenario 1: Wrinkled or Buckled Felt Underneath Shingles

• Root Cause: The organic wood fibers in asphalt-saturated felt absorb atmospheric moisture or morning dew if left uncovered before shingling. This absorption causes the felt fibers to expand, swell, and wrinkle. If shingles are nailed down over wrinkled felt, the wrinkles will telegraph through the shingles, ruining the aesthetics of the roof and creating pathways for wind-driven rain.
• Actionable Fix: Cut the wrinkles flat. Use a sharp utility knife to slice longitudinally through the center of the buckle. Push the felt flat against the deck so the cut edges overlap slightly. Apply a thin layer of asphalt plastic roof cement beneath the cut area, press the felt down firmly into the adhesive, and secure the repair with plastic cap nails spaced every 4 inches.

Scenario 2: Underlayment Blowing Off in High Winds

• Root Cause: The installer used standard 3/8-inch wire staples or short roofing nails without plastic caps. Standard staples lack the surface area required to hold organic felt against wind-uplift pressures, allowing the felt to pull clean over the fastener heads.
• Actionable Fix: Remove any torn felt sections. Cut a clean, rectangular patch of new Type II felt that extends at least 12 inches beyond the damaged area in all directions. Secure the patch using 1-inch ring-shank nails paired with 1-inch plastic caps, maintaining a strict 6-inch fastening pattern along all patch boundaries.

Scenario 3: Water Leakage Along the Lower Roof Edge (Ice Damming)

• Root Cause: In cold climates, snow melt refreezes at the cold eave line, creating an ice dam that forces standing water backward under the shingles. Standard roofing felt is not designed to resist standing water under hydrostatic pressure and will leak at the seams.
• Actionable Fix: Pull up the shingles and felt along the lowest 3 to 4 feet of the roof. Install a self-adhering polymer-modified bitumen membrane (commonly called ice and water shield) directly to the bare plywood deck. Extend this membrane from the eave edge to a point at least 24 inches inside the interior warm wall line of the home. Lap the remaining roofing felt over the top of this self-adhering membrane by a minimum of 4 inches.

Frequently Asked Questions

Can you install roofing felt over wet wood?

No, you must never install roofing felt over a wet or damp roof deck. Trapped water will eventually evaporate when heated by the sun, causing the wooden decking to rot, fasteners to loosen, and the felt above to bubble and ruin the flat profile of your shingles.

Which side of the roofing felt faces up when installing?

Always install roofing felt with the printed alignment lines and the rough, textured side facing upward. The textured surface is engineered to provide slip resistance for installers walking on the roof, while the printed lines serve as accurate visual guides for nailing patterns and horizontal overlap lines.

How long can roofing felt be left exposed to rain?

Standard ASTM D226 Type I felt should not be left exposed to the elements for more than 24 to 48 hours, as it readily absorbs moisture and will wrinkle. Thicker Type II felt can withstand exposure to wind and light rain for up to 14 days, though covering it with shingles as soon as possible is always the safest course of action.

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

You must install the metal drip edge along the eaves (horizontal bottom edges) directly to the wood deck before laying the roofing felt. Conversely, you must install the drip edge along the rakes (sloping side edges) over the top of the roofing felt to ensure that wind-blown rain cannot get underneath the outer edges of your system.

Elevate Your Roof Protection with Professional Installation

While applying roofing felt is a highly manageable task with the right safety gear and patience, errors in lap measurements or deck prep can compromise your home for decades. If you are dealing with steep pitches, complex valley intersections, or challenging winter weather, secure your investment by consulting a licensed roofing contractor today.