Mastering Roof Underlayment: A Step-by-Step Guide To Installing Felt Paper

Installing asphalt-saturated felt paper requires rolling the underlayment horizontally across a clean, dry roof deck, starting at the lowest eave and working upward toward the ridge. To prevent water infiltration, rows must overlap horizontally by at least 2 inches and vertically at end joints by at least 4 inches, secured using corrosion-resistant plastic cap nails spaced every 8 to 12 inches along the laps. Proper integration with drip edges—under the felt at the eaves and over the felt at the rakes—ensures a weather-tight barrier that meets IRC building codes and manufacturers' warranties.

Pre-Roofing Preparation: Tools, Materials, and Safety Logistics

Before laying down a single roll of felt paper, the structural integrity of the roof deck must be verified, and the correct grade of underlayment selected. Asphalt-saturated felt underlayment, commonly known as felt paper, acts as a critical secondary water barrier. Choosing between standard #15 (Type I) and heavy-duty #30 (Type II) felt depends on the roof's pitch, local climate, expected exposure time before shingle installation, and the weight of the primary roofing material.

A successful installation relies on having the proper safety gear, professional-grade tools, and high-quality materials organized on-site. The following breakdown outlines the essential equipment, technical prerequisites, and project benchmarks required for a safe, code-compliant application.

Essential Gear, Tools, and Materials

• Asphalt-Saturated Felt Underlayment: #15 or #30 felt complying with ASTM D226 or ASTM D4869 standards.
• Fasteners: 1-inch plastic cap nails (ring-shank) or 1-inch plastic cap staples (staples alone without caps are prone to tearing and wind-uplift).
• Drip Edge: Aluminum or galvanized steel drip edge flashing.
• Adhesives & Sealants: Asphalt plastic roof cement (ASTM D4586) and a caulking gun.
• Cutting Tools: Utility knife with replacement hook blades (hook blades cut cleaner through asphalt felt without dulling as quickly as straight blades).
• Layout Tools: Chalk line, measuring tape, and a straightedge.
• Safety Equipment: OSHA-compliant fall protection harness, roof anchors, lifeline, non-slip work boots, safety glasses, and heavy-duty work gloves.

Mandatory Prerequisite Standards & Knowledge

• Substrate Inspection: The roof deck must be entirely dry, clean, and free of protruding nails, splinters, or structural sagging. All APA-rated plywood or OSB sheathing must be properly spaced (typically 1/8-inch gap at panel edges to allow for thermal expansion).
• Slope Restrictions: Standard felt installation procedures apply to roofs with a slope of 4:12 (four inches of vertical rise per twelve inches of horizontal run) or greater. Low-slope roofs between 2:12 and 4:12 require double-coverage application. Slopes below 2:12 are not permitted for standard asphalt felt paper application.
• Weather Conditions: Do not install felt paper on a wet or damp roof deck. Trapping moisture beneath the underlayment leads to dry rot, mold growth, and deck warping. Ideal installation temperatures range between 40 degrees Fahrenheit and 90 degrees Fahrenheit; colder temperatures make the rolls stiff and prone to cracking, while extreme heat softens the asphalt coating, making it highly susceptible to scuffing and tearing.

Project Benchmarks & Metrics

• Estimated Duration: 4 to 6 hours for a standard 1,500-square-foot roof with a two-person crew.
• Average Material Budget: $0.15 to $0.45 per square foot (including felt paper, fasteners, drip edge, and sealants).
• Waste Factor: Calculate an additional 10% to 15% material allowance to account for overlaps, valley linings, and cutting waste.

Step-by-Step Felt Paper Installation: From Eave to Ridge

Step 1: Inspecting and Preparing the Roof Substrate

Begin by sweeping the entire wooden deck clean of sawdust, dirt, loose shingles, and old nails. Run a flat shovel or a heavy scraper across the deck surface to identify and pull or hammer flat any shingle nails left behind from a previous tear-off. Inspect the structural integrity of the roof deck panels. Replace any water-damaged, rotted, or delaminated OSB or plywood. Verify that all decking sheets are securely fastened to the rafters below using 8d ring-shank nails or deck screws. Ensure there are no gaps larger than 1/4 inch between panels; wider gaps must be filled or the affected sheets replaced.

Step 2: Installing the Eave Drip Edge

Before the felt paper is rolled out, install the metal drip edge along the eaves (the horizontal bottom edges of the roof). This placement ensures that any water backing up under the shingles or running off the underlayment flows directly over the drip edge and into the gutters, rather than seeping into the wooden fascia board.

Align the drip edge flush against the roof deck and the fascia board. Fasten the drip edge to the deck using 1-1/4 inch galvanized roofing nails spaced every 8 to 12 inches along the top flange. At the corners, cut and fold the drip edge to wrap neatly around the rake edges.

Warning: Do not install the rake (side/gable) drip edge yet. The rake drip edge must go over the top of the felt paper, whereas the eave drip edge goes underneath the felt paper. Failing to follow this order will lead to water infiltrating the rake edge of the roof deck.

Step 3: Snapping the Alignment Guide Lines

Felt paper must be laid perfectly straight to prevent wrinkles, buckles, and uneven overlapping as you move up the roof pitch. Measure the width of your felt roll (most standard rolls are 36 inches wide).

Using a tape measure, mark a point 34 inches up from the lower edge of the drip edge at both ends of the roof plane. The 2-inch difference accounts for a 2-inch overhang at the eave to ensure total coverage. Snap a blue or red chalk line across the roof deck between these two marks. This chalk line serves as the alignment guide for the top edge of your first course of felt.

Pro-Tip: For longer roof spans, snap intermediate lines every 34 inches (or the net exposure width of your specific felt paper) up the roof to ensure the upper runs remain perfectly parallel to the eave.

Step 4: Rolling and Fastening the Starter Row

Position the roll of felt paper at one end of the roof, aligning the top edge precisely with the snapped chalk line. Unroll the first 8 to 10 feet of felt along the eave. Make sure the bottom edge extends slightly past the eave drip edge by approximately 1/4 to 1/2 inch to create a drip overhang.

Once aligned, drive a cluster of three plastic cap nails at the starting edge to anchor the roll. Pull the roll taut from the opposite side to eliminate slack and prevent sagging. Continue unrolling the felt across the roof, smoothing out any bubbles or ripples by hand as you progress.

Secure the felt using plastic cap nails. Space the fasteners 12 inches apart along the top and bottom laps, and 24 inches apart in a staggered pattern along the center of the roll (known as the "field").

Keep fasteners 2 to 3 inches away from the outer edges of the felt to prevent tearing.

Step 5: Managing End Laps and Joining Rolls

When a roll of felt paper ends mid-roof, or when joining two separate rolls along a horizontal run, a vertical end lap is required. Overlap the ending roll with the start of the new roll by a minimum of 4 inches (increase this to 6 inches in high-wind zones).

Apply a thin layer of asphalt plastic roof cement between the overlapping layers of felt to create a water-resistant bond at the seam. Secure the vertical seam with a row of plastic cap nails spaced 4 to 6 inches apart along the center of the overlap. Never place end laps directly over joints in the wood decking below; stagger the end laps of successive courses by at least 6 feet to prevent localized thickness build-up and potential leak points.

Step 6: Overlapping Successive Courses Up the Roof

Subsequent courses of felt paper must overlap the lower courses to create a shingle-like shed effect for water runoff. Align the bottom edge of the second course over the top edge of the first course.

The horizontal overlap must be a minimum of 2 inches for roofs with a slope of 4:12 or steeper. Most felt rolls feature printed alignment lines spaced 2 inches from the edges to make this step straightforward.

For low-slope roofs (2:12 to under 4:12), building codes require a double-coverage installation. To achieve double-coverage:

1. Lay a 19-inch starter strip of felt along the eave.
2. Apply the first full-width (36-inch) roll directly over the starter strip, aligning the bottom edges.
3. Install the next full-width roll overlapping the previous run by 19 inches, leaving only 17 inches of the lower course exposed. Fasten each course securely using the same 12-inch perimeter and 24-inch center fastening schedule.

Step 7: Wrapping the Ridges and Hips

When the felt paper reaches the peak of the roof (the ridge) or the sloping corners (the hips), the material must wrap over the peak to provide continuous watershed protection.

Extend the final course of felt from the first side of the roof over the ridge by at least 6 inches. Cut the felt and fasten it securely on the opposite side of the roof deck.

When installing felt on the second side of the roof, run the final course over the ridge by at least 6 inches in the opposite direction, overlapping the first layer. This creates a double-layer, 12-inch protective cap over the entire ridge line. If a ridge vent is scheduled to be cut and installed, leave the decking open as required by the vent manufacturer, but run the felt over the opening temporarily to keep out debris until the vent is installed.

Step 8: Installing Valley Linings and Sealing Rakes

Valleys are high-volume water channels that require extra reinforcement. Prior to installing the main horizontal courses of felt paper, install a vertical valley liner. This can be a 36-inch-wide roll of self-adhering ice and water shield or a #30 felt roll centered directly in the valley and fastened 1 inch from the outer edges only.

When rolling out the horizontal courses of felt paper from the roof planes, run the felt across the valley, over the top of the valley liner, and past the center line by at least 12 inches. Cut the horizontal felt courses parallel to the valley center line, leaving a clean, overlapping run. Secure the edges with plastic cap nails placed no closer than 6 inches to the valley centerline, and seal the cut edges with a continuous bead of asphalt plastic roof cement.

Once all felt is laid, install the rake drip edge flashing over the felt paper along the gable ends. Drive roofing nails every 10 to 12 inches through the metal flange and underlayment into the wood deck.

Underlayment Performance Metrics and Material Specifications

The selection of underlayment material dictates the durability, exposure limits, and overall performance of the primary roofing system. While synthetic underlayments have grown in popularity, asphalt-saturated organic felt remains an industry standard due to its breathability, slip-resistant surface, and cost-effectiveness.

The following table provides a comprehensive comparison of technical parameters, code compliance, and application thresholds for standard #15 and heavy-duty #30 organic felt paper underlayments.

Common Underlayment Failures and On-Site Field Corrective Actions

Wrinkling and Buckling After Exposure to Rain

• Root Cause: Asphalt-saturated organic felt contains cellulose fibers that expand when they absorb atmospheric moisture or direct rainwater. If the felt paper is pinned down tightly and then gets wet before shingles are applied, the expanding fibers cause the paper to swell, warp, buckle, and lift away from the deck. Installing shingles directly over wrinkled felt results in unsightly humps and uneven shingle courses.
• Actionable Fix: Allow the felt paper to dry completely. In dry weather, the fibers shrink and the felt will often lie flat again. If wrinkles persist after drying, use a utility knife to slice through the center of the buckle parallel to the roll direction. Flatten the sliced felt, slide a piece of new felt underlayment beneath the cut to maintain water shedding, apply a thin layer of roof cement under the flaps, and secure the sliced edges flat using plastic cap nails.

Felt Paper Tearing or Blowing Off in High Winds

• Root Cause: Tearing and blow-offs typically stem from using inappropriate fasteners (such as standard wire staples or T50 staples without caps) or an inadequate fastening schedule. Standard staples have a small surface area and easily pull through the thin felt paper under wind-uplift pressures.
• Actionable Fix: Remove loose or torn felt pieces and inspect the deck for leftover fasteners. Install a fresh section of felt paper over the exposed area, ensuring the new piece overlaps the intact surrounding courses by at least 4 inches horizontally and 6 inches vertically. Secure the patch using 1-inch ring-shank plastic cap nails, adhering strictly to the code-required fastening pattern of 12 inches along the edges and 24 inches in the center field.

Water Leaks Along the Rake Edge (Gables)

• Root Cause: This issue is caused by an incorrect installation sequence where the rake drip edge is installed underneath the felt paper instead of over it. Rainwater running off the side of the roof slips under the drip edge flange, directly rotting the wooden barge board or rake trim.
• Actionable Fix: Carefully pry up the nails holding down the rake drip edge and the edge of the felt paper. Slip the felt paper underneath the metal rake drip edge flange. Apply a bead of asphalt plastic roof cement between the bottom of the drip edge and the top of the felt paper, then re-secure the drip edge with fresh roofing nails driven through both the metal flange and the underlayment.

Fastener Over-Drive and Blow-Through

• Root Cause: Using pneumatic coil nailers or staple guns with the air pressure set too high causes the plastic cap or nail head to slice completely through the felt underlayment. This leaves a hole in the water barrier and provides zero holding power.
• Actionable Fix: Adjust the air compressor regulator down to a lower pressure setting (typically between 70 and 90 PSI, depending on the tool and deck density) so that the plastic caps seat flush against the felt surface without dimpling or cutting the material. For any blown-through fasteners, apply a dab of roof cement over the damaged area, place a 6-inch square patch of felt over the hole, and secure the patch with a properly adjusted hand-driven plastic cap nail.

Frequently Asked Questions

Does felt paper go over or under the drip edge?

Felt paper must go over the drip edge along the eaves (the bottom edges of the roof) to shed runoff water directly into the gutters. Conversely, along the rakes (the sloping sides of a gable roof), the felt paper must go under the drip edge to prevent wind-driven rain from blowing under the edge of the underlayment.

Can you leave roof felt paper exposed to rain?

While standard #15 felt can withstand light moisture, it should not be left exposed to heavy rain as it absorbs water, wrinkles, and may tear. Heavy-duty #30 felt is highly water-resistant and can safely remain exposed to the elements for up to 14 days before shingle installation without significant degradation.

How many fasteners should be used per roll of felt paper?

For a standard 36-inch by 72-foot roll of #30 felt, you should use approximately 120 to 150 plastic cap fasteners. Space the fasteners 12 inches apart along the top and bottom overlapping edges, and place a staggered row every 24 inches down the center of the roll to prevent wind uplift.

What is the minimum roof slope required to install asphalt felt paper?

The absolute minimum roof slope for standard asphalt felt paper is 2:12. For slopes between 2:12 and 4:12, a double-coverage installation is code-required, which involves overlapping each course by 19 inches. For slopes 4:12 and greater, standard single-coverage with a 2-inch horizontal overlap is sufficient.

Protect Your Structure with Professional-Grade Roofing Execution

Selecting high-quality materials and applying them with precise overlapping and fastening schedules ensures that your roof underlayment acts as an impenetrable second line of defense against severe weather. By prioritizing correct drip edge sequencing, maintaining deck dryness, and addressing substrate flaws immediately, you guarantee a durable, long-lasting roofing installation that protects your investment for decades to come.