Master The Underlayment: How To Install Felt Paper On A Roof For Long-Term Protection

Successfully installing asphalt-saturated felt paper creates a critical, code-compliant secondary water barrier that protects the roof deck from wind-driven rain and condensation. This process requires precise overlap margins—minimum 2-inch horizontal side laps and 4-inch vertical end laps—secured with corrosion-resistant fasteners. Executing these steps correctly ensures the roof assembly complies with the International Residential Code (IRC) and provides a durable foundation for shingles, metal, or tile roofing.

Essential Prep Work: Tools, Materials, and Roof Deck Standards

Before rolling out any underlayment, you must ensure the roof deck is structurally sound and that you have the appropriate grade of asphalt felt. Traditional felt paper comes in two standard weights: 15-pound (Type I) and 30-pound (Type II). Type II felt offers superior tear resistance, better durability under foot traffic, and longer exposure tolerance before the primary roofing material is installed.

For slopes between 2:12 and 4:12, a double layer of felt paper is required to prevent water penetration from ice damming or slow drainage. For slopes 4:12 and steeper, a single layer of properly overlapped felt is sufficient.

Pre-Operation Checklist

• Essential Gear and Materials:
  
  
  • Asphalt-saturated felt paper (ASTM D226 Type I or Type II / ASTM D4869)
  • Corrosion-resistant plastic cap nails (1-inch ring shank) or pneumatic cap stapler
  • Aluminum or galvanized steel drip edge flashing
  • Utility knife with extra hook blades
  • Chalk line for aligning courses
  • Hammer or pneumatic cap nailer
  • Fall protection harness, anchor, and safety rope
• Mandatory Prerequisite Standards:
  
  
  • The roof deck must be entirely dry, clean, and free of protruding nails, structural sags, or rotted sheathing.
  • The ambient temperature should ideally be above 40°F (4°C) to prevent the felt paper from becoming brittle and cracking during unrolling.
  • Local building codes must be checked for specific wind-zone fastening requirements.
• Estimated Budget & Duration Benchmarks:
  
  
  • Material Cost: $0.10 to $0.25 per square foot.
  • Time Commitment: 3 to 6 hours for a standard 1,500-square-foot roof with a two-person crew.

Step-by-Step Roof Felt Paper Installation Guide

Step 1: Inspecting and Preparing the Roof Deck

The longevity of your roofing system depends entirely on the substrate. Inspect the plywood or oriented strand board (OSB) sheathing for delamination, water damage, or structural sagging.

1. Sweep the entire deck clean of sawdust, dirt, shingle granules, and construction debris. Even small wood chips under the felt can wear through the material over time under foot traffic.
2. Inspect for and hammer down any proud deck nails or screws. Flush fastening prevents punctures in your new underlayment.
3. Replace any soft, buckled, or rotted sheathing panels. Pay close attention to the eaves and valleys where water damage most frequently occurs.

Warning: Do not install felt paper over wet or damp sheathing. Trapped moisture cannot escape through asphalt-saturated felt, which will cause the roof deck to rot and the felt to buckle or wrinkle as it dries.

Step 2: Installing the Drip Edge Flashing at the Eaves

The drip edge directs water away from the fascia board and directly into the gutters. Its placement relative to the felt paper varies depending on whether you are working on the eaves (bottom edges) or the rakes (sloping side edges).

1. Install the drip edge along the eaves directly onto the bare wood deck.
2. Secure the drip edge with roofing nails spaced every 10 to 12 inches. Keep the nails high up on the flange, close to the top edge.
3. Ensure adjacent drip edge pieces overlap by a minimum of 2 inches where they meet.

Pro-Tip: Remember the "under-over" rule: Drip edge goes under the felt paper at the eaves to prevent water from wicking backward under the wood deck, but it goes over the felt paper at the rakes to prevent wind-driven rain from lifting the edges of the underlayment.

Step 3: Rolling Out the Starter Course

The first layer of felt paper is called the starter course. It runs horizontally along the lower edge of the roof deck, parallel to the eaves.

1. Measure up from the eave's edge the width of the felt roll minus approximately 3/8 of an inch. This slight overhang allows water to drip cleanly into the gutter rather than wicking back into the wood.
2. Snap a straight chalk line across the roof at this measured height to use as an alignment guide.
3. Unroll the first 10 feet of felt paper, aligning the top edge with your snapped chalk line and letting the bottom edge overhang the drip edge slightly.
4. Temporarily secure the starter end with a few cap nails, then continue unrolling across the deck, pulling the paper taut to eliminate any wrinkles or bubbles.

Step 4: Fastening the Underlayment

Proper fastening resists wind uplift before the shingles are installed and prevents tears when installers walk on the roof.

1. Use 1-inch plastic cap nails rather than bare staples. Plastic caps distribute the holding pressure over a wider surface area, reducing the risk of tear-out.
2. Along the lower and upper edges of the roll (the laps), space fasteners 12 inches on-center.
3. In the center field of the felt roll, space fasteners 12 to 24 inches on-center in a staggered pattern to pin the center securely to the deck.
4. Avoid nailing within 6 inches of the rake edges until the rake drip edge is installed over the felt.

Warning: Standard construction staples or hammer-tacker staples are highly susceptible to wind tear-offs and rust. Most regional building codes mandate plastic cap nails or metal cap nails for underlayment attachment.

Step 5: Managing Horizontal and Vertical Overlaps

Water flows downhill, so every subsequent course of felt paper must overlap the lower course to shed water like shingle layers.

1. Identify the horizontal lap line printed on your felt paper (usually located 2 to 4 inches from the edge). Align the bottom of the next roll with this line on the lower course.
2. If the roof slope is standard (4:12 or steeper), maintain a minimum 2-inch horizontal lap.
3. If the roof slope is low (2:12 to 4:12), apply a double-coverage system: overlap each 36-inch wide sheet by 19 inches, leaving only 17 inches exposed. Secure each layer firmly before rolling out the next.
4. Where two rolls meet end-to-end (vertical joints), overlap them by at least 4 inches (some manufacturers recommend 6 inches). Stagger these vertical end laps so they do not line up with end laps on adjacent courses.

Step 6: Flashing Valleys, Hips, and Ridges

Valleys carry high volumes of runoff, while hips and ridges are vulnerable to wind-driven rain. These areas require extra layers of protection.

1. In valleys, lay a continuous 36-inch wide vertical strip of felt paper down the center of the valley before installing any horizontal courses.
2. Run your horizontal felt courses across the roof and directly over this valley liner. Cut the horizontal courses back so they overlap the valley center by at least 12 inches.
3. For hips and ridges, wrap the horizontal courses from one side of the roof over the peak by at least 6 inches. Repeat the process on the opposing side, creating a double layer of felt paper over the entire peak. Secure the ridge overlap with cap nails on both sides of the crest.

Technical Specifications and Material Grade Comparison

To choose the best material for your roof pitch and budget, refer to this breakdown of standard roofing underlayments:

Resolving Common Roofing Felt Installation Mistakes

Scenario 1: Wrinkled or Buckled Felt Paper

• Root Cause: The felt paper was unrolled and fastened while damp, or it absorbed dew/rain overnight before the shingles were applied. The organic cellulose fibers in felt paper expand when wet, causing the paper to wrinkle. If shingles are installed over these wrinkles, the bumps will telegraph through to the shingles, ruining the flat appearance of the roof.
• Actionable Fix: Cut out the buckled or wrinkled section with a utility knife to relieve the tension. Lay a flat patch of new felt paper over the slit area, extending at least 6 inches beyond the cut in all directions, and secure it with plastic cap nails. Ensure all lap margins are maintained.

Scenario 2: Water Seepage at the Rakes and Eaves

• Root Cause: The drip edge flashing was installed in the wrong sequence, or the horizontal felt courses were run under the drip edge at the rakes. This allows wind-driven rain to slip behind the flashing and directly contact the fascia and sheathing.
• Actionable Fix: Gently pry up the rake-side drip edge. Lift the edge of the felt paper, slide the drip edge under the felt paper at the eaves but ensure the felt paper is slid under the drip edge at the rakes. Re-fasten with cap nails and apply a thin layer of asphalt roofing cement to seal any old nail holes.

Scenario 3: Underlayment Tearing Out Around Fasteners

• Root Cause: The installer used standard wire staples or a hammer tacker without plastic caps, or they walked too quickly down a steep slope, tearing the paper free of the fastners.
• Actionable Fix: Stop walking directly on unsecured areas. Drive 1-inch plastic cap nails 6 inches apart around the torn area. If the tear is larger than 2 inches, cut a repair patch that extends 6 inches past the damage in all directions. Slip the top edge of the patch under the course above it to maintain the water-shedding overlap, and nail it securely.

Frequently Asked Questions

Does felt paper go over or under drip edge?

At the eaves (the bottom edges of the roof), the felt paper must go over the drip edge to direct water off the roof deck and into the gutter. At the rakes (the sloping side edges of the roof), the felt paper must go under the drip edge to protect the edges of the underlayment from being lifted by wind-driven rain.

Can you use a staple gun for roofing felt?

Standard staple guns or hammer tackers should only be used to temporarily hold felt paper in place if the primary roofing material is being installed immediately. For lasting wind resistance and code compliance, always secure underlayment with 1-inch plastic cap nails or a pneumatic cap stapler.

How long can felt paper be exposed to rain before shingles go on?

Type I (15 lb) felt paper should not be exposed to rain for more than 24 hours, as it absorbs water easily and will wrinkle. Type II (30 lb) felt can withstand moderate rain and UV exposure for up to 14 days, though it must be completely dry before shingles are installed over it.

Do you need felt paper under metal roofing?

Yes, felt paper or a high-temperature synthetic underlayment is highly recommended under metal roofing to act as a thermal barrier and to prevent condensation from dripping onto the wood deck. Direct contact between metal panels and wood decks can trap moisture, leading to premature rot.

What is the minimum roof slope required for felt paper?

The minimum slope for felt paper is 2:12. For slopes between 2:12 and 4:12, you must install a double layer of felt paper with a 19-inch overlap. For slopes flatter than 2:12, felt paper is not code-compliant, and you must use a self-adhering, ice-and-water shield membrane.

Secure Your Next Roofing Project with Professional Integrity

Protecting your home starts with choosing high-grade underlayment materials and installing them to the highest professional standards. Take the time to snap your alignment lines, use code-approved cap nails, and secure your roof deck before the storm season arrives.