Professional Guide: How To Install Felt Paper On Roof Underlayment Systems

Installing felt paper, also known as roofing felt or tar paper, serves as the critical secondary moisture barrier protecting the roof deck from wind-driven rain and ice dam infiltration. Achieving a professional-grade installation requires meticulous attention to overlap measurements, proper fastener spacing, and strict adherence to the manufacturer’s specific ASTM D226 or D4869 standards to ensure long-term structural integrity.

Essential Preparation, Safety, and Material Requirements

Before beginning the installation of roofing felt, ensure the roof deck is clean, dry, and free of protruding nail heads or structural damage. Any moisture trapped under the felt will promote dry rot and deck deterioration. Working on a roof presents significant fall risks; therefore, fall protection systems such as harnesses, roof anchors, and OSHA-compliant toe boards are non-negotiable.

• Essential Material List:
  
  
  • Felt paper rolls (standard #15 or #30, depending on local building code requirements).
  • Plastic-cap nails or pneumatic cap staplers (crucial for wind resistance).
  • Roofing utility knife with hook blades.
  • Chalk line for maintaining perfectly horizontal runs.
  • Personal Protective Equipment: High-traction footwear, safety harness, and gloves.
• Prerequisite Knowledge:
  
  
  • Understanding the specific local building codes regarding underlayment weight.
  • Proficiency in safely navigating a sloped roof.
• Benchmark Estimates:
  
  
  • Duration: Approximately 2 to 4 hours for a standard 1,500-square-foot roof.
  • Budget: Variable based on felt weight (saturated felt vs. synthetic underlayment).

Procedural Workflow for Precision Felt Installation

Step 1: Preparing the Drip Edge and Starter Course

Begin by installing the metal drip edge along the entire perimeter of the eaves. The drip edge must be placed directly onto the roof deck before the felt paper. Once secured, roll out the first layer of felt paper along the eave line, ensuring the bottom edge overhangs the drip edge by approximately 1/2 to 1 inch. This slight overhang allows water to shed directly into the gutter system rather than curling under the deck. Secure the felt with cap nails spaced every 8 to 12 inches along the perimeter to prevent wind lift.

Step 2: Executing Horizontal Overlaps

Roll the second course of felt over the first, ensuring a minimum overlap of 2 to 4 inches. The overlap measurement is critical; follow the printed lines on the felt paper, which are specifically designed to guide the installer. If no lines exist, measure twice to ensure the overlap remains consistent. If you are working on a low-slope roof—specifically those between 2:12 and 4:12 pitch—most local codes mandate a "double-coverage" application, which requires overlapping each layer by 19 inches or more to create a secondary seal.

Step 3: Fastening and Securing the Sheets

Proper fastening is the most common point of failure. Use only plastic-cap nails, as the large cap provides the necessary surface area to hold the paper without tearing through the material. Never use standard smooth-shank roofing nails, as they have little holding power in thin felt. Space your fasteners approximately 12 inches apart along the top edge of the felt.

Warning: Never leave the felt paper exposed to the elements for longer than the manufacturer specifies. Prolonged UV exposure will cause the bitumen to degrade, resulting in cracking and a total loss of water-shedding capabilities. Aim to install the asphalt shingles or metal roofing system within 48 to 72 hours of laying the felt.

Step 4: Handling Valleys and Obstructions

When reaching a valley, run the felt from one roof plane across the valley and at least 12 to 24 inches onto the adjacent plane. This ensures a continuous water-shedding path. For chimneys, vent pipes, and skylights, cut the felt to fit snugly around the obstruction. After the primary felt is laid, use peel-and-stick flashing or high-temperature ice and water shield to wrap the penetration points, effectively sealing the gap between the felt and the structural flashing.

Technical Comparison of Roofing Underlayment Materials

Common Site Failures and Professional Field Fixes

• Failure Scenario: Buckling or "Fish-mouthing"
  
  
  • Root Cause: Felt paper was installed too tightly or during high-humidity conditions, causing the material to expand and wrinkle once secured.
  • Actionable Fix: If the wrinkles are minor, wait for a sunny day for the heat to relax the material, then re-fasten. If the wrinkles are severe, cut the buckle, flatten the overlapping edges, and apply a dab of plastic cement underneath before securing with a cap nail.
• Failure Scenario: Tear-outs at Fastener Locations
  
  
  • Root Cause: Use of standard framing or roofing nails without plastic caps, causing the paper to pull through the small nail head during high-wind events.
  • Actionable Fix: Immediately replace the failed nails with plastic-cap fasteners. If the hole has widened, apply a small patch of felt over the area using roofing adhesive to restore the moisture barrier.
• Failure Scenario: Inadequate Valley Coverage
  
  
  • Root Cause: Trimming the felt exactly at the valley center, which leads to water capillary action under the paper.
  • Actionable Fix: Install an additional 36-inch wide strip of ice and water shield directly in the valley center over the existing felt to create a waterproof channel.

Frequently Asked Questions

Is it necessary to use felt paper under all roofing types?

Yes, building codes almost universally require a layer of underlayment beneath shingles to provide a secondary barrier against water penetration and to prevent chemical interaction between the asphalt shingles and the wood deck. It also serves as a protective layer during the construction phase before shingles are applied.

Can I install new felt paper over old existing felt?

While it is technically possible to overlay if the existing layer is in perfect condition, it is highly discouraged. Old felt often hides rot or mold in the underlying deck. Always remove the old layer to inspect the integrity of the sheathing before installing a new roofing system.

What happens if the felt paper gets wet during installation?

Light rain is usually acceptable provided the felt has a chance to dry completely before you cover it. However, if the felt becomes saturated, it can warp or become brittle. If you notice pooling water on the felt, you must let it dry fully, as trapping moisture between the felt and the final roofing material will lead to premature structural decay.

Should I choose #15 or #30 felt paper for my roof?

Choose #30 felt if you live in an area with high wind or significant temperature fluctuations, as the heavier weight offers better durability and resistance to tearing. Use #15 only if it meets your local building code and you plan to install the shingles immediately.

Ensure Your Roof Longevity Today

Proper underlayment installation is the foundation of a weather-tight home, preventing costly interior damage and structural rot. Consult your local building department today to verify the specific underlayment requirements for your climate and proceed with confidence using these professional standards.