How To Install Roofing Felt Like A Professional Roofer

Installing roofing felt, also known as underlayment, creates a vital secondary weather barrier beneath your primary shingles or tiles. Achieving a watertight, long-lasting installation requires mastering proper horizontal overlapping techniques, accurate fastening intervals, and strict adherence to local building code slope requirements.

Pre-Operation & Equipment Checklist

Proper execution of a roofing underlayment installation demands precise preparation, exact tools, and adherence to safety and material standards. Roofing felt acts as the final line of defense against wind-driven rain and ice dams, meaning shortcuts during preparation will compromise the entire roof deck system.

• Essential gear, tools, and materials:
  
  
  • Asphalt-saturated organic felt paper (typically No. 15 or No. 30 weight) or modern synthetic underlayment.
  • Hammer tacker (staple gun) or plastic cap nails (cap staples/roofing nails with plastic washers) to prevent pull-through in high winds.
  • Utility knife with hook blades for clean, straight cuts along edges and valleys.
  • Chalk line, tape measure, and a heavy-duty roofing hammer.
  • Personal protective equipment including non-slip footwear, a safety harness tied to a certified roof anchor, and eye protection.
• Mandatory prerequisite knowledge and standards:
  
  
  • Verification of local building codes regarding underlayment weight requirements and ice barrier mandates (e.g., self-adhering polymer modified bitumen in cold climates).
  • Inspection of the roof deck for structural integrity, protruding nails, moisture damage, and proper ventilation clearances.
  • Weather window verification: installation must occur on a completely dry day with minimal wind to prevent tearing of the felt during application.
• Estimated budget and duration benchmarks:
  
  
  • Material costs average between fifteen to forty cents per square foot depending on whether organic felt or synthetic underlayment is selected.
  • Standard residential roof installation (approx. 2,000 square feet) typically requires four to six hours of labor for a two-person crew.

Step-by-Step Underlayment Installation Workflow

Step 1: Roof Deck Preparation and Cleaning

Begin by thoroughly cleaning the entire roof deck surface of all debris, loose nails, sawdust, and protruding fasteners that could puncture the roofing felt. Ensure the oriented strand board (OSB) or plywood decking is dry, structurally sound, and properly spaced with expansion gaps. Install metal drip edge along the rakes (gables) directly onto the bare wood deck, and along the eaves underneath the initial underlayment layer to direct water away from the fascia.

Warning: Never install roofing felt over a wet or damp wood deck, as trapped moisture will cause premature rot, delamination of the decking, and buckling of the underlayment.

Step 2: Laying the First Horizontal Course at the Eaves

Unroll the roofing felt parallel to the eaves, starting at the bottom left corner of the roof surface. Extend the felt approximately one-quarter to one-half inch past the outside edge of the metal drip edge. Ensure the sheet is completely straight and parallel to the eave line by measuring and snapping a chalk line if necessary. Fasten the bottom edge temporarily, then secure the entire course using a hammer tacker or cap nails spaced roughly twelve inches apart along the top and bottom edges, and six inches apart along horizontal lap lines.

Pro-Tip: If using traditional organic asphalt felt (No. 15 or No. 30), allow the material to relax in the sun for a few minutes before fastening to prevent wrinkling and buckling caused by subsequent temperature expansion.

Step 3: Overlapping Subsequent Courses Up the Slope

Roll out the second course of roofing felt directly above the first course, moving toward the ridge of the roof. Follow the manufacturer's pre-printed horizontal overlap lines, which typically require a minimum two-inch overlap for standard slopes and a four-inch overlap for low-slope applications (2:12 to 4:12 pitch). Always lay upper courses over lower courses (shingle style) to ensure water flows downward and over the seams without penetrating the substrate. Continue this process up the entire roof plane until you reach the ridge.

Step 4: Managing Valleys, Rakes, and Roof Penetrations

At roof valleys, run the felt from one slope across the valley center by at least twelve inches, then layer the intersecting slope's felt over it, or install a dedicated valley liner underneath the felt system depending on local architectural specifications. For vertical walls, chimneys, and plumbing vents, weave the felt tightly against flashing components and apply a bead of compatible roof cement or flashing sealant where necessary to maintain a continuous moisture barrier. Ensure all end-laps (where two rolls meet horizontally) feature a minimum six-inch overlap.

Comparative Material Analysis: Felt vs. Synthetic Underlayment

Common Site Failures and Field Fixes

• Root Cause: Fastener pull-through or tearing of the felt due to high winds during the construction phase.
  
  
  • Actionable Fix: Immediately patch torn areas with a square patch of roofing felt extending at least six inches beyond the damaged perimeter, secured with plastic cap nails and sealed with roofing mastic.
• Root Cause: Buckling, wrinkling, or bubbling of the felt paper after installation due to morning dew absorption or temperature fluctuations.
  
  
  • Actionable Fix: Cut a straight slit down the center of the bubble with a utility knife, allow the material to relax, press flat against the deck, and secure the edges with roofing nails before sealing the slit with asphalt cement.
• Root Cause: Water infiltration at horizontal laps due to insufficient overlap or installing courses against the direction of water flow.
  
  
  • Actionable Fix: Strip away the compromised section up to the next undamaged course and reinstall a new piece of felt maintaining correct shingle-style laps (upper over lower).
• Root Cause: Fasteners rusting or popping through the felt due to improper nail selection.
  
  
  • Actionable Fix: Drive new ring-shank roofing nails or cap nails adjacent to the failed fasteners and coat the old exposed holes with an elastomeric roof sealant.

Frequently Asked Questions

How long can roofing felt be left exposed to the weather?

Standard organic asphalt felt (No. 15 and No. 30) is designed as a temporary weather-resistant barrier and should be covered with shingles within one to two weeks, as prolonged UV exposure causes curling, cracking, and degradation. Synthetic underlayments offer significantly higher UV resistance and can typically remain exposed for up to six months depending on manufacturer specifications.

Do I need to use No. 15 or No. 30 felt under architectural shingles?

While building codes often permit No. 15 felt for standard shingle installations, most professional roofing contractors and manufacturers recommend No. 30 heavy-duty felt or a premium synthetic underlayment to provide superior puncture resistance, a flatter nailing surface, and enhanced long-term durability.

Can I install roofing felt directly onto spaced board roof decking?

Yes, roofing felt can be installed over spaced board decking (skip sheathing) provided the gaps between boards do not exceed the manufacturer's structural specifications and local building codes. However, solid wood decking or OSB is strongly recommended to prevent sagging and localized tearing under heavy foot traffic.

What is the correct fastening method to prevent wind damage?

You must use roofing nails with metal or plastic caps (cap nails) or a heavy-duty pneumatic hammer tacker with approved staples. Smooth-shank staples alone should be avoided in high-wind zones because they pull through the felt paper easily during severe storms.

Equip your upcoming roofing project with industry-standard materials and precise installation techniques to guarantee decades of dependable protection against water damage.