How To Put Felt On A Roof: A Professional Step-by-Step Installation Guide

Installing roofing felt correctly creates an essential waterproof barrier beneath your final tiles or shingles, protecting your roof deck from moisture damage, wind-driven rain, and ice damming. Achieving a watertight seal requires precise material selection, proper overlap management, and secure fastening techniques that comply with modern building codes.

Pre-Operation & Equipment Checklist

A successful roofing felt installation depends heavily on pre-job planning, safety compliance, and gathering the correct specialized equipment. Roofing work involves working at height, requiring proper fall protection systems including OSHA-compliant harnesses, secure anchor points, and non-slip footwear.

• Essential Tools & Materials: Heavy-duty roofing felt (either traditional asphalt-saturated organic felt or modern synthetic underlayment), a utility knife with hook blades, a roofing hammer or pneumatic cap stapler, galvanized roofing nails with plastic caps (cap nails), a chalk line, a heavy-duty broom, and a measuring tape.
• Mandatory Standards & Knowledge: Work must be performed on a dry, structurally sound roof deck made of exterior-grade plywood or OSB panels. Ambient temperatures should remain above 40 degrees Fahrenheit to ensure proper material flexibility and adhesive curing.
• Scope & Benchmarks: A standard residential roof section can typically be felted in a single day by a two-person crew, depending on pitch and total square footage. Budget approximately 10 to 15 percent extra material to account for waste, eaves, valleys, and required overlaps.

Step-by-Step Roofing Felt Installation Process

Step 1: Roof Deck Preparation and Cleaning

1. Inspect the entire wood roof deck for protruding nails, damaged or warped plywood panels, and soft spots that indicate rot. Drive in any high nails flush or slightly countersunk.
2. Sweep the entire surface clean of all sawdust, shingle granules, twigs, roofing debris, and sharp objects that could puncture the felt underlayment.
3. Install metal drip edge along the eaves (horizontal lower edges) directly against the bare wood deck. Secure the drip edge with roofing nails spaced every 8 to 10 inches.

Step 2: Laying the First Course of Felt

1. Roll out the first course of roofing felt horizontally across the lowest edge of the roof, starting at the bottom left corner and aligning it parallel to the eave.
2. Let the bottom edge of the felt overhang the metal drip edge by approximately 1/4 to 1/2 inch to ensure water sheds directly into the gutters.
3. Secure the felt temporarily by driving a cap nail near the top edge, then pull the material taut and flat to eliminate wrinkles and air pockets.
4. Fasten the course permanently using plastic cap nails or a pneumatic cap stapler, placing fasteners every 6 inches along the vertical laps and every 12 inches along the field of the sheet.

Warning: Never leave roofing felt exposed to prolonged UV radiation or heavy rain storms for longer than recommended by the manufacturer (typically 30 days maximum for organic felt, or up to 6 months for premium synthetics), as moisture degradation will compromise its waterproofing integrity.

Step 3: Managing Overlaps and Upper Courses

1. Roll out the second course directly above the first, ensuring a strict horizontal overlap of at least 2 inches (or 4 inches if using low-slope applications under 4:12 pitch). Follow any printed guide lines on the felt roll.
2. Ensure upper courses are laid in a shingle-fashion layout—meaning higher layers always overlap lower layers—so that water naturally flows downward over the seams rather than beneath them.
3. At the end of each roll or vertical wall intersection, overlap adjacent sheets by a minimum of 6 inches and seal the joint if required by local high-wind specifications.

Step 4: Detailing Valleys, Rakes, and Penetrations

1. In roof valleys, install a full vertical strip of heavy-duty felt or ice-and-water shield extending at least 18 inches to either side of the valley center line before running horizontal courses over it.
2. Trim the felt cleanly along the rake edges (sloped roof edges) using a utility knife with a hook blade, folding the excess slightly over the edge or terminating it flush with the gable drip edge.
3. Flash around chimneys, plumbing vents, and skylights by weaving the felt tightly around the flashing flanges, sealing upper edges with approved roofing cement or flashing tape.

Common Site Failures and Field Fixes

• Issue: Wrinkling and Buckling Felt After Installation
  
  
  • Root Cause: Felt was installed too tightly in cold weather and expanded when exposed to direct afternoon sun, or became saturated with dew before shingle installation.
  • Actionable Fix: Slice through large wrinkles with a utility knife to relieve tension, press the edges flat, and secure them with roofing cement and cap nails before shingle placement.
• Issue: Wind Damage and Torn Fastener Holes
  
  
  • Root Cause: Insufficient fastening frequency, using standard nails without plastic caps, or leaving the underlayment exposed to high winds for too long.
  • Actionable Fix: Replace severely torn sections entirely. For minor edge pulls, install additional cap nails through a thin strip of tin cap or repair tape.
• Issue: Leaks Penetrating Seams During Heavy Rains
  
  
  • Root Cause: Inadequate overlap margins (less than 2 inches) or running felt against the direction of water flow.
  • Actionable Fix: Lift the upper layer carefully, apply a bead of polymer-modified roofing sealant between the layers, and re-fasten securely with cap nails.

Frequently Asked Questions

Which direction should roofing felt be installed?

Roofing felt must always be installed horizontally, starting at the lowest point of the roof (the eave) and working your way up toward the ridge. This ensures that every upper course laps over the lower course, creating a shingle effect that sheds water away from the deck.

Can I use staples instead of roofing nails with plastic caps?

While standard narrow-crown staples are sometimes used by contractors for temporary holding, they easily pull through standard felt underlayment during high winds. Building codes and best practices strongly recommend using galvanized roofing nails with plastic caps (cap nails) to prevent the underlayment from tearing loose.

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

Traditional asphalt-saturated organic felt (#15 and #30) is designed as temporary weather protection and should be covered with shingles within 30 days. Premium synthetic underlayments offer greater durability and can typically withstand UV exposure and weathering for up to 6 or 12 months depending on the specific manufacturer specifications.

Do I need felt underlayment under metal roofing?

Yes, felt or synthetic underlayment is vital under metal roofing systems. It acts as a crucial slip sheet to prevent friction between the metal panels and the wood deck, dampens sound from rain, and catches any condensation that forms beneath the metal surface.

Is #15 or #30 felt better for my roof?

The choice between #15 and #30 felt depends on your roof pitch, climate, and budget. #30 felt is thicker, heavier, and more resistant to foot traffic and tearing, making it ideal for steeper roofs or areas prone to severe weather, whereas #15 is standard for most traditional residential shingle applications.

Protect Your Roof Investment Today

Ensure your roofing project stands the test of time by utilizing premium materials and adhering strictly to manufacturer installation specifications. Start planning your underlayment layout today to guarantee a permanently dry, secure home structure.