How To Attach Roofing Felt: The Master Installation Guide
Attaching roofing felt correctly requires laying courses horizontally from the eave to the ridge with strict 2-inch to 4-inch horizontal laps and 6-inch end laps, secured using corrosion-resistant roofing nails or heavy-duty staples. This underlayment acts as a vital secondary weather barrier beneath asphalt shingles, protecting the roof deck from wind-driven rain and moisture infiltration during construction.
Pre-Operation and Equipment Checklist
Before beginning any roofing installation, gathering the correct tools, safety gear, and materials ensures compliance with building codes and manufacturer warranties. Roofing felt, also known as asphalt-saturated underlayment, comes in standard weights—most commonly ASTM D226 Type 1 (No. 15) and Type 2 (No. 30). Selecting the appropriate weight depends on your local climate, roof pitch, and the primary shingle manufacturer's specifications.
- Essential Gear and Tools: Utility knife with hook blades, chalk line, roofing hammer or pneumatic cap stapler, metal tape measure, broom, and a caulking gun with flashing cement.
- Safety Equipment: OSHA-compliant personal fall arrest system (harness, rope, and secure anchor), non-slip footwear, safety glasses, and heavy-duty work gloves.
- Materials: Rolls of roofing felt (15lb or 30lb), 1-inch galvanized roofing nails with plastic caps (cap nails) or heavy-duty staples, and galvanized drip edge metal.
- Technical Prerequisites: The roof deck must be completely clean, dry, and structurally sound (plywood or OSB panels must be free of gaps exceeding 1/4 inch and have all protruding fasteners driven flush).
- Time and Budget Benchmarks: A standard 1,500-square-foot roof deck requires approximately 4 to 6 hours for underlayment installation, with material costs typically ranging from $100 to $300 depending on felt weight and square footage.
Step-by-Step Underlayment Installation Workflow
Step 1: Deck Preparation and Drip Edge Installation
Begin by thoroughly sweeping the roof deck to remove all sawdust, grit, nails, and debris that could puncture the felt underlayment. Install aluminum or galvanized steel drip edge along the eaves directly over the roof deck, securing it with roofing nails spaced every 8 to 10 inches.
Pro-Tip: Always install the drip edge under the underlayment along the eaves, but reverse this at the rakes by placing the drip edge over the felt to prevent wind from catching the edge of the paper.
Step 2: Rolling and Positioning the First Eave Course
Unroll the first course of roofing felt horizontally along the bottom edge of the roof, starting at the lower-left corner. Align the bottom edge of the felt so it overhangs the drip edge by approximately 1/4 inch to 3/8 inch, allowing water to shed cleanly into the gutters without contacting the wooden fascia or roof deck. Ensure the roll runs parallel to the eave line by snapping a chalk line as a reference guide.
Step 3: Fastening the Underlayment
Secure the first course using plastic cap nails or a pneumatic cap stapler, which provide superior holding power and prevent the felt from tearing in high winds. Drive fasteners along the top edge, the bottom edge, and down a vertical center line spaced approximately 12 inches apart.
Warning: Never use standard smooth-shank nails without plastic caps, as wind uplift can easily pull the felt right over the fastener heads, ruining the underlayment before shingles can be applied.
Step 4: Overlapping Subsequent Courses Up the Roof Deck
Roll out the second course of felt directly above the first, ensuring a minimum horizontal lap of 2 inches (or following the printed lap lines on the felt). For roof pitches between 2:12 and 4:12 (low slope), increase the horizontal overlap to 19 inches to create a double-layered moisture barrier. For vertical end laps where two rolls meet, overlap the material by a minimum of 6 inches and seal the seam with a bead of roofing cement if applying in high-wind zones. Continue this overlapping pattern progressively upward until you reach the roof ridge.
Step 5: Detailing Around Roof Penetrations
When encountering roof penetrations such as plumbing vents, chimneys, or skylights, cut the felt around the obstacle and weave the underlayment tightly into the surrounding step flashing or pipe boots. Ensure the upper layers of felt always lap over the flashing flanges of lower components, maintaining a continuous downward shedding plane for any moisture that breaches the primary shingle layer.
How To Put A Felt Roof On at Timothy Brower blog
Roofing Underlayment Material Comparison
| Material Specification | ASTM D226 Type 1 (No. 15 Felt) | ASTM D226 Type 2 (No. 30 Felt) | Synthetic Polypropylene Underlayment |
|---|---|---|---|
| Weight per Square | ~7.5 to 11.5 lbs per 100 sq ft | ~16 to 26 lbs per 100 sq ft | ~2 to 4 lbs per 100 sq ft |
| Tear Resistance | Low to Moderate | High | Extremely High |
| UV Exposure Limit | Maximum 30 Days | Maximum 30 Days | 6 Months to 1 Year |
| Walkability / Traction | Moderate (slick when wet) | Good (firmer body) | High (anti-skid coatings) |
| Primary Application | Standard slope shingle roofs | High-end or commercial steep slope | Premium architectural shingle systems |
Common Site Failures and Field Fixes
Even experienced installers encounter environmental and mechanical challenges when attaching roofing felt. Addressing these issues immediately prevents long-term substrate degradation.
- Root Cause: Sudden rainstorms saturate traditional felt paper before shingles are installed, causing the organic or fiberglass mat to wrinkle and buckle.
- Actionable Fix: Cut along the top of the raised wrinkle with a utility knife, apply a liberal amount of asphalt roofing cement beneath the flap, press flat, and secure with cap nails on both sides of the cut.
- Root Cause: High winds tear the felt away from the roof deck because fasteners were spaced too far apart or lacked plastic caps.
- Actionable Fix: Strip away severely damaged sections entirely and replace them with a patch that extends at least 12 inches beyond the damaged area in all directions, fastening securely with cap nails every 8 inches.
- Root Cause: Incorrect overlap measurements result in exposed deck areas or water penetration paths beneath horizontal seams.
- Actionable Fix: Do not attempt to patch narrow gaps; roll out an entirely new overlapping course over the deficient area, ensuring minimum code requirements for lap widths are strictly met.
Frequently Asked Questions
Which direction should roofing felt be installed?
Roofing felt must always be installed horizontally, starting at the lowest point (the eaves) and working your way up toward the ridge. This directional layering ensures that water flowing down the roof runs over the overlaps rather than catching underneath an exposed edge.
Can roofing felt be left exposed to the weather?
Traditional asphalt-saturated felt paper is designed as a temporary weather-resistant barrier and should ideally be covered with shingles within two weeks to 30 days. Extended exposure to UV rays and rain will degrade the organic or fiberglass mat, causing brittleness and tearing.
Do I need to use plastic cap nails, or are staples acceptable?
While pneumatic staples are frequently used for speed, building codes and manufacturers strongly recommend using plastic cap nails or ring-shank roofing nails with tin caps. Caps distribute holding pressure over a wider surface area, preventing the fastener head from tearing through the felt under high wind loads.
Should roofing felt be installed over the entire roof deck?
Yes, full-deck coverage is required by modern building codes for all standard sloped shingle roofs. Underlayment provides essential secondary protection against wind-driven rain, ice damming, and moisture migrating backward beneath the shingle joints.
How do I handle valleys when applying roofing felt?
In roof valleys, install a continuous 36-inch wide strip of heavy-duty underlayment (such as No. 30 felt or a self-adhering ice and water shield) directly down the center of the valley first. Then, run your horizontal courses of felt into the valley, trimming them cleanly and sealing the edge with roofing cement.
Protect Your Investment with Professional Installation Standards
Mastering the correct techniques for attaching roofing felt guarantees a watertight foundation for your final shingle installation and extends the lifespan of your entire roofing system. Ensure your next project meets strict structural tolerances by prioritizing proper lap dimensions, secure fastening patterns, and weather-tight flashing integration.
