Roofing felt is a water-resistant underlayment installed between roof decking and shingles. When properly selected and installed, it helps protect a home from moisture during construction and can provide a secondary water-shedding layer beneath the finished roof. The right approach depends on the roof’s slope, local building code, roofing material, and product instructions. This guide explains how to install roofing felt under asphalt shingles, what tools and materials are needed, and which details commonly affect performance.
What Roofing Felt Does
Roofing felt, also called asphalt-saturated felt or asphalt roofing underlayment, is traditionally made from a fibrous base saturated with asphalt. It is installed over roof sheathing before shingles. The felt is not a substitute for shingles, flashing, or a watertight roof design. Instead, it serves as a secondary layer that helps shed water if wind-driven rain or melting snow gets beneath the shingles.
Underlayment can also reduce direct exposure of the roof deck to moisture while shingles are being installed. However, felt is not intended to remain exposed indefinitely. Sunlight, wind, rain, and foot traffic can degrade it, so the roofing material should be installed within the exposure period stated by the manufacturer.
Roofing Felt Versus Synthetic Underlayment
Traditional felt remains an option for many asphalt-shingle roofs, but synthetic underlayment is also widely used. Synthetic products are typically made from woven or nonwoven polymers. They may be lighter, easier to handle, and more resistant to tearing during installation, but performance and installation requirements vary by product. Neither category is automatically right for every roof.
| Feature | Roofing felt | Synthetic underlayment |
|---|---|---|
| Material | Asphalt-saturated fibrous base | Polymer-based sheet material |
| Handling | Can wrinkle or tear when wet or mishandled | Often lighter and more tear-resistant, depending on the product |
| Installation | Requires careful alignment and specified laps | May use printed alignment lines or product-specific fastening patterns |
| Exposure limits | Must be protected within the stated time limit | Exposure limits vary substantially by manufacturer |
| Compatibility | Commonly used beneath asphalt shingles when permitted | Must be approved for the roofing material and roof design |
Roofing felt is commonly identified by nominal weight, such as 15-pound or 30-pound felt. These traditional labels do not mean every roll has precisely that weight, and they are not a complete measure of performance. Products may also be labeled to standards such as ASTM D226 or ASTM D4869. Check the product packaging and technical data sheet rather than choosing by weight alone.
Check Code, Roof Design, and Product Instructions First
Before buying materials, confirm what the local building department requires. The International Residential Code is adopted and amended by jurisdictions in different ways, so the applicable rules can vary by location. Code provisions may address underlayment type, installation method, roof slope, ice-barrier requirements, and fastening. The roofing manufacturer’s instructions also matter: a shingle warranty or installation specification may require a particular underlayment or layout.
Low-slope roofs, areas with substantial snowfall, and regions exposed to wind-driven rain may have requirements that differ from those for a typical roof. Some roofing assemblies require self-adhered ice-and-water protection at eaves, valleys, or other vulnerable areas. Do not assume ordinary roofing felt meets every code or weather-protection requirement. Confirm the project details with the local building department or a qualified roofing professional.
Materials and Tools
For a basic felt installation under asphalt shingles, the materials generally include roofing felt approved for the project, compatible roofing nails or staples if permitted, drip-edge flashing, and any required self-adhered membrane. Valleys, chimneys, skylights, vents, and wall intersections may need additional flashing components. Use fasteners specified by the underlayment manufacturer and applicable code.
Common tools include a tape measure, chalk line, utility knife, hammer or roofing nailer, and a sturdy ladder. A roofing nailer should be set to drive fasteners flush without tearing the felt. Safety equipment may include slip-resistant footwear, eye protection, gloves, and fall-protection equipment suitable for the roof and job conditions.
How to Install Roofing Felt
Prepare the roof deck
Start with a clean, dry, sound roof deck. Remove old nails, debris, and protrusions that could puncture the underlayment or prevent shingles from lying flat. Replace damaged or rotted sheathing and confirm that the deck is properly fastened. Do not cover wet wood or install felt over an unstable surface. Check the weather forecast and avoid working on a wet, icy, or wind-exposed roof.
Install required edge protection
Install drip edge and any required eave protection in the sequence specified by local code and the product instructions. A commonly used detail places metal drip edge at the eaves beneath the underlayment, with drip edge at the rakes installed over the underlayment. Requirements can differ, so follow the approved roof assembly rather than treating this sequence as universal. Where ice-barrier membrane is required, its placement relative to the drip edge must also follow local code and manufacturer directions.
Start at the eave
Begin at the lowest roof edge and work upward toward the ridge so upper courses overlap lower courses. Roll out a manageable length along the eave and align it with the roof edge. Keep the sheet straight and flat, without wrinkles, bulges, or sagging. Some products specify a small overhang or exact alignment at the eave; follow the instructions for the particular roll and flashing detail.
Fasten the first course
Fasten the felt using the specified fastener type, length, and spacing. Place fasteners where they will be covered by the next course when the instructions call for that method. The required spacing depends on the product and conditions such as wind exposure. Avoid sparse fastening that allows the sheet to lift, but do not add fasteners indiscriminately: excess or misplaced fasteners can create leak paths or conflict with the product’s fastening pattern.
Overlap the next courses
Install each higher course over the one below it, maintaining the required horizontal lap and end lap. Many felt installations use overlaps in the range of a few inches, but the exact dimensions depend on the product, roof slope, and code. Do not rely on a generic measurement if the roll instructions specify something different. Stagger end joints between neighboring courses where practical, and keep laps flat and fully seated.
Use the printed lines on the roll, if provided, to help maintain consistent laps. At valleys, hips, ridges, and roof penetrations, use the layout and flashing details required for that assembly. Avoid placing an exposed end joint where it can direct water toward a vulnerable opening. Do not cut or patch around a vent or chimney without integrating the underlayment with the appropriate flashing.
Continue to the ridge
Work across the roof in courses, checking alignment and fastening as installation proceeds. At the ridge, overlap the material from the opposite roof plane as directed by the product instructions. If the ridge is vented, preserve the ventilation opening required by the vent and roofing system. Do not cover a ridge vent opening with underlayment unless the vent manufacturer’s instructions specifically call for that detail.
Roof Areas That Need Extra Attention
Water is more likely to enter at transitions and penetrations than across an uninterrupted roof slope. Valleys, eaves, rakes, chimneys, skylights, plumbing vents, and roof-to-wall intersections need carefully coordinated underlayment and flashing. The underlayment should not be treated as a replacement for metal flashing or a properly installed membrane. Follow the shingle, flashing, and membrane manufacturers’ instructions as one complete system.
In cold climates, ice dams can force water beneath shingles at the eaves. Where required, a self-adhered ice-barrier membrane is installed in the locations specified by code. Its required width and placement depend on local conditions and code language. A standard roll of felt generally does not provide the same protection as an ice-and-water membrane.
Common Roofing Felt Installation Mistakes
- Installing over a wet or damaged deck: Trapped moisture and weak sheathing can undermine the roof assembly.
- Reversing the overlap direction: Upper courses should shed water onto lower courses, not channel it beneath them.
- Using inconsistent or inadequate laps: Laps must match the product instructions and applicable code.
- Leaving wrinkles or loose edges: Raised areas can interfere with shingle installation and may catch wind.
- Using the wrong fasteners: Fastener type, length, and spacing must suit the product and roof deck.
- Ignoring exposure limits: Underlayment should be covered within the manufacturer’s stated time.
- Skipping flashing details: Felt alone cannot make valleys, chimneys, or penetrations watertight.
- Mixing incompatible products: Confirm that the underlayment, membrane, flashing, and shingles are approved for the intended assembly.
Safety During Roof-Felt Installation
Roofing work carries serious fall and weather risks. OSHA requires employers to provide fall protection for workers on many construction projects; applicable requirements depend on the work and site. A homeowner working on a roof should also use appropriate fall protection and avoid working alone. Ladders must be stable and correctly positioned, and tools and materials should be secured against sliding or falling.
Felt can become slick when damp, dusty, or covered with loose granules. Wind can lift a roll or sheet unexpectedly. Stop work if conditions become unsafe, and never handle long sheets in strong gusts. If the roof is steep, high, complex, or has fragile sections, hiring a qualified roofing contractor is a safer choice.
How to Inspect the Finished Underlayment
Before shingles cover the roof, inspect the entire surface for loose edges, tears, holes, wrinkles, open seams, and missed fasteners. Confirm that course overlaps are consistent and that valleys and penetrations have been handled according to the approved details. Repair damaged areas with a method approved by the underlayment manufacturer; a casual patch may not provide a durable repair. The roof should be covered with shingles or other approved roofing material within the product’s exposure limit.
Keep a record of the underlayment product, installation date, and any manufacturer instructions used. This information can help with inspections, future repairs, or warranty questions. If the project requires a building permit, schedule inspections as required before concealing work.
When to Hire a Roofing Contractor
Professional help is advisable when the roof is steep or high, the decking needs structural repair, or the design includes multiple valleys, skylights, chimneys, or complex wall transitions. A contractor can also confirm how local code applies to underlayment, ice barriers, flashing, and ventilation. When comparing bids, ask which underlayment will be installed, how it will be fastened and overlapped, and how the contractor will protect the roof from weather during the work.
For basic projects, the central principle is straightforward: use a product approved for the roof system, install it over a sound and dry deck, overlap courses in the water-shedding direction, and follow the exact code and manufacturer requirements. Those details help roofing felt perform as intended beneath the finished roof.
Sources and Further Guidance
- International Code Council for model-code information; local adoption and amendments determine the rules that apply.
- Occupational Safety and Health Administration for construction fall-protection guidance.
- National Roofing Contractors Association for roofing guidance and contractor resources.
- Consult the technical data sheet and installation instructions for the specific underlayment, shingles, flashing, and membrane selected for the project.