Felt can help protect a shed roof from rain and wind, but the right product and installation method depend on the roof’s design. In the United States, “roofing felt” may mean an underlayment placed beneath shingles or a mineral-surfaced roll roofing product intended to remain exposed. Knowing the difference is essential. This guide explains how to choose shed roof felt, prepare the surface, install it with suitable overlaps and fasteners, and spot problems that can shorten the roof’s life.
What “Felt a Shed Roof” Means
To felt a shed roof is to cover its roof deck with a felt-based roofing material. The phrase can describe two different jobs: applying underlayment before installing shingles or another roof covering, and installing a roll roofing product as the finished roof surface. These materials are not interchangeable, even though both may be called felt.
Roofing underlayment is generally a layer beneath the primary roof covering. Traditional asphalt-saturated felt and modern synthetic underlayments are commonly used this way. They provide a secondary water-shedding layer, but are not automatically designed to stand up to prolonged exposure as the finished roof.
Mineral-surfaced roll roofing is made to be exposed. It typically has a weather-resistant surface and is installed in courses over a prepared deck, following the product’s instructions. A shed owner should check the label and technical instructions rather than relying on a product’s informal name.
Choose the Right Shed Roof Felt
Start by identifying the planned roof assembly. If the shed will have asphalt shingles, the felt or synthetic underlayment goes beneath the shingles. If the shed will have an exposed roll-roof covering, choose a product specifically labeled for that use. Using underlayment as a permanent exposed roof can lead to rapid deterioration, leaks, or a warranty problem.
Compare the manufacturer’s instructions for the roof pitch, deck material, fastening method, overlap, and weather exposure. The minimum pitch allowed varies by product. A low-slope roof may need a different assembly from a steep one, and some coverings are not approved for very low pitches at all.
- Asphalt-saturated felt: A familiar underlayment option, sold in different weights or grades. It can wrinkle or tear if mishandled or left exposed too long.
- Synthetic underlayment: A lightweight, often tear-resistant underlayment. It still must be installed according to its specified use and is not a substitute for exposed roll roofing.
- Mineral-surfaced roll roofing: A finished roof covering for certain applications. Its installation details, such as side laps, end laps, and fastening, depend on the product.
Do not select a product solely because it is inexpensive or described as “waterproof.” Roofing materials have different exposure limits and installation requirements. In addition, local building codes and the shed’s location may affect what is permitted. Wind, snow, ice, and wildfire exposure can also influence the appropriate roofing system.
Measure the Roof and Gather Materials
Measure each roof plane from eave to ridge and from one gable edge to the other. For a single-slope shed roof, measure the full slope length, not just the horizontal distance from the front wall to the back wall. Add the areas of all roof planes, then allow extra material for laps, edges, cuts, and mistakes.
Rolls differ in width, length, and usable coverage. The packaging or installation sheet should state coverage and required overlap. Because overlaps reduce the area covered by each roll, the roof’s total square footage alone does not determine the number of rolls. Check the product’s coverage calculation before buying.
Common supplies may include the selected roofing product, compatible roofing nails or cap fasteners, a tape measure, chalk line, utility knife with spare blades, straightedge, and a hammer or manufacturer-approved fastening tool. Some roll roofing systems call for roofing cement or a specific adhesive. Use those only where and how the product instructions specify; excessive or incompatible cement can damage materials or trap moisture.
For a new roof, inspect the deck before covering it. It should be sound, securely fastened, clean, and dry. Replace rotten or delaminated sheathing and repair protruding nails. For a reroof, remove old covering when needed to expose and inspect the deck. A new layer over damaged wood hides the underlying problem rather than fixing it.
Prepare the Shed Roof Safely
Roofing work involves falls, sharp tools, and materials that can shift in the wind. Work in calm, dry weather, and avoid icy, wet, or excessively hot conditions. Use a stable ladder on firm ground, keep the work area clear, and wear slip-resistant footwear, gloves, and eye protection. A harness and properly secured fall-protection system may be appropriate; follow applicable safety guidance.
A small shed roof may look easy to reach, but its edge can still cause a serious fall. Do not overreach or carry a large roll while climbing. Plan how materials will be moved to the roof, and have another person nearby when practical. Stop work if wind makes the material difficult to control.
Confirm that the deck and roof framing can support the work. Do not walk on unsupported sections or fragile panels. If the roof has structural damage, unusual framing, or a steep pitch, a qualified roofer can assess the work before installation.
How to Install Roofing Underlayment Beneath Shingles
The following sequence is a general overview for underlayment, not a replacement for the shingle and underlayment instructions. Product requirements and local code govern details such as eave protection, overlap dimensions, fastening, and permitted exposure.
- Prepare the deck. Remove debris, ensure the roof sheathing is dry and sound, and check that edges and seams are properly supported.
- Begin at the eave. Place the first course square to the roof edge. The exact position depends on the roofing assembly and edge details. In areas where ice dams are a concern, local code may require a specific ice-barrier membrane at the eaves rather than relying on felt alone.
- Lay the next courses upward. Work toward the ridge, keeping the material flat and aligned. Overlap each course by the amount stated in the product instructions. Correct laps help water shed down and over the layer below.
- Fasten as specified. Use the approved fastener type, length, and spacing. Fasten the material flat without tearing it or leaving loose areas. Some products require cap fasteners; others specify different methods.
- Detail edges and penetrations. Follow the roof-system instructions around eaves, rakes, valleys, vents, and other openings. Underlayment alone does not make a poorly detailed roof watertight.
- Install shingles promptly. Underlayment is usually a temporary layer, not a finished roof. Observe the maximum exposure period on the product label and cover it with the specified roofing as soon as practical.
How to Install Exposed Roll Roofing
Exposed roll roofing is installed as the roof covering, so its layout and fastening matter directly to weather resistance. Read the complete instructions before unrolling material; some products require a specific starter course, substrate, adhesive, or fastening pattern. Do not assume that instructions for one brand or roll type apply to another.
- Check pitch and substrate. Confirm that the product is approved for the roof’s slope and that the deck or base layer meets its requirements. A low-slope shed roof needs particular attention to drainage and approved materials.
- Plan the course layout. Decide where the first course and side laps will fall so seams are positioned as directed. The manufacturer may specify whether courses run from the eave upward and how to stagger end laps.
- Align and secure the starter course. Position the material straight and fasten it using the specified fasteners and spacing. Keep it smooth without stretching or trapping folds.
- Install subsequent courses. Maintain the stated side and end laps. Apply roofing cement or adhesive only if required, at the prescribed locations and amounts. Use the correct fastening pattern, especially at edges and laps.
- Finish the ridge and edges. Follow the product’s details for ridge caps, rake edges, eaves, and any wall or flashing connections. Improvised folds or exposed fasteners can become leak paths.
Exact lap widths and nail spacing should not be guessed. They vary with the roll roofing product, roof pitch, and installation system. The product label, technical data sheet, and local code are the controlling references.
Common Installation Mistakes
- Using underlayment as the finished roof: Underlayment may not be designed for permanent sun and weather exposure. Select a finished covering intended for that purpose.
- Installing over wet or damaged decking: Trapped moisture and weak sheathing can cause rot, poor fastening, and premature failure.
- Reversing or reducing laps: Incorrect overlaps can let water travel beneath the material. Follow the specified course direction and dimensions.
- Leaving wrinkles or loose sections: Folds can interfere with the finished covering and create places where water collects. Keep material flat without pulling it so tightly that it distorts.
- Using the wrong fasteners: Fasteners that are too short, corrode easily, or are installed incorrectly may not hold. Match fasteners and spacing to the roofing instructions.
- Relying on cement to fix poor detailing: Cement is not a substitute for proper laps, flashing, or approved installation. Use compatible products only where directed.
- Leaving the roof exposed too long: Temporary underlayment can be damaged by sunlight and weather beyond its rated exposure period. Install the roof covering within the stated window.
- Ignoring drainage: Debris, blocked gutters, or a poorly designed low-slope roof can keep water on the surface and increase leak risk.
Roofing Felt and Building-Code Considerations
Building-code requirements depend on location, roof design, and the adopted code edition. The International Residential Code includes provisions for roof coverings and underlayments in residential construction, but local authorities determine which code and amendments apply. A shed may also be subject to local rules concerning permits, accessory structures, setbacks, or fire-resistance requirements.
Before work begins, check with the local building department when the shed is attached to a home, unusually large, used as habitable space, or covered by a project requiring a permit. A product’s installation instructions are also essential: code compliance does not eliminate the need to follow the listed system’s requirements.
In regions with snow and ice, wind-driven rain, hurricanes, or wildfire risk, roofing details can be especially important. Ice barriers, wind-rated fastening, fire classifications, and approved roof coverings may apply. A local building official or licensed roofing contractor can clarify requirements for a particular property.
Inspect and Maintain a Felted Shed Roof
Inspect the roof periodically, especially after severe wind or heavy storms. Look for lifted edges, loose or missing fasteners, cracked or worn surface material, open seams, punctures, and debris. Check the underside of the roof and the shed interior for water stains, damp wood, or mold-like growth.
Keep leaves and branches from accumulating, and maintain clear drainage paths. Trim nearby branches that rub against the roof. Repair small defects promptly using a repair method compatible with the roofing product. If leaks recur, the deck feels soft, or damage covers a large area, a patch may not be enough; the roof assembly may need professional evaluation or replacement.
Roofing felt is one part of a complete roof system. A durable shed roof also depends on a sound deck, correct slope, properly installed flashing and edge details, compatible fasteners, and a covering suited to local conditions. For product-specific directions, consult the manufacturer’s installation guide and the local building department. Useful references include the International Code Council’s International Residential Code and the roofing product’s current technical documentation.