Felt underlayment is a water-resistant sheet installed beneath roofing shingles or other roof coverings. It provides a secondary layer of protection between the roof deck and the exterior material, helping limit damage if water gets past the roof covering. Often called roofing felt or tar paper, it is made from a fibrous base saturated or coated with asphalt. Its performance depends on the product type, roof design, local code, and installation—not simply on whether it is labeled “felt.”
What Felt Underlayment Is
Roofing felt is a roll-form underlayment laid over a roof deck before shingles, tiles, or certain other coverings are installed. Traditional felt combines a base mat of organic fibers or fiberglass with asphalt, which helps make the sheet water-resistant. Some products also include mineral fillers or surface treatments.
Despite the common name “tar paper,” modern roofing felt is generally asphalt-based, not coated with coal tar. The terms are often used casually as if they mean the same thing, but the product’s label and technical data sheet are more reliable than the nickname.
Felt underlayment is not the finished roof and is not intended to replace shingles, flashing, or a properly built drainage system. It is a secondary layer of protection beneath the primary roof covering. Depending on the roof system, it may help protect the deck during construction and reduce the effects of wind-driven rain or water that gets beneath shingles.
What Felt Underlayment Does
A roof covering sheds most rain, but wind, damaged shingles, ice, or installation gaps can allow some water beneath it. Underlayment creates an additional barrier between that water and the wood roof deck. It can also help keep the deck covered while roofing work is in progress, although exposure limits vary by product.
Felt may help separate asphalt shingles from the roof deck and provide a more uniform surface. It can also reduce the chance that resin or other substances from the deck will affect the roof covering. However, its role is limited: it does not correct leaks caused by poor flashing, defective shingles, inadequate drainage, or improper roof ventilation.
Underlayment can be especially important around roof penetrations, valleys, eaves, and other areas where water management is critical. Those locations often require additional materials or specific installation details, such as flashing or self-adhered ice-barrier membrane. Felt alone is not a substitute for those components.
Common Types of Roofing Felt
Asphalt-Saturated Organic Felt
Traditional organic felt uses a paper-like mat made from cellulose fibers, often including recycled material. Asphalt saturates the mat to make it water-resistant. It is generally less dimensionally stable than fiberglass-based felt and can wrinkle or tear if it gets wet, is handled roughly, or remains exposed longer than the manufacturer allows.
Fiberglass-Reinforced Felt
Fiberglass-based felt uses a glass-fiber mat, which can improve dimensional stability compared with an organic mat. Products differ in construction and performance, so a “fiberglass” label does not by itself establish that one roll is suitable for every roof. The manufacturer’s specifications and the roof-covering instructions still matter.
ASTM Type I and Type II Felt
Roofing felt specifications commonly refer to ASTM D226, a standard covering asphalt-saturated organic felt. Under that standard, Type I is a lighter class and Type II is a heavier class. Building plans and product labels may also use the familiar shorthand “15-pound” and “30-pound,” historically associated with roll weight.
Those weight names should not be treated as exact modern measurements of the roll’s weight or as a guarantee of performance. Product construction and labeling have changed over time. When a specification calls for a particular type, the product should be checked against the applicable standard and its current technical documentation.
Synthetic Underlayment
Synthetic underlayment is a different category, generally made from woven or nonwoven polymer materials. It is often lighter and more resistant to tearing than traditional felt, and some products tolerate longer exposure to weather. Those benefits vary by product and do not make synthetic underlayment automatically appropriate for every roof.
Synthetic products may have different surface textures, fastening requirements, and limits on exposure. Some are designed for use beneath asphalt shingles; others are intended for specific roofing materials. Installers should follow the product instructions and the roofing manufacturer’s requirements rather than assume that felt and synthetic underlayment are interchangeable.
Felt Underlayment Compared With Synthetic Underlayment
| Feature | Felt Underlayment | Synthetic Underlayment |
|---|---|---|
| Typical material | Fibrous mat saturated or coated with asphalt | Polymer-based sheet or fabric |
| Handling | Can wrinkle or tear, especially when wet | Often lighter and more tear-resistant, depending on product |
| Weather exposure | Exposure limits are often relatively short | Some products permit longer exposure |
| Cost | Often a lower-cost option | May cost more, depending on product and market |
| Compatibility | Common beneath asphalt shingles when permitted | Must be approved for the roof covering and application |
This comparison describes common tendencies, not universal rules. A premium felt product may outperform a basic synthetic product in a particular application, while a high-quality synthetic may offer better tear resistance or exposure tolerance than felt. Product data, roof slope, climate, and installation details are more useful than the category name alone.
Where Felt Underlayment Is Used
Felt is commonly installed beneath asphalt shingles on residential roofs. It may also be specified in some roof assemblies with other coverings, but material compatibility is essential. For example, metal roofing, tile, slate, and low-slope systems can have distinct underlayment needs. A product approved beneath shingles is not necessarily appropriate beneath metal panels or a roof covering that can retain heat.
Roof pitch matters, too. Low-slope roofs face a greater risk of water pooling or backing up beneath the covering. They may require special underlayment arrangements, additional layers, or a different roofing system altogether. The minimum slope and underlayment method should be taken from the roof-covering manufacturer’s instructions and the locally adopted building code.
In cold regions, code or manufacturer instructions may call for an ice barrier at eaves and other vulnerable locations. An ice-barrier membrane is typically a self-adhering waterproofing material, not ordinary felt. It may be installed in addition to underlayment, according to local requirements and the roof design.
How Roofing Felt Is Installed
Installation starts with a sound, clean, and properly prepared roof deck. The underlayment is rolled out in the sequence and direction specified for the roof system, usually beginning near the eaves and progressing upward. Each course overlaps the one below it so water is directed down toward the eaves rather than into a seam.
Fasteners must be appropriate for the product and roof deck. Their placement and spacing depend on the underlayment, roof slope, wind conditions, and whether the finished roofing will be installed promptly. Wrinkles, loose edges, or tears can complicate shingle installation and create pathways for water, so damaged areas should be handled according to the manufacturer’s repair instructions.
Valleys, eaves, ridges, walls, chimneys, plumbing vents, and other transitions need careful detailing. Underlayment is only one part of those details; properly installed flashing is essential. The final shingle or roof-covering installation should follow its own layout and fastening requirements, even when the deck is already covered with felt.
Roofing felt can become slippery, particularly when wet, dusty, or steeply pitched. Roofing work also involves fall hazards. Installation is best handled by trained professionals using appropriate fall protection and following applicable safety rules.
How to Choose the Right Underlayment
The best choice is the product approved for the roof covering and conditions. Before buying, check the roofing manufacturer’s installation instructions, the underlayment technical data sheet, the project specifications, and local code requirements. A contractor or building official can clarify which edition of a code applies in a particular jurisdiction.
- Roof covering: Confirm that the underlayment is approved for shingles, metal, tile, or the specific material being installed.
- Roof slope: Verify the minimum slope and any special low-slope requirements.
- Climate and exposure: Check wind, rain, snow, and the allowed time the product may remain uncovered.
- Code and project details: Confirm required layers, fastening, and ice-barrier provisions.
- Installation conditions: Consider handling, tear resistance, surface traction, and whether the product can be installed safely.
Price should be compared as part of the complete roof system. A lower-cost roll may require more careful handling or faster coverage, while a more expensive product may provide useful installation or exposure benefits. The most expensive option is not automatically the best fit.
How Long Felt Underlayment Lasts
Felt is normally covered by the finished roofing material, so it is not designed to function as a permanent exposed roof. Its usable life depends on the roofing system, installation quality, moisture exposure, ventilation, and the condition of the covering above it. An underlayment’s stated exposure period is not the same as a guarantee that it can serve as a finished roof for that length of time.
If a roof leaks, the visible interior stain does not identify whether the felt has failed. Water may enter through damaged shingles, flashing, valleys, or roof penetrations and travel along the deck before appearing inside. A qualified roofer can inspect the entire assembly and identify the source rather than relying on the underlayment label alone.
Common Questions About Felt Underlayment
Is roofing felt waterproof?
Felt is generally water-resistant, but it should not be treated as an indefinitely waterproof exposed roof. Seams, fastener holes, tears, and prolonged water exposure can compromise its protection. The roof covering and properly detailed flashing remain the primary defenses.
Is felt underlayment the same as tar paper?
“Tar paper” is a common informal name for roofing felt, but the name can be misleading. Most modern roofing felt is asphalt-based. Product packaging and technical information should be used to confirm the material and intended application.
Can felt be left exposed?
Only for the exposure period specified by the manufacturer, and only in the conditions allowed by its instructions. Sunlight, wind, rain, and foot traffic can damage underlayment. A roof should not be left unfinished on the assumption that felt is a permanent covering.
Does every roof need felt?
Many pitched roofs require an underlayment, but the material and installation method vary. Some systems permit synthetic underlayment or other approved products; specialized roofs may require different assemblies. Local code and the roof-covering manufacturer determine the applicable requirements.
Reliable Guidance and Product Information
For a specific roof, the most useful references are the locally adopted building code, the roofing manufacturer’s installation instructions, and the underlayment manufacturer’s technical data. The International Code Council provides information about model building codes, while the ASTM D226 standard page describes the scope of the felt specification. Model-code provisions may be amended locally, so local requirements should be confirmed before work begins.
In practical terms, felt underlayment is a secondary protective layer beneath a roof covering—not a substitute for good shingles, flashing, or skilled installation. Choosing an approved product and installing it according to the roof system’s instructions helps it perform its intended role.