Tar paper is a familiar name for the dark sheet installed between roof decking and shingles. In modern roofing, that layer is usually called roofing felt or asphalt-saturated felt, though synthetic underlayments are also common. Its main purpose is to provide a secondary layer of protection beneath the roof covering. It can help limit water intrusion, protect the roof deck during installation, and separate shingles from the sheathing. It is not a substitute for shingles, flashing, or proper drainage, and its performance depends on the product, installation, and roof design.
What Is Tar Paper on a Roof?
Traditional roofing felt is made from a fibrous base—often cellulose or fiberglass—that is saturated or coated with asphalt. The asphalt helps make the sheet water-resistant, while the base gives it structure. Rolls are laid over the roof deck, typically in horizontal courses, before shingles or another approved roof covering are installed.
“Tar paper” is a common, older term, but most current products are asphalt-based rather than made with coal tar. Roofing labels generally use terms such as asphalt-saturated felt, roofing underlayment, or a product designation such as No. 15 or No. 30 felt. Those weight labels are traditional categories; actual product thickness, strength, and performance can vary by manufacturer.
Roofing felt is one kind of underlayment. Other options include synthetic underlayment made from woven or nonwoven polymer, and self-adhering membranes designed for particular locations. These products are not automatically interchangeable: the roof covering manufacturer’s instructions, local building code, and roof conditions determine which materials are suitable.
The Main Purposes of Tar Paper
It Provides a Secondary Water-Resistant Layer
Shingles are the primary roof covering, but wind-driven rain, snow, ice, or damaged shingles can allow water to get beneath them. Underlayment creates a secondary line of defense over the roof deck. Properly installed felt can help shed limited water toward the eaves and reduce the chance that moisture immediately reaches the sheathing.
That protection has limits. Standard felt is not a waterproof roof on its own, and it is not intended to remain exposed for long periods unless the product is specifically rated for that use. Open seams, tears, loose fasteners, or prolonged exposure can compromise it. A roof still needs sound shingles or another approved covering, properly detailed flashing, and functioning drainage.
It Helps Protect the Roof Deck During Installation
Roofers may install underlayment before shingles, leaving the deck temporarily exposed to weather while the work proceeds. Felt can reduce exposure to brief showers and help keep construction debris off the sheathing. The amount of protection depends on how quickly the roof is covered and whether the underlayment is approved for the expected exposure period.
Underlayment should not be treated as permission to leave an unfinished roof open indefinitely. Rain can enter at laps, fastener holes, valleys, edges, or damaged sections. Contractors often schedule roofing work around weather and cover unfinished areas when needed.
It Separates Shingles From the Roof Deck
Felt creates a layer between the shingles and wood sheathing. That separation can help reduce direct contact and friction as roof materials expand, contract, or move slightly with temperature and moisture changes. It also provides a more uniform surface over the deck than bare sheathing alone.
Some shingle manufacturers specify underlayment as part of the roof system. Following those installation requirements can help ensure the assembly performs as designed and may be necessary to maintain applicable product warranties. The exact requirements depend on the shingle type and manufacturer.
It Can Help Limit Wind-Driven Moisture and Dust
When installed with the required overlaps and attachment, underlayment can reduce the amount of wind-driven moisture or dust that reaches the roof deck through small gaps in the roof covering. It may also help manage minor incidental water beneath shingles. It does not seal every roof opening, however, and it cannot compensate for poor flashing, loose shingles, or an improperly built roof.
What Tar Paper Does Not Do
Roofing felt is often misunderstood as a complete waterproof barrier. It is a backup layer, not the roof’s primary weatherproofing system. It does not stop leaks caused by defective flashing, failed valleys, damaged shingles, or water entering around chimneys, vents, skylights, and wall intersections.
- It does not replace shingles. Felt is generally intended to be covered by an approved roof covering.
- It does not repair roof damage. Covering a leak without correcting its source can trap moisture and conceal deterioration.
- It does not provide roof ventilation. Attic ventilation is a separate system involving intake and exhaust paths.
- It does not eliminate ice dams. Ice-dam protection may require a code-compliant self-adhering membrane, appropriate insulation and ventilation, and effective drainage details.
- It is not a substitute for flashing. Flashing directs water around vulnerable transitions and openings.
Tar Paper, Synthetic Underlayment, and Ice-and-Water Membrane
Homeowners may hear several underlayment terms during a roof replacement. The products have different materials and uses, so the best choice depends on the roof, climate, covering, local code, and manufacturer instructions.
| Product | Typical role | Important considerations |
|---|---|---|
| Asphalt-saturated felt | Traditional underlayment beneath shingles and other approved coverings | Can wrinkle or tear if mishandled; exposure limits and installation details vary by product. |
| Synthetic underlayment | Lightweight underlayment used beneath many roof coverings | Often has higher tear resistance and may be easier to handle, but surface traction, exposure rating, and approved uses differ. |
| Self-adhering membrane | Sealing protection in specified vulnerable areas, such as eaves or valleys | Adheres to the deck and can provide stronger water protection; placement and required coverage depend on code and design. |
In many regions, asphalt shingles are installed over either felt or an approved synthetic underlayment. Some roof designs use more than one type—for example, a self-adhering membrane at vulnerable eaves with felt or synthetic material over the remaining deck. A contractor should identify the exact product and explain why it is appropriate for that roof.
Where Underlayment Is Installed
Underlayment is generally installed on the roof deck beneath the roof covering. For asphalt shingles, courses are commonly placed parallel to the eaves, with the upper course overlapping the lower course so water flows down and over the lap. The specific lap width, fastening pattern, and treatment at edges and penetrations depend on the product instructions and applicable code.
Special attention is needed around eaves, valleys, chimneys, plumbing vents, skylights, and roof-to-wall intersections. These areas are more vulnerable to water entry and typically require coordinated underlayment and flashing details. At eaves in areas subject to ice dams, local requirements may call for a self-adhering membrane extending a specified distance inside the exterior wall line. The rule varies by jurisdiction and roof configuration.
The International Residential Code includes requirements for underlayment and ice-barrier protection in its roofing provisions. Local governments may adopt different editions or amendments, so the code enforced by the local building department—not a general online summary—controls a specific project.
Does Every Roof Need Tar Paper?
Many common residential roof assemblies use an underlayment, and building codes or roof-covering instructions may require one. However, the precise material and configuration depend on the roof covering. Asphalt shingles, metal panels, tile, and other materials can have different requirements, and some systems use specialized underlayments.
For a roof replacement, the contractor should verify the condition of the existing deck and follow the new roof covering manufacturer’s instructions. If the roof deck is damaged, wet, or structurally unsound, installing new felt over it does not solve the underlying problem. The affected sheathing may need repair or replacement before the roof system is installed.
How Long Can Roofing Felt Stay Exposed?
There is no single safe exposure period for every product. Some underlayments are intended to be covered promptly; others are labeled for longer exposure. Sunlight, heat, wind, rain, slope, and foot traffic can all affect performance. A product’s label and technical data sheet should state its exposure limitations and installation conditions.
Long exposure can lead to deterioration, wrinkling, water intrusion at laps, or damage from workers and weather. If a roofing project is interrupted, the contractor should inspect and repair or replace compromised areas before installing shingles. Covering damaged underlayment does not restore its original performance.
Common Installation Problems
- Incorrect overlaps: Reversed or insufficient laps can allow water to travel beneath the underlayment rather than over it.
- Wrinkles and buckling: Uneven felt can interfere with shingle installation and create an inconsistent surface.
- Excessive or misplaced fasteners: Fasteners can create openings, especially when installed outside the product’s approved pattern or left exposed.
- Poor detailing: Valleys, eaves, ridges, and roof penetrations need details compatible with both the underlayment and roof covering.
- Leaving it exposed too long: Weather and ultraviolet exposure can damage products that are not rated for extended exposure.
- Covering a wet deck: Trapping moisture beneath a roof covering can contribute to wood deterioration or indoor moisture problems.
These problems are reasons to use a qualified roofing contractor and confirm that the installation follows the product instructions and local requirements. The National Roofing Contractors Association offers consumer information on roofing systems and contractor considerations.
How to Evaluate Underlayment During a Roof Project
A homeowner can ask the contractor for the underlayment product name, type, and technical data sheet. It is also useful to ask whether the proposal includes a self-adhering membrane at eaves or valleys, how roof penetrations will be detailed, and how long the underlayment may remain exposed before the roof covering is installed.
Before work begins, the contractor should explain whether the existing roof covering will be removed and how damaged decking will be handled. During installation, photos of the exposed deck and underlayment can document work that will later be hidden. After completion, the final invoice or project records can identify the materials used.
The purpose of tar paper on a roof is to add a protective layer beneath the roof covering. It can provide backup water resistance, help protect the deck during construction, and separate the shingles from the sheathing. Its value comes from being part of a properly designed roof assembly—not from acting as a standalone waterproof roof.