“Tar paper” is a familiar name for the dark sheet installed beneath shingles, but most modern products are asphalt-saturated roofing felt—not paper coated with coal tar. This secondary layer helps protect a roof deck from moisture while shingles are being installed and during some wind-driven rain events. Understanding the material, its limits, and installation requirements helps homeowners compare roofing bids and make better decisions about roof replacement or repair.
What Is Tar Paper Roof Underlayment?
Tar paper roof underlayment is the common name for asphalt-saturated felt, a roll roofing material made with a fibrous base, typically organic felt or fiberglass, saturated or coated with asphalt. It is installed over roof sheathing and beneath the roof covering. In the United States, contractors and homeowners may also call it roofing felt, felt underlayment, or asphalt felt.
The phrase “tar paper” is imprecise. Traditional roofing felt is generally made with asphalt, not coal tar. Coal-tar products exist for specialized roofing applications, but they are not interchangeable with ordinary shingle underlayment. Product labels and technical data sheets—not the casual name—identify the material and its intended use.
Underlayment is a secondary water-shedding layer, not a substitute for shingles, flashing, or a properly designed roof. It can limit water intrusion if shingles are damaged or moisture gets beneath them, but it should not be treated as a permanently exposed waterproof roof. Wind, prolonged exposure, incorrect fastening, and standing water can compromise it.
What Roofing Underlayment Does
Roof shingles shed most rain and snow, while underlayment covers the roof deck and provides an added layer of protection. It can help reduce the chance that incidental water reaches the sheathing, including during the short period between removing an old roof and installing new shingles. Underlayment also separates the roof covering from the deck and can help protect the deck during construction.
Its performance depends on the entire roof assembly. Roof slope, local climate, flashing, ventilation, shingle installation, and building-code requirements all affect water management. Correctly installed underlayment cannot compensate for a leaking chimney flashing, poorly installed valleys, or a roof that drains improperly.
Underlayment also is not the same as an attic vapor retarder or insulation. Those components manage heat and moisture from inside the building. A roofing underlayment’s primary role is to help manage water from above the roof deck.
Common Types of Asphalt Roofing Felt
15-pound and 30-pound felt
Roofing felt has traditionally been described as 15-pound or 30-pound felt, based on historical product-weight conventions. These names do not always correspond to the actual weight of a modern roll, so they should not be used as a precise measure of thickness or performance. The package’s ASTM designation, coverage, installation instructions, and technical data are more useful for comparing products.
In general, heavier traditional felt has often been selected for added durability or particular roofing applications. However, a “30-pound” label does not automatically make a product appropriate for every roof or superior to every synthetic underlayment. Compatibility with the roof covering, local code, and manufacturer specifications matters.
Organic and fiberglass-reinforced felt
Older organic felt uses a cellulose-based mat, while many newer felts use a fiberglass mat. Asphalt treatment helps the sheet resist water, and fiberglass reinforcement can improve dimensional stability. Product characteristics vary by manufacturer. Felt can wrinkle or tear if it becomes wet, is left exposed too long, or is handled roughly, so installers should follow the stated exposure limit and fastening instructions.
Felt Versus Synthetic Underlayment
Synthetic underlayment is made from polymer materials rather than an asphalt-saturated fiber base. It has become a common alternative to traditional felt, particularly on new construction and roof replacements. Neither category is automatically the best choice for every project. The right selection depends on roof design, budget, weather exposure during construction, and the shingle manufacturer’s requirements.
| Feature | Asphalt-Saturated Felt | Synthetic Underlayment |
|---|---|---|
| Material | Fiber mat saturated or coated with asphalt | Polymer-based sheet, with product-specific construction |
| Handling | Can wrinkle, tear, or become less reliable when wet | Often lighter and more tear-resistant, depending on product |
| Exposure | Must be covered within the manufacturer’s stated limit | Also has a stated exposure limit; “synthetic” does not mean permanent |
| Installation | Requires specified laps and approved fasteners | Requires product-specific laps, fasteners, and fastening patterns |
| Cost | Often a lower-cost material option | May cost more, but prices vary by product and market |
Comparisons should include more than the roll price. Labor, the number of rolls needed, fastening requirements, exposure period, and compatibility with the roofing material can affect overall project cost. A synthetic product may be easier to handle on a large roof, while felt may be suitable for a project where the roof covering will be installed promptly and the selected product meets all requirements.
How to Choose the Right Underlayment
Start with the roof covering manufacturer’s installation instructions. Shingle, metal, tile, and other roofing systems can have different underlayment requirements. The product should be approved for that application and installed as part of the roof system—not selected solely because it is familiar or inexpensive.
- Check local building codes. Requirements may vary by location, roof slope, climate zone, and roof covering. Confirm the rules with the local building department or a licensed roofing contractor.
- Review the product standard. Labels may identify an applicable ASTM standard, such as ASTM D226 for asphalt-saturated organic felt, ASTM D4869 for asphalt-saturated felt underlayment, or ASTM D6757 for non-asphaltic felt underlayment. Verify the current product documentation and code requirements; not every standard applies to every roofing system.
- Consider the weather during installation. If the roof may remain uncovered for a period, select a product rated for that exposure and protect the building as directed. Exposure limits differ by product and conditions.
- Account for slope and water risks. Low-slope areas, eaves in ice-dam regions, valleys, and penetrations may require additional or specialized protection under code or manufacturer instructions.
- Confirm compatibility. Some roof coverings, self-adhered membranes, and metal systems have specific requirements. Ask the contractor to identify the exact product and explain where it will be installed.
In snow-prone areas, local codes may require an ice barrier at eaves. This is commonly a self-adhered, water-resistant membrane, not simply an extra strip of ordinary tar paper. The required placement and distance from the roof edge depend on code and local conditions. Valleys and other vulnerable details may have additional requirements.
Basic Installation Principles
Roof underlayment installation involves working at height and should generally be handled by trained roofing professionals. The steps below describe common principles, not a substitute for the product instructions, building code, or safe-work practices.
- Prepare the deck. The roof sheathing should be sound, clean, and ready for the roofing system. Damaged or soft sheathing needs evaluation and repair before it is covered.
- Install edge and water-management details. Drip-edge placement, eave protection, and underlayment sequencing can vary by roof assembly and local code. The contractor should follow the required detail for the specific project.
- Lay courses with proper laps. Felt is commonly installed in horizontal courses, beginning near the eaves and progressing toward the ridge. Headlaps and sidelaps must meet the product instructions and applicable code.
- Fasten as specified. Use the approved fastener type and spacing. Loose, overdriven, or improperly placed fasteners can damage the sheet or leave it vulnerable to wind.
- Keep the material flat and protected. Wrinkles, tears, and poorly sealed laps can create trouble beneath the roof covering. Replace damaged areas as directed rather than assuming shingles will correct them.
- Install the roof covering promptly. Do not exceed the product’s maximum exposure period. If weather delays the work, protect the roof according to the manufacturer’s guidance.
Exact details vary. For example, underlayment placement around valleys, ridges, walls, and penetrations depends on the roofing system and the applicable instructions. A single generic installation diagram should not override a product’s technical documentation or local code.
Common Mistakes and Warning Signs
Several problems can reduce underlayment performance even when the material itself is suitable. Common mistakes include leaving felt exposed too long, using the wrong fasteners, failing to follow lap requirements, installing over damaged sheathing, and treating ordinary felt as a substitute for an ice barrier where one is required.
Homeowners reviewing a roofing proposal can ask the contractor to state the underlayment’s brand, product name, type, and installation location. They can also ask whether an ice barrier is included, what exposure limit applies, and how the installation will comply with local code and the roof-covering manufacturer’s instructions. A proposal that only says “tar paper” may not provide enough detail to compare bids accurately.
On an existing roof, underlayment generally cannot be inspected without removing the shingles or other roof covering. A leak does not by itself prove that the felt failed; water may enter through flashing, a valley, a damaged shingle, or another detail. A qualified roofer can trace the source rather than recommending underlayment replacement without inspection.
Cost and Service-Life Considerations
Material prices vary by product, roof size, location, and market conditions, so a universal cost estimate can be misleading. The underlayment is only one part of a roofing project; labor, tear-off, disposal, deck repairs, flashing, ventilation, and the roof covering can make up a larger share of the total price.
Underlayment is normally replaced when the roof covering is replaced because it sits beneath the shingles or other material. Its expected performance depends on product quality, installation, exposure, and the roof assembly. It should not be described as having a guaranteed service life independent of the roofing system, nor should it be left exposed as a long-term roof unless the manufacturer specifically approves that use.
Frequently Asked Questions
Is tar paper required under shingles?
Requirements depend on the roof covering, slope, location, and applicable building code. Many shingle systems call for underlayment, but the specific product and installation details must be checked against code and the shingle manufacturer’s instructions.
Can tar paper get wet?
Roofing felt can tolerate some moisture, but it is not intended to remain wet or exposed indefinitely. Wetting can contribute to wrinkling, tearing, or installation problems. Follow the product’s exposure limits and keep the roof protected during construction.
Is 30-pound felt better than synthetic underlayment?
Not in every situation. The labels describe different materials and do not provide a direct, universal performance comparison. Product specifications, code approval, roof-covering compatibility, installation quality, and weather exposure are better measures.
Does tar paper stop roof leaks?
It can provide a secondary layer of water protection, but it is not a repair for damaged shingles, failed flashing, or poor drainage. Persistent leaks require finding and correcting the source of water entry.
Reliable Sources for Roof Underlayment Requirements
Product instructions and local regulations should guide project decisions. Useful references include the International Code Council for model building codes, the ASTM International for material standards, and the selected roofing manufacturer’s current installation guide. Model codes are not automatically adopted everywhere, so the local authority having jurisdiction determines which requirements apply.