Is Roofing Tar Paper Waterproof is a common homeowner question when evaluating roof underlayment options. This article explains what tar paper is, the difference between waterproof and water-resistant underlayments, installation considerations, lifespan, and when to choose alternatives like synthetic underlayment or peel-and-stick membranes.
| Feature | Tar Paper (Asphalt-Saturated Kraft) | Synthetic Underlayment |
|---|---|---|
| Waterproof | No (Water-Resistant) | Usually Water-Resistant; Some Variants More Waterproof |
| Tear Resistance | Low | High |
| UV Exposure Limit | Short | Longer |
| Typical Use | Temporary Protection & Traditional Underlayment | Long-Term Underlayment |
What Roofing Tar Paper Actually Is
Roofing tar paper, also called asphalt-saturated felt or building felt, is a traditional roofing underlayment made by saturating a paper or fiberglass base with asphalt. It comes in various weights commonly referred to as 15-pound and 30-pound felt, which historically denoted approximate weight per square but now reflect thickness and durability.
Tar paper’s composition—a fibrous base plus asphalt—gives it water resistance and a degree of vapor retardation, but not true waterproofing performance comparable to modern membranes.
Waterproof vs Water-Resistant: Why The Distinction Matters
Waterproof materials block liquid water under pressure or standing conditions. Water-resistant materials repel or shed water but can allow moisture infiltration over time, when exposed to prolonged wetting, or under positive hydrostatic pressure.
Tar paper is water-resistant, not fully waterproof. It sheds rain during normal conditions but can absorb moisture, swell, or allow seepage if exposed to water for extended periods or under ponding situations.
Performance Factors That Determine Water Protection
Several factors influence whether tar paper will keep a roof dry: the grade (15 vs 30 pound), proper installation, overlap and fastener patterns, exposure to UV, and duration of exposure before shingles are installed. Weather events and roof slope also affect performance.
Higher weight (30-pound) felt offers better water resistance and tear strength than 15-pound felt, but neither is designed for prolonged waterproofing or long-term exposure without shingles or a secondary membrane.
Common Uses For Tar Paper In Roofing
Tar paper is typically used as a secondary barrier under shingles to provide temporary protection during installation, help prevent wind-driven rain infiltration, and act as a limited vapor retarder. It also serves as a separator between wood sheathing and shingles.
In many regions, building codes accept tar paper as underlayment for asphalt shingle roofs, but codes may also allow or require improved underlayment options in specific conditions like steep slopes or high-wind zones.
How Tar Paper Fails As A Waterproof Barrier
Tar paper can fail for multiple reasons: seams and fastener penetrations allow wetting, prolonged UV exposure degrades asphalt, ponding water saturates the felt, and wind-driven rain can push water under seams.
Fastener holes and overlapping details are common entry points for moisture, so even well-installed felt is not an impermeable membrane during severe weather or long exposures.
Alternatives That Provide Better Waterproofing
Synthetic underlayments, rubberized asphalt peel-and-stick membranes, and self-adhered membranes offer improved water resistance and durability compared with traditional tar paper. Each has advantages and trade-offs in cost, slip resistance, and application.
- Synthetic Underlayment: Made from polypropylene or polyester blends; lightweight, tear-resistant, and more UV-stable than felt.
- Peel-and-Stick Rubberized Asphalt: Adheres to the deck to form a continuous waterproof barrier; excellent at flashing valleys, eaves, and roof penetrations.
- Ice and Water Shield: Specialized self-adhered membrane for low-slope zones and eave areas to prevent ice dam-related leaks.
When Tar Paper Is Still A Reasonable Choice
Tar paper remains a cost-effective option for properly ventilated, well-drained steep-slope roofs where the primary roofing layer (shingles, tiles) will be installed promptly. It can be suitable in climates without prolonged precipitation or in low-budget repair scenarios.
For short-duration protection during installation or for properly maintained roofs with rapid shingle installation, tar paper is an acceptable underlayment in many cases.
Installation Best Practices To Maximize Water Resistance
Good installation reduces moisture risk. Install felt parallel to the eave, start with an overhang recommended by manufacturer codes, and ensure proper overlaps—generally 2 to 4 inches on laps and 6 to 12 inches in valleys depending on slope and local code.
Use the correct nails and spacing, avoid creating ponds, and minimize exposure time to sunlight, since heat and UV can oxidize the asphalt and reduce effectiveness.
How Long Tar Paper Lasts Before It Needs Replacement
Tar paper’s lifespan as an exposed temporary layer is limited. Manufacturers typically recommend limiting exposure to 30 to 90 days; real-world durability varies with UV intensity and weather. When left exposed beyond manufacturer guidance, felt can dry out, crack, or lose adhesion, increasing leak risk under shingles.
For durable performance, install the primary roof covering soon after the underlayment is applied. If replacement is needed, inspect for brittleness, tears, and saturation before reusing.
Diagnosing Leak Risks And Inspecting Underlayment
When assessing an existing roof, check for sagging, dark stains, or rot in the sheathing, and inspect attics for water stains. Lift shingles at flashings and valleys to view felt condition, and probe for softness or delamination of the paper.
Visible saturation, mold growth, or torn felt are signs that the tar paper has failed and needs prompt repair or replacement.
Cost Comparison: Tar Paper Versus Alternatives
Tar paper is generally the least expensive underlayment option by material cost, but installation labor and potential long-term maintenance can affect total lifecycle cost. Synthetic underlayments and peel-and-stick membranes command higher upfront prices but can lower risk of leaks and reduce rework costs.
| Underlayment Type | Typical Cost | Durability |
|---|---|---|
| 15# Tar Paper | Low | Short |
| 30# Tar Paper | Low-Medium | Medium |
| Synthetic Underlayment | Medium | Medium-Long |
| Rubberized Peel-And-Stick | High | Long |
Codes, Standards, And Regional Considerations
Building codes vary by jurisdiction. Some local codes permit traditional asphalt felt, while others encourage higher-performance underlayments for steep-slope roofs or climates with ice dams and heavy precipitation. Always consult local building codes and manufacturer specifications before selecting an underlayment.
Areas prone to ice dams, hurricanes, or prolonged exposure during reroofing often require or recommend more robust membrane systems.
Practical Recommendations For Homeowners
Choose an underlayment based on roof slope, climate, budget, and how long the roof will be exposed during installation. For maximum leak protection at critical areas, consider combining materials—use peel-and-stick at eaves and valleys and synthetic underlayment across the field.
If the primary concern is waterproofing, tar paper alone is not sufficient; use a self-adhering membrane at vulnerable roof zones.
Frequently Asked Questions About Tar Paper And Waterproofing
Is 30-Pound Felt Waterproof?
30-pound felt is more water-resistant than 15-pound felt due to added thickness and asphalt content, but it remains non-waterproof and can fail if exposed to standing water or prolonged wetting.
Can Tar Paper Be Used Under Metal Roofs?
Yes, tar paper can be used under some metal roofing systems, but compatibility depends on manufacturer recommendations. For high-performance metal roofs or low-slope installations, synthetic underlayments are often preferred.
How Long Can Tar Paper Be Left Exposed?
Exposure limits vary; many manufacturers advise limiting exposure to 30-90 days. Extended exposure increases risk of UV degradation and loss of protective qualities.
Repair And Replacement Tips
Small tears can be patched with compatible roofing cement or a patch of felt adhered with roofing cement; larger or saturated areas require replacement of the underlayment and possibly the sheathing. When reroofing, consider upgrading to synthetic underlayment if budget permits.
Address flashing and penetration details carefully—these are the weak points where tar paper cannot substitute for a continuous waterproof membrane.
Key Takeaways About “Is Roofing Tar Paper Waterproof”
Tar paper is a traditional, cost-effective, water-resistant underlayment but not a fully waterproof membrane. It provides temporary protection and contributes to a roof system when covered by primary roofing materials, yet it has limits in ponding scenarios, extended exposure, and severe weather.
Homeowners and contractors should weigh climate, local code, and lifecycle costs when deciding between tar paper, synthetic underlayments, and peel-and-stick membranes to ensure adequate protection for the roof deck and interior spaces.