Tar Paper on Deck Joists: Installation, Purpose, and Alternatives

Tar paper, also known as roofing felt, is commonly used in deck framing to provide a moisture barrier and reduce wood decay. When placed under decking boards or against joists, tar paper can help control moisture wicking and improve the longevity of the structure. This article explains the role of tar paper on deck joists, best installation practices, common pitfalls, and viable alternatives that homeowners and builders should consider for durable, code-compliant decks across the United States.

What Is Tar Paper And Why Use It On Deck Joists

Tar paper is a heavy, paper-based material infused with asphalt or bitumen to resist moisture. In decking, it acts as a vapor barrier, preventing ground or ambient moisture from seeping into the wood framing and carrying salts or minerals that can cause rot and fungal growth. Tar paper can also deter wood-sucking insects in some environments. While not a substitute for proper flashing and ventilation, it offers an extra line of defense in humid climates or areas with poor drainage.

Key benefits include: moisture mitigation, reduced wood movement, and a cleaner separation layer for fasteners and flashing. Its effectiveness depends on proper installation, coverage, and compatibility with other deck components. Tar paper should not be used in ways that trap moisture or create standing water against the wood.

Installation Guidelines For Tar Paper On Deck Joists

Preparation is crucial. Clean the joists, ensure there is adequate ventilation, and correct any water drainage issues before applying tar paper. Use a high-quality, asphalt-impregnated felt that is rated for exterior use. Cut sheets to fit between joists, with minimal overlapping and a snug fit around ledger boards and beams.

Placement typically involves laying tar paper over the top surface of the joists or beneath the deck boards, depending on whether the paper is intended to shield the wood from ground moisture or to protect against run-off from above. When used under decking boards, ensure the paper does not cause cupping or prevent proper drainage. Secure with corrosion-resistant fasteners or staples, keeping them flush to avoid creating bumps that can impede deck boards.

Important notes include: do not layer tar paper in a way that traps water; ensure overlaps shed water away from the structure; avoid contact with soil where possible; and confirm compatibility with any chemical treatments used on the wood. If the deck is built over concrete or grade, consider a continuous moisture barrier system that meets local building codes rather than relying solely on tar paper.

Common Mistakes And How To Avoid Them

Common errors include using tar paper as the sole moisture barrier, improper overlaps that create pockets of water, and failing to address drainage and ventilation. Some installers also neglect to seal around penetrations such as post anchors and bolts, enabling moisture ingress. To avoid these issues, implement a comprehensive moisture-management plan that incorporates flashing, proper ventilation space, and, where appropriate, a more modern moisture barrier system.

• Overlapping orientation should promote drainage away from the structure.
• Check for compatibility with composite or treated lumber, as some products may interact with tar paper.
• Regularly inspect for tears or delamination and replace damaged sections promptly.

Alternatives To Tar Paper For Deck Joists

Several alternatives may provide superior or more durable moisture protection depending on climate, wood species, and deck design. A few common options include:

• Roofing Felt Alternatives: Modern synthetic underlayments or polyolefin-based barriers can offer better tear resistance and longevity while maintaining moisture control.
• Rot-Resistant Barrier Systems: Specifically designed deck moisture barriers installed beneath the decking or between joists, often combined with drainage mats.
• Ventilated Deck Systems: Elevating decking with air gaps allows for natural evaporation and reduces moisture buildup, often paired with proper flashing and drainage.
• Sealants and Finishes: Water-repellent treatments or sealants applied to the joists can complement barriers, though they do not replace a dedicated moisture barrier.
• Code-Compliant Flashing: In many jurisdictions, code requires comprehensive flashing and edge detailing; these requirements may render tar paper unnecessary or redundant.

Choosing the right solution involves evaluating local climate (humidity, rainfall, freeze-thaw cycles), deck usage, and maintenance expectations. For most new builds, engineers and code officials increasingly favor continuous, durable moisture barriers over traditional tar paper alone.

Maintenance And Longevity

Proper maintenance extends the life of a deck with tar paper or its alternatives. Schedule annual inspections to check for water infiltration, damaged barrier sections, and evidence of mold or rot on joists. Clean debris that can trap moisture, ensure drainage paths remain clear, and verify that ventilation gaps remain unobstructed. Replacing damaged barrier sections promptly helps prevent widespread wood deterioration.

Longevity depends on climate and installation quality. In coastal or high-humidity regions, barrier systems with durable materials and mechanical fasteners outperform tar paper in resisting moisture intrusion over time. For homeowners, documenting installation details and maintaining a log of inspections facilitates code compliance and resale value.

Environmental And Building Code Considerations

Tar paper is generally considered low-to-moderate environmental impact, though some products contain bitumen and asphalt components. Local building codes typically require approved moisture barriers and proper flashing for decks attached to houses. Builders should verify that the chosen barrier material meets relevant standards (ASTM, governing codes) and is compatible with treated lumber, fasteners, and any finishes used. In addition, some jurisdictions restrict the use of tar paper in high moisture zones or mandate alternative barrier systems. Always consult the current local code and consider third-party inspections to ensure compliance.

Summary Of Practical Guidance

Tar paper can play a role in protecting deck joists, but it is not a universal solution. For most U.S. installations, prefer a comprehensive moisture management plan that includes flashing, ventilation, and durable barrier materials designed for exterior use. When tar paper is selected, ensure proper sizing, overlaps that promote drainage, and compatibility with other deck components. Evaluate alternatives based on climate, maintenance expectations, and code requirements to achieve a durable, safe, and long-lasting deck.