What is Joist Tape?
Joist tape (also called deck joist flashing tape) is a self-sealing waterproof membrane—typically butyl or rubberized asphalt—applied to the tops of deck joists, beams, and ledgers. It creates a moisture barrier, seals around screw penetrations, and helps prevent rot in framing members, extending the life of the deck structure.
Why Use Joist Tape on Deck Framing?
- Blocks water and debris: Sheds runoff from fasteners and prevents standing water in the joist/board interface.
- Seals penetrations: Self-seals around screw holes to limit water intrusion and staining around fasteners.
- Protects ledgers & beams: Helps reduce moisture ingress at the most critical structural points.
- Corrosion mitigation: Creates a barrier between treated lumber and metal fasteners/hangers to help reduce chemical interaction.
- Longer service life: Keeps framing drier—an easy, low-cost upgrade for a higher-durability deck.
Butyl vs. Asphalt Joist Tape
Both are waterproof, self-sealing membranes. Butyl formulations generally adhere at a wider temperature range, remain more stable in heat, and leave less residue during install. Rubberized asphalt tapes are cost-effective and widely used. Choose based on climate, install temperature, and budget.
Where & How to Apply
Common Locations
- Joist tops and beam tops (full length)
- Ledger boards (wider widths to cover top face and rim interface)
- Post caps, blocking, and other water-collecting transitions
Basic Installation Steps
- Clean, dry, and square the surface. Cut tape to length.
- Peel the release liner and apply centered on the member’s top.
- Press firmly and roll tight so edges bond down; slit/overlap neatly at hangers.
- Install decking soon after; follow the product’s UV exposure limit.
Selection Tips
- Width: 2–4 in. is typical for joists; 4–6+ in. for ledgers and beams.
- Temperature: Check minimum install temp; butyl often adheres better in cooler weather.
- Compatibility: Safe for pressure-treated lumber; confirm with manufacturer if using specialty materials.
- UV window: Most tapes require decking installation within a stated exposure period.
FAQs
Do I need joist tape on every joist?
It’s recommended wherever deck boards contact framing. Continuous coverage across all joists, beams, and ledger tops provides the best moisture defense.
Can joist tape be used with composite or PVC decking?
Yes. It’s commonly used under both wood and composite/PVC boards to protect the framing below regardless of deck-board type.
What width should I choose?
Use 2–4 in. for standard joists so edges turn slightly down both sides. For ledgers and beams, 4–6+ in. helps cover the full top surface and the joint at the rim.
Will it stick in cold or hot weather?
Follow the labeled temperature range. Butyl tapes typically bond better in cool weather and stay stable in heat; rubberized asphalt is best installed in moderate temps.
Can I leave joist tape exposed to sun?
Only for the product’s specified UV exposure window. Install decking promptly after taping to ensure long-term performance.
Does joist tape replace metal flashing?
No. Use joist tape on horizontal tops and penetrations; use code-approved flashing for wall/ledger interfaces and other transitions as required.