If you’re planning a deck project, joist tape is often highly recommended to extend the lifetime of the deck’s substructure. The adhesive serves to prevent water damage that would lead to wood rot.
Joist tape is not just regular tape branded for a particular purpose. The specialized purpose is catered to through more characteristics than simply the ability to stick to a surface.
While duct tape may be good for sealing leaks in some applications and can be water-resistant on non-porous surfaces (e.g., metal or plastic), it’s risky to use it for waterproofing porous surfaces like wood. This makes it an unreliable alternative to joist tape in wood deck applications.
The Purpose of Joist Tape
Joist tape (amazon link), or flashing tape, is a waterproof adhesive that protects deck joists, beams, and ledger boards from water, air, and moisture.
Specifically in deck construction, joist tape prevents water from remaining on the tops of joists where they are covered by decking. This simple solution can add decades to your deck’s life.
Think of it as an insurance policy that adds years before a deck substructure begins rotting and splitting. If joist tape is not used, many decks will have wood rot around 8-10 years post-construction.
At a low price (especially compared to replacing the deck substructure), joist tape is an affordable way to protect the investment you put into your outdoor home space.
It functions by adhering to the top of joists and beams (leaving the other three sides of the wood to breathe), as well as protecting screw holes from moisture to ensure the deck foundation doesn’t rust and decay before your deck, which is weatherproofed.
An added benefit to applying joist tape is it creates greater fastener-to-wood connections by holding the fastener in place more securely as an extra adhesive layer for the screw to drive through.
Outside of deck applications, joist tape can be used on window and door openings of new buildings to provide additional weatherproofing.
Required Characteristics of Joist Tape
Compatible With Your Application
While most joist tapes are compatible with several materials, it’s important to check that they will adhere to the specific material you’re working with.
For example, some tapes are compatible with wood, plywood, oriented strand board, and concrete while others might be better for metal, plastic, and glass.
Joist tape prevents wood rot by capping the joist with waterproof material to prevent water damage.
As its main purpose, joist tape is required to be waterproof, so it’s typically made of acrylic, butyl, or bitumen material.
Besides the material itself being waterproof, the bond between the tape and the wood also has to be strong enough for water not to easily get trapped in between.
Stickier tape is also better for creating a tighter seal around screws, which will keep moisture out of the fastener-to-wood connection and combat rusting.
In very warm climates, high temperatures might cause the adhesive in the tape to melt, so finding a joist tape specialized to resist high-temperature oozing will ensure that this does not happen.
While not essential per se, joist tape can have other features that save time and money, such as not sticking to itself, preventing splitting from freeze-thaw cycles, or being easy to reposition.
Some other features like non-skid surfaces improve the traction of the deck frame for easier walking during construction.
How Does Duct Tape Compare?
While duct tape (amazon link) has some of the same characteristics as joist tape, in the most common joist tape application—wood decks—it falls short.
Compatible With Your Application
If you’re using duct tape for “waterproofing” a non-porous structure such as glass, metal, plastic, or varnished wood, it should work similarly to joist tape. As seen earlier, some joist tapes are even made for such surfaces.
However, there are two problems with this.
First, porous surfaces such as wood or concrete and duct tape aren’t very compatible, so the tape is more likely to unstick and, therefore, not perform the joist tape function: protection from the elements.
Second, duct tape is not technically waterproof.
During a 60-day experiment where the duct tape was applied to a surface and then exposed to rain, cold weather, and sunny spells, duct tape on porous surfaces (like wood and brick) came unstuck much sooner than on non-porous applications.
Because most decks are made with porous wood, duct tape isn’t up to par with joist tape in this area. Additionally, because the adhesive is more likely to come unstuck when exposed to water, using duct tape might actually accelerate water damage by trapping moisture between the tape and wood.
Again, on non-porous surfaces, duct tape can be a very strong adhesive. It is often used to seal leaks or make temporary repairs—even for applications that will be exposed to varying weather.
However, as discussed, water exposure can compromise the adhesive strength of duct tape when applied to surfaces like wood.
In terms of heat resistance, duct tape can stay adhesive up to 140 °F (60 °C), so if temperatures and added sun heat in your region don’t reach that temperature, you can trust duct tape’s strength.
Is There a Good Substitute?
If you’re looking for a substitute for joist tape for deck construction, you may be better off applying some coats of bitumen paint to cap the substructure rather than using duct tape.