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What Does Joist Tape Do?

By Dave Kile

Timber, even treated lumber, deteriorates surprisingly quickly when wet. Did you know that more than 90 per cent of a deck’s timber substructure starts to split and rot around 8-10 years? Rotting timber is a big problem with a simple solution: Joist flashing tape.

What does it do? Simply put, joint tape protects by covering the timber most susceptible to rot: the bearers and joists that are just below your deck boards. You may not see them when you’re standing on your deck, but they are critical to its structural integrity.

Builders trust Trex Protect joist tape because it ensures the deck will last. They say: 

  • “I love Trex Protect. I put it on my own home.”
  • “It keeps the screws in place.” 
  • “It’s like shingles on the roof. Every single board is covered.”

Deck Joist Tape: Brief Introduction

When you install joist flashing tape, you protect your deck’s joists, double joists, rim joists, bearers, the ledger board, and stringers with a thin, waterproof membrane. Deck flashing tape can:

  • Help to reduce future maintenance  
  • Give your deck a longer life  
  • Protect your deck’s structural integrity 
  • Hold decking screws and deck fasteners tighter and stronger

Why do You Need Joist Tape?

You probably already know that water on timber equals trouble. Rot can begin quickly when timber stays wet. Many homeowners focus on preventing timber rot by sealing their deck boards. This is important, yet, whether you have sealed your timber deck boards or installed composite decking boards, your timber deck substructure is exposed. 

In time, moisture penetration — rain and snow falling between your deck boards’ spacing will create a wet substructure. Leaves and other debris that get trapped on or between the boards can make the situation worse. By creating a moisture barrier and waterproofing with joist tape, your timber will last longer.

Timber can also be damaged when the metal hardware and fasteners in your deck get wet and corrode. This is an even greater danger since the U.S. banned chromate copper arsenate as a preservative for treated lumber. Now companies commonly use alkaline copper quaternary (ACQ) and copper azole (CA-B and CA-C) in lumber treatment, which contains much higher concentrations of copper and can be twice as corrosive for the zinc in galvanized fasteners. Deck joist tape prevents this chemical reaction by creating a barrier between the metal hangers and treated timber. Thus, while flashing tape protects your timber, your deck screws and fasteners will last longer, too.

Types of Joist Tape

There are two primary forms of joist tape: asphalt and butyl. Butyl offers many benefits:

  • First, butyl rubber self-adhesive provides a secure bond without the oozing of asphalt adhesive.
  • Second, its peel-and-stick super grip creates a tight seal. This quick animated video shows how simple it is to install, even for a DIY homeowner.
  • Butyl tape also seals around screw holes and nails to prevent corrosion and to keep fasteners in place.
  • A quality butyl tape won’t bunch up, bind, or tear as easily during installation.

Things to Consider While Selecting One

If you’ve decided to add joist tape to your new deck build, which should you choose? Here are three important factors:

  1. Material: As mentioned, joist tape comes in acrylic and butyl. Butyl-based tape offers the best protection and durability with the easiest installation. Look at how a quality butyl tape holds up in a three-month water test.
  2. Installation: With Trex Protect butyl tape, you’ll have less trimming and a better fit on your boards. It’s not too thick, which would be difficult to install, or too thin, which is not durable. Tape adheres best to dry surfaces. Press firmly onto the boards; using a hand roller may help to smooth out any wrinkles.
  3. Price: For just a few dollars more, you can install a durable butyl tape that will give you peace of mind for your deck build. A good manufacturer will back their product: Trex Protect comes with a 25-year warranty. Use this materials estimator to determine how much tape you will need to buy. 

 If you’re still unsure about types of joist tape, check out this comparison on Decks.com, which includes DeckWise.

Because you must apply deck flashing tape on top of the joist, it is not usually installed on old decks. However, when it’s time to replace your old deck, make flashing tape a priority part of your construction.

What does joist tape do? In summary, it protects, it helps your deck last longer, and it gives you peace of mind. A deck is a perfect home improvement: build it to last.

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