What is Flashing Tape?

Flashing tape is a self-adhesive, waterproof membrane that sticks to timber framing to create a moisture barrier at vulnerable connection points. On a deck, it is typically applied to the tops of joists, bearers, rim joists and the ledger board so that water cannot pool on the timber, seep into fastener penetrations or sit against metal hardware where it can lead to corrosion and rot. By sealing these high-risk areas, flashing tape helps the substructure last longer and supports a safer, more durable deck.

Prevent Deck Rot with Flashing Tape

Consider the timber holding up your deck and the timber connecting your deck to your home. These are two critical spots where water leaking through your deck boards can cause hidden damage unless your bearers and joists are protected. 

Using quality flashing tape can protect the top of the joist, rim joist, and ledger board from developing rot and timber decay. A butyl-based tape blocks moisture and helps deck screws hold longer, and is more durable than acrylic or asphalt-based tape. Flashing tape acts as a barrier between the timber and galvanized metal, such as joist hangers. It also helps seal deck fasteners, prevents moisture penetration, eliminates splitting from freeze and thaw, and provides a non-skid surface to make it safer during installation.

Learn more about how choosing the right flashing tape during construction can add years of life to your deck. 

Types of Flashing Tape 

When choosing a flashing tape, be sure to consider these essential factors:

Acrylic, Butyl, or Bitumen?

You can look at metal flashing tape for some purposes, but for timber, you’ll want to use an adhesive tape that is more rubbery. Flashing tape typically comes in three types: acrylic tape (G Tape), butyl, and bitumen. Bitumen, made from petroleum and asphalt, will dry out over time, reducing flashing effectiveness. That leaves butyl or acrylic. Butyl rubber tape has many advantages over the acrylic-based tape. Butyl tape, such as Trex Protect, is stickier, endures less stain, has less high-temperature oozing, and can be applied in a wide range of temperatures. Butyl tape is also more rubbery than acrylic-based tape, allowing it to flow better around the screws to create a tighter seal.  

Benefits of Flashing Tape

Easy Installation

Any contractor or DIY homeowner will tell you to buy a butyl tape that is easy to install and requires little trimming. A self-adhesive butyl tape that lies directly on the boards requires minimum cuts with little overlap.

Strength

When applying flashing tape to your bearers and joists, you want to be sure to purchase a tape that is not too thin and not too thick. With thin tape, you risk the long-term durability of the tape. A thicker tape can be challenging to install. You want a tape that will last as long as your beams and joists. Trex Protect is available in two widths 50mm X 20m for joists and 100 mm x 20 m for bearers, and 279.4 mm x 15.24 m for rim joists. 

Guaranteed results

Trex Protect is guaranteed to withstand the test of time. It comes with a 25-year warranty, so it lasts as long as the decking it supports. Acrylic tape comes with only a 300-day warranty.  

Flashing Tape for Decks

The average deck begins to experience water damage within 8 to 10 years. High-performance Trex Protect fights the rot and guards against costly future water damage — at a low cost. Adding flashing tape when building your deck is also an effective way to protect your deck and safeguard your decking cost investment from damaging moisture, helping the substructure last as long as the decking it supports. For an average-sized deck, measuring 3.6m x 7.3m, five rolls of Trex Protect Joist Tape and one roll of Trex Protect Bearer Tape will be all you need. 

To find out exactly how much Trex Protect Joist and Bearer Tape your project requires, use our Materials Estimator.

Similarly, wall flash is used when constructing a new deck to protect the ledger connection from moisture. And if you’re installing a Trex RainEscape under-deck drainage system, easy-to-install, self-adhesive Trex RainEscape Wall Flash seals the space between your house and ledger board to keep water out. Wall flash is best used when constructing a new deck and is another small investment with significant returns. Trex RainEscape Wall Flash comes with a 25-year warranty from Trex, the world’s #1 composite decking brand. See how Trex RainEscape Wall Flash protects in this video.

For more detail around ledger protection and step-by-step instructions on how to install deck flashing, be sure to review the guidance before you start fixing the ledger and deck boards.

How to Install Flashing Tape

Installing self-adhering flashing tape is a relatively simple process, with four main steps. 

Begin by preparing your surface. TTrex Protect Joist, Bearer and Rim Tape is applied to horizontal and vertical surfaces. Trex Protect Tape has three widths making it convenient to apply to joists, double bearers and rim joists. Use 50mm x 20m Joist Tape for joists, 100mm x 20m Bearer Tape on all double bearers and 279.4 mm x 15.2 m for rim joists. 

When pulling deck boards onto the deck, use a piece of plywood or wafer board to overhang the edge of the deck. This method will prevent the product from rolling up and pulling up from the joist.

1. Clean the joist surface, ensuring it is dry and free of debris. 
2. Open the wrapper and remove the backing while applying the tape directly to the surface. Run your hand over the tape and firmly press it onto the joists.
3. After covering the joists and beams with tape, cut the tape to length with a utility knife or scissors.

As you plan your deck build, don’t forget the flashing tape. You have many years ahead to enjoy your deck! Don’t let timber rot become the enemy of your beautiful outdoor living experience.

Common Mistakes to Avoid When Installing Flashing Tape

Even a good product can underperform if it is installed incorrectly. Avoid these common mistakes when working with flashing tape on a new or refurbished deck:

• Applying tape to damp, dirty, or rough timber.
  Flashing tape needs a clean, dry, smooth surface to bond properly. Dust, sawdust, loose fibres, or moisture can prevent good adhesion and create air pockets where water can collect. Always brush or blow off debris and allow the timber to dry fully before you start.
• Leaving gaps at joints, cuts, and corners.
  If tape sections do not overlap or wrap cleanly around corners, water can find its way under the membrane and sit directly on the timber. Make sure seams overlap, and press the tape firmly into inside corners, over joist ends and around the sides of rim joists to create a continuous barrier.
• Using the wrong type of tape for deck framing.
  Bitumen and some general-purpose tapes can dry out, crack or ooze in high temperatures, reducing long-term performance. For deck substructures, a butyl-based product like Trex Protect is formulated to stay flexible, self-seal around fasteners and withstand the temperature swings decks endure.
• Not rolling or pressing the tape firmly into place.
  Simply laying tape on top of a joist is not enough. Use your hand, a small roller, or a block of timber to press the tape firmly onto every surface so the adhesive makes full contact. This helps it seal around any fastener penetrations and reduces the risk of water tracking underneath.
• Leaving flashing tape exposed to sunlight long-term.
  Most flashing tapes are designed to be covered by decking boards. Extended UV exposure can shorten the life of the tape, so install your decking as soon as practical after taping, and never use joist tape as a finished surface.

Frequently Asked Questions

Here are answers to the most common questions about choosing, installing, and maintaining flashing tape for long-lasting deck performance.

How long does flashing tape last?

Lifespan depends on the adhesive technology and product quality. Bitumen-based tapes typically offer little to no long-term warranty support and are more prone to drying out and becoming brittle over time. Acrylic tapes can provide short-term performance but often carry very limited warranties—sometimes as little as 300 days. High-quality butyl flashing tapes are engineered for longer service life, with products offering multi-year warranties that range from 10 to 20 years, depending on the manufacturer. Trex Protect, as a premium butyl tape designed specifically for deck substructures, carries an industry-leading 25-year limited warranty when installed and covered correctly. Choosing a tape with a warranty that matches the expected life of your deck framing is a good benchmark.

Can flashing tape be installed on an existing deck?

Yes, but access is the deciding factor. The best time to install flashing tape is during new construction or when you are re-decking and the joists, bearers and ledger board are fully exposed. On an existing deck where the boards are still in place, your options are limited to areas you can reach, such as exposed joist ends, stair stringers, or spots where boards are being replaced. If you are resurfacing a timber deck with new boards or composite decking, it is an ideal opportunity to clean the framing and add flashing tape before reinstalling the surface.

Can flashing tape be exposed to sunlight?

Flashing tape is designed as a concealed moisture barrier, not as a finished surface. Most manufacturers, including Trex Protect, require that the tape be covered by decking boards and not left exposed to UV for extended periods for the warranty to remain valid. Short-term exposure during construction is generally acceptable, but you should aim to install the decking soon after taping.

Do I need flashing tape for composite decks as well as timber decks?

Yes. Composite or PVC deck boards still allow water to drain through the gaps, so the timber substructure beneath is exposed to the same moisture as it would be under a timber deck. Flashing tape protects joists, bearers, rim joists and ledger boards from that repeated wetting and drying cycle and helps prevent corrosion where metal connectors meet the timber. Using joist and bearer tape under composite decking is a simple way to help the frame last as long as the surface boards.

Does flashing tape affect my deck warranty?

Always check the warranty documents from your decking and framing suppliers and follow their installation instructions so that moisture-protection products and fasteners all work together as intended. Substructure protection helps reduce the risk of rot and structural movement that can impact board performance. Using a compatible flashing tape as recommended during construction, such as Trex Protect with its industry-leading 25-year limited warranty, typically supports long-term deck performance and helps protect your investment.

Sources

Decks.com. (2024, July 15). Types of Deck Flashing.
Decks.com. (2023, November 3). Joist Tape: What Is It and Do I Need It?.
NADRA.org. (n.d.). Five Steps to Building a Water-Resistant Deck.
NRLA.com (2024, October). Water Management for Wood Decks: A Retailers Guide.