A Firm Foundation: Protect Your Deck Before It’s Too Late

You’ve thought of all the essentials for your new deck: the perfect colour boards, stair lights that add ambiance and safety, sleek railings and a multi-level design. Your deck boards are guaranteed for 25 years and you’re ready to kick back, relax and repeat.

Here’s the thing: your deck boards will really only last as long as the foundation upon which it’s built. To protect that timber substructure, you need to think of the details, such as flashing tape, before the deck boards go on.

Why worry about joists and bearers?

The long-term life of your timber depends greatly on protecting its substructure against moisture. Wet timber equals deteriorating timber. Even worse, it may become a victim of dry rot or wet rot — more on that in a minute. Unfortunately, the timber in most decks begins to break down after as little as eight to 10 years if left unprotected. With most decks constructed from timber, this is a consideration for almost all deck builds.


Along with concerns about rot, you must also consider corrosion. Flashing tape acts as a barrier between your deck timber and galvanised metal used for joist hangers and it helps seal deck fasteners to prevent moisture penetration.

Creating a barrier between the timber and metal is important because lumber is often treated with preservatives containing alkaline copper quaternary (ACQ) and copper azole (CA-B and CA-C). These preservatives contain higher concentrations of copper and can be twice as corrosive for galvanised fasteners as older, banned preservatives.

Flashing tape prevents corrosion by creating a barrier between the metal and lumber. If left unprotected, the copper in the treated timber will leach out of the timber, corroding the joist hangers holding your deck together.

Deck Tape

How can I stop the rot?

First, and foremost, during construction, install flashing tape on the tops of your joists and bearers. Flashing tape is a small additional cost in deck construction and can be simply installed by a builder or homeowner.

With flashing tape, you’ll have a deck substructure that is less likely to rot, timber protected against the metal hardware corrosion, and decking screws tightly held in place — all for a relatively small investment. Because it comes in a self-adhesive format, it is easy for DIY enthusiasts as well as professional builders. Adding joist tape during construction also allows a timber deck to breathe, as it is applied to the tops of the joists and bearers.

While flashing tape comes in butyl and asphalt varieties, butyl tape offers several advantages: it endures a wider range of temperatures, is stickier, and is easy to install. Many butyl tapes come with a guarantee, so you have peace of mind about your deck’s substructure.

How do you apply flashing tape?

Once you’ve bought your high-quality butyl tape, installation is relatively simple:

  1. Prepare to install. Look at the weather and make sure it will be dry and above 4˚C.
  2. Cut the tape to the desired length and remove the tape backing to reveal the adhesion side. Cut the ends to the needed length.
  3. Apply the tape to all the joists and bearers. Push out any bubbles or wrinkles; use a hand roller if needed.

What is wet rot and dry rot?

Wet rot and dry rot are similar in that they both get their start in wet timber. However, there the similarities end.

Wet rot occurs when high moisture leads to the natural decay of timber. Water feeds and attracts this rot fungus called Coniophora Puteana, breaking down the timber’s cell walls and feeding on its nutrients. Common causes of wet rot include plumbing leaks, malfunctioning gutters, condensation penetrating a wall or timber post.

Rotting Wood

Dry rot, on the other hand, appears to be dry, as the fungi (called Serpula Lacrymans) target cellulose in the timber’s structure. While water helps it begin, once started it feeds on the moisture created from digesting the timber. That makes dry rot the more serious to treat. While you can end wet rot by cutting off the water supply, dry rot can continue growing.

Both can lead to significant cost, often requiring professional eradication. When replacing rotten deck boards, be thorough in your inspection of the compromised areas.

Why is deck ledger flashing important?

The ledger board, where your deck meets your house, requires special attention. According to Decks.com, if your ledger board is rotten, you will likely need to replace your entire deck.

Flashing can cover this crucial connector in your deck’s construction and keep it watertight. The first deck board is often mounted slightly away from your house, leaving a small gap where water can become trapped. Over time, this trapped water will cause your timber to rot. Seal this space with flashing.

What can I do after construction?

Maintenance will be key in continuing to protect your deck. Here are some quick tips in order to help minimise damage:

  1. Regularly sweep and clean your boards to prevent buildup of damp leaves or debris.
  2. Be sure to put a tray beneath any flower pots sitting on the deck.
  3. Seal any exposed timber, especially deck boards on an annual basis.
  4. Inspect your timbers regularly for damage. This checklist can help.
  5. Look for signs of insect damage and get treatment if needed.
  6. Remove vegetation that permanently shade the deck, and remove creepers from rails or other components, as these can trap moisture.
  7. Address any damaged timbers right away.
  8. Seal your decking boards annually or install composite decking, which is made of recycled plastics and doesn’t need to be sealed.

As you begin construction, be sure to install flashing tape and invest in quality materials. Taking steps now to protect your dream deck will allow you to enjoy it for many barbecues and afternoons reading in the hammock — for years to come!

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Kiron Barnden

Registered Builder, TrexPro and director of Norik Constructions Pty Ltd, which specializes in home renovations and Trex decks in the eastern suburbs of Melbourne.

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