Pro Deck Installation Tip: Deck Joist Waterproofing

Applying tar paper or flashing tape to deck joists is easy and will extend the life of your deck.

Deck Joists 101

Deck joists are boards usually made out of pressure treated lumber. These boards come in standard sizes (2×6, 2×8, and 2×10) and are a crucial element in the structural stability of your deck. Joists systems are the frames that the decking will be attached to. This foundation is usually attached to the side of a house, or some other structure. The joists are hung from a header board, which is attached to the house. At the opposite end, there is a rim joist. The rim joist is the end piece of the joist system that caps all of the ends.

Is There Any Way to Waterproof Deck Joists?

Like any other wood, joists made of pressure treated pine are susceptible to decay due to water intrusion. To increase the life of your deck framing, there is a simple step that you can take. Although it will extend the time to complete your new deck, this material is proven to prolong the life of the joist system.

Adhering either tar paper or flashing tape to the top of your joists is easy to do. If you happen to have any tar paper or roofing material lying around, feel free to use these materials. They work in the same way builder’s paper does.

You will want to leave the least amount of openings for water to possibly enter. So, when you’re applying the strips, cut the adhesive to be at least a half-inch wider than the width of the joist. The edge of tape will wrap around the corners of the joist. This extra coverage over the edges will divert the waters path, prevent moisture from penetrating the wood, and reduce the chances of rapid decay.

Now that you have a better understanding on how to protect the joists from water damage, you can build your deck with confidence You know that you are putting in the extra steps necessary to ensure that your deck looks gorgeous and will last for many years.


A solid foundation is the most important part of any deck building project. If there is no dependable structure to uphold all of the hard work and time that was put into the project, you’re just creating the possibility for future headaches and messes. Water intrusion can be the most damaging element to any deck structure. This invasion of moisture can seep into the foundation and settle in crack and begin to eat away at the framework. This will cause rapid decay and compromise the stability of your deck. Knowing how to waterproof the joists will save you possible problems in the future, and is a very simple step to practice. After all, if you’re taking the time to build a beautiful deck made of Advantage Decking, then why not ensure the entire structure is as structurally sound as possible.

2 thoughts on “Pro Deck Installation Tip: Deck Joist Waterproofing”

  1. Great article, thank you for the info! I have a question. Instead of using tarpaper or flashing tape would asphalt shingles work? I have quite a few leftover from my roof. Thank you for your input.

  2. We always recommend using flashing paper because it creates a very tight bond with the wood in a continuous line and is what has worked for installation.

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