Deck Joists Should They Be Pressure Treated?


Any good home project involving lumber begins with making a good decision about the type of wood to use.

When it comes to deck joists, pressure-treated wood is the way to go. In fact, pressure-treated lumber is preferred for most of your deck’s structure, from the posts and beams to the joists.

But with so many lumber and other material options out there for deck projects, why should you go the pressure-treated route? Let’s take a closer look.

Why Should Deck Joists Be Pressure Treated?

Many contractors and home builders call using pressure-treated lumber for your deck joists the “logical choice.” It’s easy to see why.

Deck joists form the backbone of your deck frame, the repeated board that forms the structural base of your desk that is then attached to a ledger with joist hangars.

You don’t want to just pick any type of lumber to handle that job. It’s not just the deck joists that should be made from pressure-treated lumber. You can also use it for the posts, beams, and other parts of your deck. 

In general, pressure-treated lumber supports more weight and can span longer distances than other woods that are usually used in deck building, including redwood and cedar. Another big benefit: pressure-treated lumber is much less expensive than other woods used in deck construction.

Pressure-treated joists also help your deck last longer since it is usually exposed to whatever nature throws at it — termites, sun, rain, snow, and more.

That’s why homeowners should be careful when deciding between pressure-treated lumber for deck joists or reusing old non-treated framing for deck boards.

Old framing often has stress signs, like splinters or cracks and soft spots.  Warping and decay at the ends of boards are common. You’ll never want to use rotted or decayed wood for joists, posts, or beams in your deck.

Using pressure-treated deck joists ensures that you are starting out with a deck in tip-top shape, and you’ll avoid many of the common problems associated with older lumber that is not pressure treatedOpens in a new tab..

In general, it is best to use pressure-treated wood for any exterior projects.

But do be careful. For projects close to the ground, you should use pressure-treated wood graded for ground contactOpens in a new tab.. There are 12 levels of treated wood, but only a few a rated for ground contact. Click the link to read more about wood treated for ground contact.

Other Materials to Consider

In addition to pressure treating deck joists, many recommend using materials such as a wood protectant or joist tape, which both help protect the ends of lumber from damage caused by moisture.

Joist tape, made of either asphalt that has been rubberized or butyl, can be placed on top of the joists to also protect it from water and deafen the transmission of sound. This is just one of 15 ways to prevent joists from rottingOpens in a new tab..

If you use the right materials, your deck should last at least 15 years — often up to 30.

Pressure-Treated Wood Ratings and One Big Thing To Avoid

If you’ve settled on pressure-treated lumber, you may encounter its rating system. All pressure-treated lumber is rated, based on the preservative pounds retained in each cubic foot. 

What does it all mean? The higher the rating, the better the protection against moisture, insects, and fungi. Wood that’s used in areas where rot is more likely — like an outdoor deck — usually will contain more preservatives.

A cheat sheet on common ratings:

• Above-grade (.25 or .15) is usually used for decking, railing, and fence materials.

• Ground-contact (.40) is typical for decking, joists, beams, and posts.

• Below-grade (.60) is often used for posts that are buried partially below grade and for other wood foundations that are permanent.

Decking board should be tagged with the concentration number and the type of treating solution that has been used.

As you can see, not all treated lumber is created equally. One big thing to avoid when it comes to pressure-treated lumber for deck joists: heartwood. 

Heartwood is wood from a tree’s center. It’s denser so it doesn’t incorporate pressure treatment as easily as wood from a tree’s outer edges, called sapwood. Though it may be tough to find posts that don’t use heartwood, you can likely find it in treated lumber in 2×20 and deck board forms.

Another slight drawback to treated lumber: the boards will shrink after installation if they are dried in a kiln. Treated lumber may also be more inclined to crack once it’s set in place. That’s an easy fix: just use a water repellent every year or so.

In the end, though, pressure-treated deck joints have far more pros than cons compared to the alternatives.

Ryan Nickel

A Red Seal carpenter, passionate about building decks to be enjoyed.

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