Should Deck Joists Be Pressure Treated?

Most, if not all, deck joist materials are made of wood. One of its disadvantages is its susceptibility to moisture. Now, you may ask, should wooden deck joists be pressure treated? We researched the answer to this topic, and here is what we got. 

Pressure-treated wood is the preferred choice for deck joists. Pressure-treated lumber is chosen for the majority of the deck's structural components, including the posts, beams, and joists.

Continue reading as we discuss why it is important for joists to be pressure treated and when you should use them.

We'll also talk about how long pressure-treated deck joists last, ways to make them last longer, and the benefits of using them. Additionally, we'll also tackle pressure-treated wood grades. 

A complete decking frame fixed to posts ready for decking boards in a garden, Should Deck Joists Be Pressure Treated?

Why The Need To Use Pressure-treated Deck Joists?

Deck joists are the structural backbone of your deck frame; they are the repeated boards that make up your desk's base and are connected to a ledger using joist hangars.

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Worker installing wood floor for patio

You shouldn't choose any kind of lumber at random for that task. You can use pressure-treated wood for more than simply the deck joists. Additionally, you can utilize it for your deck's posts, beams, and other components.

In general, pressure-treated lumber supports more weight and can span longer distances than other types of wood that are usually used in deck building.

Another big benefit: pressure-treated lumber is much less expensive than other woods used in deck construction.

Because your deck is typically exposed to the elements, including termites, the sun, rain, snow, and more, pressure-treated joists also help it survive longer.

When Should You Use Pressure-Treated Wood?

Unsure whether to use pressure-treated deck joists? Here are some scenarios where you need to use this kind of lumber for your deck joists:

When There Is Direct Contact To Moisture

If there is direct contact between the wood and anything that could provide moisture, use pressure-treated wood.

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Repairing and installing new decking around a Swimming Pool.

This refers to posts that are buried or in contact with concrete, but it also encompasses any lumber that comes in contact with masonry or concrete because it is porous and absorbs water like a sponge.

Before wood-decaying fungus and other organisms begin to cause the wood to deteriorate, untreated wood can absorb up to 18% of the moisture in the air it comes into contact with.

On the other hand, pressure-treated lumber is typically safe for up to 28% moisture.

Where It Is Out Of Reach From Humans And Pets

You could absorb something that could eventually make you ill if you touch the wood and pick up these chemicals, then wipe your eyes or eat some food.

Because of this, builders advise that you only use pressure-treated lumber for the deck's structural support and switch to untreated wood for the deck's surface while using non-pressure-treated wood indoors.

How Are Woods Pressure Treated?

The lumber is treated by placing it in a large, cylindrical chamber called a retort, which has an airtight door on one end, and injecting waterborne chemicals under pressure into the wood.

Just like a colored translucent stain soaks into and becomes a part of wood trim within a home, the preservative chemicals are driven deeply into the cellular structure of the wood and become part of the wood.

The lumber is taken out of the retort after the chemical has been injected into it, and it is placed on a drip pad to catch any chemical preservative that leaks out of the lumber. In the subsequent batch, this liquid is recycled and used again.

Completely drip off the excess chemical before using the lumber. In the hot summer heat, this might just take a few days, but in the damp, chilly winter conditions, it might take up to two weeks.

How Long Do Pressure-Treated Woods Last?

pressure treated 2x12x16 resting on stands

Pressure-treated lumber lasts for a very long time, whether you use it for framing or even for decking.

Without care or maintenance, pressure-treated lumber exposed to freeze-thaw and wet-dry cycles will survive around nine years. The same deck can endure for more than 40 years with the right care and treatment.

Many manufacturers offer a 30-year rot and bug warranty on their treated wood.

How to Extend the Life of Treated Deck Joists

Your deck joists can endure longer in several key ways, including:

  • Apply a moisture-resistant stain or sealant to the joists before installation.

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Applying protective varnish on a wooden boards.

  • Use metal joist hangers and provide ventilation with at least 1/8 inch between the sides of the bearing joists and the end of the lateral joists.

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  • Install decking boards using concealed fasteners to avoid contact with the joists and maintain a constant 14-inch space between them.
  • Keep the deck clean frequently to avoid debris collecting moisture onto the boards.
  • When building a new deck, cover the joist tops with butyl-based flashing tape.

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What Are Pressure-Treated Wood Grades?

The following are the wood grades in which pressure-treated lumbers are sold:


This is the highest rating for decking with a 3/4-radius edge.


Very few flaws are present in the highest possible grade. All knots must be soundly enclosed, and the slope must be at least 1/12 grain. It looks fantastic and has a high degree of consistency.

Number 1

This grade will not have any splits that are wider than the board's width. One hole is allowed every 3', and knots cannot be bigger than 2-3/4".

Number 2

The grain slope must be at least 1/8. Boards may not have splits that are wider than 1.5 times the board's width. One hole may be drilled every two feet in wane corners; however, knots may not be greater than 3-1/2".

Number 3

This grade is of the lowest quality. This grade is unfit for use in deck construction.

The fewer cracks or knots, often known as flaws, the higher the grade. For decks, you should generally pick boards of the Number 2 grade or higher.

Moreover, the retention level of pressure-treated wood reveals whether the chemicals used to treat it can be held on rather than leached.

The scale goes from 0.25 for wood that is above ground to 2.5 for wood that has been submerged in seawater.

What Are The Types Of Pressure-Treated Wood?

There are 3 types of pressure-treated lumber. These are: 

1. Above-ground

If exposed to dampness or standing water, some pressure-treated wood types will leach off the preservative that was used to treat them.

You may use these above-ground woods frequently when rot is not the main issue but rather pest resistance.

2. Ground Contact

Applying protective varnish on a wooden boards.

Ground contact pressure-treated wood can persist for up to 40 years, even in wet environments, and is defined as lumber that has been treated such that it effectively retains the chemicals employed to preserve it.

When wood comes into contact with the ground or plants, is within six inches of the surface, or is positioned in, on, or above freshwater, it is said to be in-ground contact.

3. Marine Lumber

You can typically use this particularly durable pressure-treated wood for docks, seawalls, and other seaside structures.

summer, pier, jetty, Desk, Dusk, Sunset

What Are the American Wood Preservers' Association (AWPA) Use Categories Of Pressure-Treated Wood?

For categorizing wood usage, the AWPA offers a 12-tier system that ranges from UC1 up to UC5 (wood that is submersible in salt water). Listed in the table below are the categories and their corresponding descriptions or uses:

Category Description/U
UC1 Dry Interior
UC2 Damp Interior
UC3A Above-Ground Exterior, Covered with Rapid Water Runoff
UC3B Above-Ground Exterior, Uncoated, or Poor Water Runoff
UC4A General Use
UC4B Heavy Duty
UC4C Extreme Duty
UC5A Northern Waters (Salt or Brackish Water)
UC5B Central Waters (Salt or Brackish Water)
UC5C Southern Waters (Salt or Brackish Water)
UCFA Indoor Above-Ground Fire Protection
UCFB Outdoor Above-Ground Fire Protection

Note that you can use UC4 for ground contact lumber and UC5 for marine lumber.

You can increase your structure's lifespan, and you can protect your investment better if you use the right kind of treated wood. Depending on the type of contact, UC4A, B, or C is the optimum pressure-treated wood for ground contact.

UC4A has more chemicals per cubic foot of wood than UC1, 2, or 3 categories; UC4B has more chemicals than UC4A, and UC4C has even more chemicals.

What Are The Advantages Of Using Pressure-Treated Wood?

Utilizing pressure-treated wood has these advantages because of its chemical and preservative treatment:

  • Pressure-treated wood is resistant to the microorganisms that cause wood to rot thanks to the chemical compounds used in the process.
  • Similar to this, treatments ward off hazards posed by common insects like termites and carpenter ants.
  • Although pressure-treated wood is more costly than untreated wood up front, it frequently proves to be more cost-effective in the long run due to its long lifespan.
  • Like other types of wood, pressure-treated wood is light and simple to cut and secure to other supports.

background of treated wood planks

In Closing

When you use pressure-treated wood for your deck joists, you can ensure the longevity and durability of your structure. Also, you can save on repair costs in the long run compared to when you use untreated wood. 

Before leaving, you can check out these related posts on this page: 

Can You Use Pressure-Treated Wood For Framing?

How Often Should You Stain The Deck?

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