What is Pressure Treated Lumber?


You have heard that you should always use treated lumber for a outdoor project. But what is pressure-treated wood?

Pressure-treated lumber, is wood that is professionally treated and infused with a compound of chemical preservatives to prevent wood decay and rot frequently caused by insects or moisture exposure.

Pressure-treated wood is used primarily for outdoor use, especially in backyard decks, furniture, and fences. When wood is pressure treated, it not only makes it less vulnerable to rot and general water damage, but some treatments make it fire-retardant.

Let’s take a closer look at pressure-treated lumber, including how its created, its benefits, and different types of wood to choose from.

The Pressure-Treated Lumber Process

Combinations of different types of chemical preservatives are infused into the wood to make it pressure-treated. Air is then removed from the wood by a vacuum and replaced by the preservatives by putting it into a holding tank, a horizontal cylinder made from steel that’s called a retort) that’s depressurized.

It is the most effective way for wood to become less susceptible to insects and disastrous rot.

Chemical treatments are driven strongly into the lumber by using 160 pounds of pressure. The lumber is then placed on a drip pad for it to dry before it is shipped off to a lumber supplier.

What are Pressure Treated Lumber Chemical Preservatives?

There are 4 common types of chemicals used to preserve outdoor wood.

  • MCA (micronized copper azole)
  • CA (copper azole) 
  • ACQ (alkaline copper quaternary) 
  • CCA (chromated copper arsenate) 

Source: US Forest Products LaboratoryOpens in a new tab.

MCA and CA are the most common treatment for outdoor home projects like fences and decks. Its biggest advantage as a wood preservative is its minimal environmental and health impact compared to other types of treatment. But they also require higher levels of treatment for the same level of protection compared to other compounds.

ACQ includes of the addition of ammonium, making the wood less edible for fungi and insects. Reducing rot more effectively with smaller quantities.

CAA is reserved for wood used in extreme environments like saltwater docks. It once was much more common for preserving wood, but its health and environmental concerns have reduced its popularity.

Pressure-Treated vs. Untreated

Pressure-treated lumber is a relatively new innovation. While wood has long been used as the preferred material for deck projects, it has just been in the past century that pressure-treated lumber has been available and preferred for outdoor work.

Pressure-treated wood is far more durable than untreated wood and lasts longer than natural wood. It also resists elements far better than untreated. Pressure-treated wood is usually less expensive than other natural rot resistance woods, like redwood and cedar.

But even with the additional cost of treatment, it will save you money and dirty work trying to replace rotten joists under your deck.

Especially when compared to untreated wood, which will deteriorate quickly outdoors compared to pressure-treated wood. Though some natural, unfinished wood has natural chemicals that fend off insects and harsh weather, they are often more expensive and have less structural strength.

Making pressure-treated wood the first choice for most outdoor projects. Especially wood.

Types of Pressure Treated Lumber

Pressure-treated lumber comes in a range of applications and sizes, as well as brand names. However, there are two main types you’re likely to find in a store when planning your backyard deck.

Above-ground wood that is pressure-treated should only be used for areas of the structure that are more than 6 inches (150mm) off the ground. Where the wood will have proper drainage and under deck ventilationOpens in a new tab.. Plus above ground wood has less protection against fungi which is in the soil.

There’s also ground-contact treated wood, which can be used in contact with the ground or above it. Ground-contact woodOpens in a new tab. is more protected compared to above-ground wood with twice the level of chemical retention.

It should be used when the wood is less than 6 inches (150mm) above the ground or poorly ventilated. It is good to use for applications that are difficult to replace or maintain.

There are also grades placed on pressure-treated woodOpens in a new tab. that’s based on their overall appearance. Those with higher grade numbers have fewer cosmetic blemishes and knots (they also come with a higher price).

Uses for Pressure Treated Lumber

The most common places in a home to find pressure-treated lumber? Decks and fences. Outdoor structures are made for pressure-treated wood, so you’ll find them on anything from walkways to docks and anywhere that is regularly exposed to natural elements.

Landscaping projects, such as flower beds, often use pressure-treated lumber. If the lumber is touching something edible, like a vegetable garden, you can use a liner.

When using pressure-treated lumber, it can be stain or paint — just make sure they are dry.

Ryan Nickel

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

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