What is Pressure-Treated Wood? Types, Pros, Cons, & FAQ
Pressure-treated wood is wood that has been treated with certain chemicals to help it resist natural decay, mildew, rot, insects, and fires. It’s an important component of exterior building projects, especially those which come in contact with the ground. But how is it made? And how does this affect how builders use it? Read on to learn more.
How Does it Work?
Pressure-treated wood is created by placing cut lumber into a large steel cylinder called a vacuum pressure seal. The wood is sourced from regional lumber mills to both save on costs and prevent the transmission of pests. The wood must be dried to a certain point since wood that is too wet will reject the additional moisture of the treatment chemicals. Some woods naturally resist the chemicals even when they’re dry enough. For these woods, small slits are cut into the sides of a board along its entire length to encourage absorption of the solution.
With properly prepared wood in the chamber, operators pump all the air out, creating a vacuum. The chamber is then filled with the desired chemicals to a certain pressure level to force the solution into the wood. When the cycle is complete, the chemicals are pumped out and the wood is removed and placed on large drip pads to dry for 24 to 48 hours. It is then tagged with a label describing what solution and in what concentration it was treated and sent to retailers. Often, it’s still too wet when it reaches its destination, so hardware stores must let it dry even further before it’s ready to be sold.
What are the Different Types of Pressure-Treated Wood?
There are three main chemical solutions used to treat wood.
These are water-based mineral salt solutions that help retain the color of the wood (since it can discolor during pressure treatment) and resist mold, mildew, fungi, and insects. They aren’t good for use in constantly wet environments though, as water will eventually begin leaching the chemicals into the surrounding environment.
Alkaline Copper Quaternary (ACQ)
This is a more environmentally friendly solution containing copper and ammonium alkyl. It’s relatively safe and less prone to leaching but still shouldn’t come into contact with human or animal food. Unlike the borates, it does tend to change the color of the wood.
The final category of treatment chemicals, these are solutions primarily designed as fire retardants. They’re used more in industrial applications and less applicable for residential use.
Beyond the differences in chemical solution, pressure-treated wood also differs in the concentrations with which it’s treated, dividing it into two main use categories.
This wood has a lower concentration of treatment solution and thus less protection. It can only be used in applications more than 6 inches from the ground and where the wood has proper ventilation and drainage. It’s a good choice for situations where the wood is easy to access and thus simple to maintain and replace and has a useful lifespan of up to 10 years.
Ground-contact wood has twice the level of chemical retention and protection as above-ground wood giving it a useful lifespan of up to 40 years. Thus, it can be used in applications where it will make contact with, or be buried in, the ground as well as above-ground uses. Any time wood is less than 6 inches from the ground, ground-contact wood must be used. It’s also the better choice for locations where the wood is harder to maintain and replace.
Where is it Used?
Pressure-treated wood is used primarily in exterior applications or in foundations–areas where wood is more exposed and thus susceptible to decay or infestation. Decks and fences are the two most common uses of pressure-treated wood.
The treatment process makes it more expensive than non-treated wood, so isn’t necessary for heavily protected, interior spaces where regular boards can be used. In fact, if in contact with metal piping, the chemicals from the treated wood can leach and corrode the metal pipes. Thus, if you use treated wood in protected, interior spaces, you’ll probably have to also use more expensive galvanized piping. It’d be better just to use untreated wood and save money all around.
Ground-contact treated wood is a good choice for raised garden beds, suitable for both ornamental as well as edible plants. If growing edible plants, however, it’s a good idea to line the interior side of the treated wood with plastic to prevent leaching into the plant soil. Make sure you don’t line the bottom of the bed as this will prevent proper drainage–only the areas making contact with the treated wood are necessary.
Advantages of Pressure Treated Wood
Pressure-treated wood provides numerous benefits when used in appropriate applications. Wood, being an organic material, will naturally decompose when exposed to moisture and wood-eating bugs. Anytime wood makes contact with the ground the risk of decomposition increases dramatically. This, of course, poses serious problems when you’re trying to live in a house made of wood, both for safety and expense.
Treating the wood with preservative chemicals resists this natural cycle of decay. The chemicals also act as a deterrent against wood-eating insects like carpenter ants and termites. When used smartly, pressure-treated wood thus greatly increases the lifespan of a given structure and likewise reduces the maintenance and expense necessary for upkeep.
Disadvantages of Pressure Treated Wood
As with everything, there are also some drawbacks to using pressure-treated wood that are good to consider. Prior to 2004, the main chemical used in treatment was arsenic. For obvious reasons the EPA banned its use and, since that date, new construction has been considerably safer. However, arsenic was cheap, and copper is expensive, meaning the price of pressure-treated wood has increased dramatically since then.
Even though the new chemicals are far safer than previous formulations, they can still leach into their surrounding environment making them unsuited to certain applications. As stated previously, this makes treated wood a bad choice for home interiors where the chemicals can corrode other components, such as piping. Any aquatic use is also highly discouraged as the chemicals will cause issues for wildlife and water quality. Instead, it’s better to use greenwood for dock pilings. Greenwood is a special, rot-resistant hardwood that’s untreated and thus much safer for marine wildlife.
Another downside is disposal. Because of the chemicals in question, treated wood must be taken to the dump to dispose of properly. It won’t decay in a timely manner, so it’s not a good option for use in a compost pile (also the chemicals will wreak havoc on your friendly decomposer bugs). You absolutely should never burn treated wood. Doing so will vaporize the chemicals allowing exposure to your mouth, nose, eyes, and lungs and causing serious issues.
Frequently Asked Questions (FAQs)
Can I Paint Pressure Treated Wood?
Pressure-treated wood can be difficult, though not impossible, to paint. Even once it’s dry enough to sell in stores, it still contains moisture that it will eventually lose. This will result in the wood shrinking and cracking any paint you’ve applied in the process. It also may be moist enough to reject the paint in the first place. If you want to paint, you’ll have to wait until the wood is dry enough–which can take several months.
You can tell when the wood is dry enough to paint by sprinkling a small amount of water on it. If the droplets are readily absorbed, you’re good to go, but if they bead on the wood’s surface, there’s still too much moisture. Alternatively, if you want to apply some sort of color, but don’t want to wait months for the wood to dry enough, you can use sealers or stains. The wood will take them far more easily than paint and you won’t be left with any cracked surface issues as the board dries.
Is Pressure Treated Wood Safe?
The chemicals used in treatment are safe for contact in small amounts, though prolonged exposure can cause irritation and other issues. If you’re doing a DIY project, take typical precautions such as wearing gloves and washing your hands after handling. It’s also a good idea to wear a mask and eye protection while cutting treated wood, and as previously stated, never burn treated wood. Children should also not be allowed to come into contact with treated sawdust. Treated wood is also safe for use in gardens–though organic hardliners would disagree. You can line the wood with plastic to give the soil an added level of protection from its chemicals.
Pressure-treated wood has served as a vital tool in the carpenter’s toolbox for decades. It makes our modern lives easier by reducing the maintenance needed to keep our houses standing for long periods of time. With the knowledge of how to use it properly, you too can craft a beautiful, sturdy addition to your home.
Featured Image Credit: bricoydeco, Pixabay