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What Exactly is Pressure-Treated Lumber?


Approximately six billion board feet of pressure-treated lumber is sold annually, yet most DIYers know very little about this ubiquitous green-tinted wood. Here are a few facts:


Pressure-treated lumber is an exterior-grade wood that’s chemically treated to resist rot, decay, and termites and other wood-boring insects. It’s milled from softwood trees (typically southern yellow pine) and used for decks, fences, sheds, picnic tables, swing sets, and other outdoor projects.


Waterborne chemical preservatives are pumped deep into the wood’s fiber under extremely high pressure. Field tests conducted by the U.S. Forest Service have shown that treated lumber can resist decay and termite attack for nearly 40 years.

However, not all treated wood is created equal. The level of rot resistance is directly related to the amount of chemical preservatives in the wood. Lumber that’s stamped “Above Ground Use” should be use only where it won’t touch the ground, such as deck railings or fence boards. Lumber designated for “Ground Contact” can be placed directly on or in the ground.


To make sure you’re building with the right stuff, check the lumber’s label or stamp for its chemical retention level. This number represents the minimum amount of preservative retained in the wood and is expressed in pounds of preservative per cubic foot of wood. The higher the number, the more rot resistant the wood is.


The American Wood Protection Association has established the following retention levels for treated lumber:

.25 for above ground

.40 for ground contact

.60 for wood foundation

2.5 for saltwater contact


For more than 60 years, the chemical used to treat lumber was chromated copper arsenate (CCA). However, due to health concerns, CCA-treated lumber for residential use was banned back in 2003. (It’s still used for telephone poles, docks, boardwalks, and other large-scale commercial projects.)


Today, pressure-treated lumber is treated with an inorganic chemical, not arsenate. The most common chemicals used are Alkaline Copper Quaternary (ACQ), Copper Azole (CA), Sodium Borate (SBX), and Micronized Copper Quaternary (MCQ). These newer types of treated woods may be less toxic, but they also contain higher levels of copper so they’re much more corrosive than the old CCA-treated lumber. Many pressure-treated lumber manufacturers recommend using only stainless steel or hot-dipped galvanized nails, screws, bolts, anchors and connectors. And since these new wood treatments are especially corrosive to aluminum, it’s best to use vinyl or copper flashing, or to wrap the wood in a protective rubberized membrane.


Here are a few things to keep in mind when working with pressure-treated lumber:

Wear gloves when handling treated wood, and wash up thoroughly before eating or drinking. Always wear safety goggles and a dust mask when cutting, drilling or sanding treated wood. Cut treated wood outdoors, not in an enclosed space. Never burn treated wood.


Allow treated wood to dry thoroughly before staining or painting. Test dryness by sprinkling the wood’s surface with water. If the water beads up, the wood is too wet and you must wait before applying a finish. If the water soaks into the wood, then it’s dry and ready for stain or paint. If you choose not to stain or paint, then apply clear wood preservative annually to maintain the wood’s water repellency.


Before driving in a nail or screw, drill a pilot hole to prevent splitting the wood. This is especially important when fastening near the end of a board. Most treated lumber will shrink slightly across its width over time as it dries out. Take this small amount of shrinkage into account when laying decking or fence boards. After being outdoors for six to 12 months, treated lumber will develop cracks, called “checks,” along the surface of each board. These hairline cracks are a normal part of the drying-out process.

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