Pressure treated lumber is wood that has been chemically treated for protection from fungus, termites and other insects. Most wood that is exposed to the elements (especially ground contact) should be pressure treated.
Because sapwoods are more easily penetrated by chemicals than heartwoods, southern pine, which has a high percentage of sapwood, is the most readily available pressure treated wood.
Pressure treated wood can be left unpainted, but paint or stain will extend its life. It should be thoroughly dried before painting or staining. Newly installed wood can be left to dry for up to three months. If the wood is left unpainted or stained, a clear wood preservative can be applied annually.
Over time, pressure treated lumber will develop cracks or “checks” along its surface. This is a normal part of the drying process.
There are a couple types of common chemical treatments that are used in pressure treated wood:
- Chromated Copper Arsenate (CCA)
In this process, copper is used as the primary fungicide, with arsenic as a secondary fungicide. It had been in use for decades, but in 2004 this process was restricted through a voluntary agreement with the EPA and it is no longer being produced for residential or general consumer use.
Several processes have been developed to replace CCA treated wood. Because the processes listed below use high levels of copper, which is corrosive to steel, it is recommended that deck hardware and fasteners should be protected with a hot-dipped zinc coating (like ZMAX) or be made of stainless steel.
- Alkaline Copper Quaternary (ACQ)
This process uses copper and aluminum compounds acting as insecticide/fungicide. It has come into wide use following restrictions on CCA.
- Micronized Copper Quat (MCQ)
This process uses copper that has been augmented with azole biocides.
- Copper Azole (CA-B & CA-C)
This process uses copper that has been ground to micro-sized particles and suspended in water rather than being dissolved in chemicals.