Last Updated: Jul 2, 2021
Pressure-treated lumber has generally gotten a bad reputation for its toxic chemicals, notably arsenic and chromium. However, in the early 2000s, copper arsenate (CCA) treatment was phased out for residential use across North America, and new, potentially less toxic substitutes filled in the niche. As we learn more about the toxicity of various chemicals, our science advances to discover safer alternatives. Treated lumber has been a beneficiary of modern science, finding less toxic options. However, there's still progress needed to create a genuinely non-toxic and sustainable lumber treatment. This article focuses on the most common pressure-treated systems that use copper as the main ingredient.
Pressure-treated wood decking is different from untreated wood decking due to increased resistance to moisture, rot, fungal decay, insects, and physical damage. Untreated wood is treated with pressure and vacuum technology that forces preservatives deep into the wood grains during the manufacturing process. Pressure-treated lumber is labeled with the type of chemicals it was treated with and whether it is rated for ground contact or used above ground.
This article focuses on copper-based preservatives that result in lumber rated for ground contact applications. Our article on non-metallic treated wood covers lumber rated for above-ground use.
Treated wood decking is typically composed of pine, spruce, or fir lumber. These species are not naturally rot-resistant but are abundant and locally available species. The lumber is then treated with different chemicals, most commonly ammonia copper quaternary (ACQ). Other treatments are copper azole (CA), micronized copper quaternary (MCQ), and micronized copper azole (MCA). The micronized versions are newer advancements that reduce corrosion potential to fasteners. Still, studies are ongoing on using these nano-sized copper particles and their environmental impacts.
ACQ is a water-based treatment that contains no arsenic or chromium and can be used in above-ground or ground-contact applications. ACQ is approved by the American Wood Protection Association (formerly the American Wood Preservers' Association).
It's pretty challenging to find a universal answer to this regarding any decking material. There are pros and cons to each of them. For treated wood decking, it depends on the type of chemical treatment, where and how the wood is harvested, and how far that wood had to travel to get to you.
On the lumber front, wood is a renewable resource that reduces our demand for metals and petroleum products. Wood is the only renewable resource available for decking. There is a perception that cutting trees is always a bad thing, but we need wood products. If we stop domestic production of wood products, they will be outsourced to developing countries. Often, these countries have not yet implemented any strict environmental or social regulations. From a global perspective, it is likely best to source lumber and manages forests domestically and responsibly rather than outsource these materials. However, wood is not always sustainably sourced; it's a dynamic material in which harvesting is quite complex. Some wood can come from unsustainably managed forests where they clear-cut large swaths of land. Other wood could be coming from old-growth forests that are multiple centuries old or from boreal forests or tropical forests where regeneration is difficult. Thankfully, there are certifications such as Forest Stewardship Council (FSC), Sustainable Forestry Initiative Program (SFI), Canadian Standards Association (CSA) that set rigorous standards for sustainable forest management. We discuss these and others in our Rise Guide to Wood Certification Programs and Sustainable Forestry.
On the chemical front, ACQ is a big step up over CCA primarily because it eliminates arsenic. The EPA claims that replacing CCA with ACQ was one of the most dramatic pollution prevention advancements in recent history. However, because of the toxicity of copper to aquatic organisms and ACQ may leach more copper than CCA, it is not recommended for applications near marine ecosystems. It is recommended to seal the lumber once a year to reduce leaching. During manufacturing, ACQ generates more air pollution than CCA did, mainly in the form of ammonia.
Copper mining is necessary to create pressure-treated lumber, and its negative environmental and human health impacts are well documented. On the other hand, treated lumber reduces the number of cut down trees because it increases their service life.
Based on its use of copper oxide and quaternary ammonium, ACQ is a relatively low risk to human health in the average level of exposure we have with it after installation. Further research on ACQ chemicals' components determined no acute or chronic adverse effects on adults or children in contact with ACQ-treated lumber. The main concern is during cutting and the sawdust that comes with it. Users should ensure that they take precautionary measures and wear gloves, safety glasses, and a mask when cutting and handling the lumber.
While the research shows low risk for adverse effects on adults and children, no precaution level for young children is too great. So it is still recommended to be mindful of children crawling on treated lumber decks and subsequent hand-to-mouth behaviors.
Manufacturers recommend that you seal the lumber after installation. If pressure-treated wood decking has not been sealed, you should avoid walking on it barefoot.
Treated lumber decking is typically the most affordable decking option for initial installation. The cost per square foot is generally between $5.75 and $12.50. There is an added cost of annual resealing that is recommended you should take into account. Manufacturers marked ACQ under brand names Preserve®, Preserve Plus®, NatureWood® by Osmose, ProGuard™, and Wolmanized Natural Select by Arch Treatment.
If you maintain your pressure-treated deck, it can last 50 years. A roof over the deck will allow it to last much longer. A trick to increasing the lifespan of any wood deck is to increase the distance from the ground. By keeping ample space between the deck and the ground, the wood will absorb less moisture from the soil and dry out quicker.
You can cut treated lumber with any standard wood cutting tools. Sawdust is the primary source of health concerns with handling treated lumber, so always wear a dust mask, goggles, and gloves when cutting and handling the wood. You should also wash the clothes you wear while cutting the lumber separately from other clothing before use. Always wash exposed areas of your skin after cutting the materials, as dust can settle on you.
Power washing on low pressure is acceptable to cleaning off decks. It would be best if you did this before the annual touch-up of the sealant.
Treated lumber should be resealed once a year. You should stain every two to three years. If you chose to paint instead of applying a stain, repaint as necessary.
Newer formulations of wood treatment are eliminating metals like copper from the ingredients list. These recent advancements are exciting developments to improving the sustainability of treated lumber by reducing the potential for leaching of metals from the lumber over time. These brands are EL2 (sold as Ecolife-branded lumber) and PTI (sold as Wolmanized EraWood).
Treated wood decking is an affordable decking option with a variety of advantages and disadvantages. If forests are managed holistically, wood products are among the most sustainable and renewable building materials available. Despite this, there are warranted concerns about the chemicals used in pressure treated wood, the mining and manufacturing necessary to creating these chemicals, and their debated toxicity to humans and aquatic life.
If you choose to build a wood deck, whenever possible and appropriate, use locally sourced and naturally durable, untreated wood, particularly in areas of the deck where people or animals will come into contact with wood or in areas near water. Inherently durable species are western red cedar, redwood, and douglas fir, western larch in western North America, white oak, eastern red cedar, northern white cedar, black locust, and catalpa in eastern North America. You can surface seal untreated wood decking with non-toxic products such as Vermont Natural Coatings' Penetrating Water Proofer.
To learn more about various types of decking, check out more of our decking articles:
Based in Washington State, Tom's education focuses on holistic land management that sustainably grows renewable building materials in a way that replenishes natural systems. His interest is in building systems that combine old techniques and modern science.