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Sustainability is a growing and increasingly important consumer trend. In survey after survey, consumers report they are willing to pay more for sustainable products. When it comes to decks and other outdoor projects, they don’t have to. Among the options for building decks, preserved wood is typically the most cost competitive.
Wood also has a strong sustainability story in its own right. Wood products come from trees, our only truly renewable resource. In North America, at least, all wood products come from responsible sources-sustainably managed forests. The U.S Green Building Council reaffirmed this recently when it amended its LEED rating system to give equal footing to wood products certified by any of the major certification bodies-FSC, SFl and Responsibly Sourced.
Wood products also serve as storage units for carbon sequestered by trees as they grow. The stored carbon remains in the wood until it begins to biodegrade. That boosts the sustainability quotient for preserved wood. The whole point of pressure-treating with preservatives is to keep the wood from decaying for decades longer than it would otherwise, providing plenty of time for new trees to grow.
What about the chemicals?
The preservatives in treated wood sold for residential use do not migrate out of the wood and contaminate rainwater runoff or nearby soils. Two separate studies at Oregon State University have proven this emphatically.
In one, researchers collected rainwater runoff and soil samples from below a newly-built preserved wood deck in Oregon’s Willamette Valley where rainfall averages 47” per year. All the wood used to build the deck had been treated with copper azole (CA-C), the preservative found in nearly all residential-use treated wood.
They tested the samples for copper levels, as copper is the primary active ingredient in CA-C. Samples were collected over multiple years. The first samples collected showed copper levels slightly higher than control samples due to preservative that had dried on the surface of the wood and washed off in the rain. Copper levels in subsequent samples, including those taken during year four of the study, were statistically indistinguishable from the baseline samples.
Another study sought to determine whether preservatives from treated wood used to build planter boxes might migrate from the wood into the soil and ultimately be absorbed by food plants grown in the garden boxes. The researchers took soil samples from multiple locations in the planters and also analyzed the roots and leaves of the plants growing in them.
Soil samples taken from within an inch of the wood after the first growing season showed incrementally higher copper levels than the control samples. Soil samples from other areas in the planters showed no measurable difference. There also was no evidence in any of the plant samples to indicate the plants had somehow absorbed copper that had originated in the wood.
Samples taken after subsequent growing seasons were indistinguishable from the first-year samples, leading the researchers to conclude the initial incrementally higher copper levels in samples adjacent to the wood must have been caused by residual preservative that had dried on the wood’s surface and then washed off.
Customers can be confident the preserved wood they’re buying is both sustainable and safe. There is no cause for concern that the preservatives might migrate from the wood and contaminate the surrounding environment.
Details of both the deck study and the garden box study can be found at preservedwood.org.
