If you think all preservative-treated wood is created equal, think again. Not all treated lumber is the same, and choosing the wrong type for the job can saddle you with expensive problems.
“My experience is that some contractors assume all of the green treated wood for sale in the store is suitable for any outdoor application,” says Jake Munson, Director, Wood Preservation at ProWood. “That’s not necessarily the case. The appropriate use differs by product.”
Retention Levels Matter
That difference is expressed by a metric called retention level, or how much chemical preservative the wood retains after being treated. Retention level is also called preservative loading, which is measured in pounds per cubic foot (pcf). The retention level determines the proper Use Category: Above Ground, Ground Contact, or Ground Contact Heavy Duty.
The concept is simple. Preservative treatment shields the wood from rot and fungal decay as well as from damage by insects such as termites. Lumber that is in contact with the ground or generally exposed to high moisture levels is more at risk of suffering such damage and thus needs more preservative.
What happens if you use wood with an inadequate preservative level? Let’s say you’re building a deck or a fence and mistakenly use posts treated for above-ground use. “Depending on the type of soil, it might be only a few years before fungus starts breaking them down,” says Munson. “That's because above-ground preservative loadings are a fraction of what we put into wood for ground contact.”
There are also spots where lumber that will never actually touch the ground should still be treated for ground contact. An example would be the floor joists on a deck that is built just a few steps above ground level: Because the joists are so close to the ground, they will be difficult or impossible to replace without disassembling a large section of the deck. Choosing ground contact-rated lumber with a high retention level is cheap insurance.
Callbacks aren’t the only consequence of making the wrong lumber choice. Lumber warranties are contingent on proper use, and using boards with an inadequate retention level can void the warranty. If you’re not sure whether the wood will be exposed to moisture, go with ground contact-rated wood.
Know the Numbers
The American Wood Protection Association (AWPA) specifies how much preservative needs to be retained in the wood for each Use Category. This varies by the type of preservative. For instance:
The retention levels for for Micronized Copper Azole (MCA) treated wood are:
- Above Ground (UC3B): 0.06 pcf
- Ground Contact General (UC4A): 0.15 pcf
- Ground Contact Heavy Duty (UC4B): 0.23 pcf
By comparison, the retention level for Ecolife (EL2) wood with a UC3B above-ground rating is 0.019 pcf.
Because retention levels vary with the type of preservative, the contractor should specify treated wood by Use Category. It’s easy to confirm that you’ve made the right choice: The Use Category and the retention level are listed on the tag stapled to the end grain of every piece of treated wood.
When placing an order with the supplier, it’s important to specify the correct Use Category—but the crew receiving the lumber should be trained on how to read these tags so they can make sure the supplier sent the right boards. “The framers should check the use category to ensure that the material was treated for the conditions it will see,” says Munson. Some suppliers only carry ground contact-rated wood, but checking the tag is still good practice.
End tags from a reputable manufacturer also include other useful information. “The tag should specify the use category and should confirm that the company follows AWPA standards,” says Munson. “There should also be a check mark confirming that the product meets building code specifications.”
Choosing treated wood with the proper retention level is a fundamental first step for any outdoor project, and making the right choice is a simple matter of reading the tag. You will be rewarded with long-lasting, trouble-free structures and a reputation as a quality contractor.
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