Drying Pressure Treated Lumber

Advice on the complications involved in kiln-drying preservative-treated lumber. April 2, 2013

Does anyone have experience drying pressure treated SYP? We need to dry 15,000 - 20,000 bf of 2x lumber per week. Do the treating chemicals cause any more problems than the acid from red oak? Any other problems to watch out for? I was thinking of running it through a pre-dryer first, then a 140* - 160* kiln. I read that the chemicals slow down the drying process. How fast will it dry down to 17 - 19%? Also, how much airflow should we use?

Forum Responses
(Sawing and Drying Forum)
From contributor B:
It's tough to answer precisely because the pressure treating process can vary. For example, depending on time and level of vacuum and pressure, and sequence, final MC can range from around 40 to 100%. The cycle can also affect dripping of solution from the wood during drying. Also, is the pressure treated CCA or one of the newer formulations? The chemistry differs. Using the predryer first and then the kiln is a good plan, though treaters who do KDAT go straight to a kiln. Lower initial temps help avoid excessive dripping. Drying can typically be on the order of about four or five days.

From Professor Gene Wengert, forum technical advisor:
You can dry it just like drying green SYP, although more slowly to avoid opening up any checks that will expose untreated wood. However, any runoff should be collected and put into the treatment tank (that is, recycled). In my opinion, redrying is best (environmentally speaking) if done with a DH unit and all the collected water is recycled. However, I have seen many steam kilns drying it, as well as air drying. I do believe that the final product is better if kiln dried. In fact, some preservatives require kiln drying in order to complete the treating process so that the chemical is bound in the wood better and then leaching and runoff are not going to happen. I seem to recall that poles have a redrying requirement and also have limits on the temperatures used in redrying to avoid strength losses. You might want to check this out perhaps with the AWPA or AWPB. Redrying must also be done carefully to avoid opening up any checks that will expose untreated wood. You will get a few pieces that warp badly. You also need to determine the final MC required... drier means more warp, longer times, more expense and even brittleness if over-done.

From Professor Gene Wengert, forum technical advisor:
I forgot to answer your specific questions.

Do the treating chemicals cause any more problems than the acid from red oak? Yes. You cannot let them run out onto the ground.

Do not over-dry. Avoid higher temperatures. Warp in a few pieces.

Running it through a pre-dryer first, then a 140* - 160* kiln is a lot of extra handling. Kiln redrying time is usully only three or four days in a kiln.

The chemicals do not slow down the drying process. The cooler temperatures do mean a bit slower drying.

Air flow depends on the width of the load, but probably no less than 600 FPM.

These are general answers. Specifics depend on the chemicals and your requirements.

From the original questioner:
Thanks for all the answers. The company I am working for is buying kiln dried treated lumber and every truckload has bundles with the moisture content too high. They are going to solve the problem by drying their own. I know how to dry hardwood fairly well, but treated is new to me. They don't know anything about drying.

From contributor D:
The chemical process is important. With CCA, it is best to get the temperature over 130F before doing any drying. If you start drying before the wood is hot enough to assure fixation, the water will evaporate and leave the salts on the surface which is not safe. The chemicals in CCA will partially fix without hitting 130F but 5-20% will never go through fixation. In a test on poles, we found that poles left out for even two years still had unfixed chemical which would be deadly for linemen. So the chemical used is a vital question.