Drying Mixed Thicknesses of Lumber

Tips on how to air-dry and kiln-dry lumber of different sawn thicknesses. October 1, 2009

Question
I currently have a mixed load I'd like to dry. It’s the same species but a combination of 4/4 and 8/4. I'm running a dehumidification kiln. How would it work to double stack the 4/4 (two pieces together without a spacer) to make it the same relative thickness as the 8/4 lumber? Would both the 8/4 and the double stacked 4/4 dry at close to the same rate without a lot of degrade?

Forum Responses
(Sawing and Drying Forum)
From Gene Wengert, forum technical advisor:
Do not double stack. The reason is that the core will always be a little bit different than the shell, so with the 4/4 pieces, you will have a gradient, face to face, and that means warping after drying. It tales 2.5 times longer to dry 8/4 than 4/4.

You should consider shed drying the 8/4 and that will shorten the drying time. You can also shed dry the 4/4. With shed drying, you can still not mix, but drying will be fast enough that you can do two separate loads.

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From contributor T:
I have dried two inch and thicker in the same load. I place the thick lumber on the bottom of the stack. When the 4/4 is done, I remove it. If the thick lumber is not down to where you want it, leave it until it is. Refill the kiln and carry on with another load. Usually it will be ready the second time. I also stand short thick pieces against the back wall so the air will circulate around them. This may not be exactly by Hoyle, but it works for me. Usually the wood I am talking about is well air dried, and stored inside, before kilning. Get yourself a good moisture tester.

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From Gene Wengert, forum technical advisor:
The secret to contributor T's success is that the wood is well air-dried first.