Air-Drying and Kiln-Drying Methods for Wide Plank Flooring

A discussion of the most cost-effective and quality-conscious way to dry wide planks for use as flooring. September 6, 2010

Which method of drying (air or kiln) is best for wide plank floors? I'm selling wood floors, mostly 10-12 inch and wider, and from what I have been told, air drying is essential for the wide planks; otherwise the risk of cracking is much higher. I was also told that if the board is dried fast (kiln) it will take in more moisture and do it faster than slowly dried (air). I heard that from one supplier who does air drying. Another one, who uses kiln only, says that it is absolutely not needed. What method is the right one?

Forum Responses
(Sawing and Drying Forum)
From contributor W:
Having sold and installed wood flooring, I use only kiln dried. Even MC and less chance of insect s and their eggs. As far as taking on moisture, if they are at the same MC, I don't think there is any difference.

From contributor S:
For me, kiln dry is the only way to go. This way you know for sure it's dry. In wintertime the air dry will shrink a fair amount.

From the original questioner:
I mean air drying first and then kiln drying, of course. Not just air or just kiln.

From contributor J:
I would only use kiln dried lumber in an interior application. After lumber is sawn, it needs to get stickers between the rows within a few days to prevent staining. It is then typically air dried to remove free water while waiting for a target moisture content before going in the dry kiln. The lumber is then dried according to a schedule that takes into account the original moisture content. Should be no difference in final product if all processes are done properly. Main reason green lumber is not immediately put in the kiln is cost.

From contributor B:
If it's dried too fast it's going to split, cup, check, honeycomb - and/or any combination of degrades - forget the MC uptake after kiln drying. Moisture uptake will be a product of storage conditions after kiln drying.

If the kiln dried is done correctly, it will yield the same product whether you start with green or air dried lumber. With kiln dried (assuming your kiln operator knows what he's doing), you can control drying conditions to ensure you limit the amount of degrade. With air dried, you take your chances much more so.

We used to take green lumber straight off the mill and send it to the kiln. Now we carefully air dry the lumber before sending to the kiln. We do this because it saves us lots of time (kiln run time per charge) and plenty of coin (thanks again for that advice, Gene). I'm going to say that nearly all kilns you go to are going to want to air dry the lumber before they send it into the kiln, but at these green and high MC, that's the critical time in drying (relative to degrade), so that's where I'd concentrate my attention - get it air dried carefully at the high MC, get it kiln dried correctly, and properly store the kiln dried RC lumber until ready for molding. The wide stuff is a pain, but sure makes great looking flooring - worth the effort in my book!

From the original questioner:
And for how long do you typically need to air dry the wood before sending it to the kiln?

From contributor A:
What kind of wood? Most of the wide plank flooring I do is SYP and in 8" + sizes and often goes from the saw to the kiln. We do this more for color than any other reason.

With hardwoods (which we limit to 6" width) we air dry first due to cost concerns. Dry wood is dry wood. Now there are things before drying that effect the wood, like boiling/steaming, aging and internal stresses that may show up later. Sycamore that has spalted will lay flatter than green logs when sawed and dried. But when at 8% MC, they are both at 8% MC, and given moisture, will swell.

From Professor Gene Wengert, forum technical advisor:
If you own the kiln and it is empty, load it with lumber at whatever MC you have. Appreciate that partially dried lumber can have two colors...the outside is often lighter than the inside. If you are short on kiln capacity, then load it with 20% MC, remembering that drying time is determined by the wettest pieces that go into the kiln. In any case, proper air drying and proper kiln startup are critical for high quality.

From contributor U:
We don't have any certain period of time to air dry before the kiln. I like to see it drop at least to 25% or so (that way I can get an accurate reading of the MC with the probes and moisture meter). And yes, I'm talking about hardwoods. Softwoods I'd just as soon see go to the kiln within a week of sawing during the summertime, when they are so prone to stain.

Again, the main reason we air dry first is less kiln time, which means less money spent per charge. And this is most critical with hardwoods, since they take the longest to dry and must be dried slower than softwoods.