Limits of Air Drying

Even in the driest parts of the U.S., air-dried lumber typically won't get below about 10% moisture content. March 10, 2008

I sawed some ash about 1 1/2 years ago, air dried in a barn. The moisture tester says 6%, and I jointed some boards flat and planed them down to 3/4". The next day some of the boards were no longer flat, maybe 1/2" gap under center when laid on table. Is ash just a warpy wood, or what is my problem?

Forum Responses
(Sawing and Drying Forum)
From Professor Gene Wengert, forum technical advisor:
Wood only changes size or shape over time because its moisture is changing. Your tester is not correct or the humidity around the wood is greatly different from 30% RH (= 7% MC).

Incidentally, in most of the USA, the driest that wood will get in an unheated barn or other structure is about 11% MC.

From contributor J:
Gene, could you also blame this on exposure to a radiant heat source, such as direct sunlight, that would heat a board and drive out moisture without having a dramatic effect on surrounding RH levels?

From the original questioner:
I live in north central Kansas, right on the edge of the great American desert, and my neighbors also get their wood below 6% air drying. Not all the boards warped - just some. And no sunlight in the barn or the shop came over the boards.

From contributor D:
I'd be interested to know where on the lumber you are taking the moisture readings: end grain, middle of board, etc.?

From contributor V:
In response to the first part of your question, I stack bundles of ash outside unprotected in 8' high stacks to air dry before going into the kiln. I don't use any weight on top of the bundles. The top row of boards that is exposed to the sun will sometimes end up with an unusable board from warping, but the second row on down almost never has a warped board. Overall ash is a very stable wood.

Try drying flatsawn sycamore sometime. Even with over 1000 lbs on top I have had boards on the bottom of an 8' stack raise almost an inch off of a stick before. I would usually figure at least 5% loss from warping.

From Professor Gene Wengert, forum technical advisor:
In order to air dry to 6% MC, the average outside RH would have to be about 22% RH. Even Denver is not this dry on the average. (Dodge City averages about 12% EMC and Denver about 10% EMC. Air drying to within 2% MC of the EMC would be possible when air drying is 6 months using 4/4 lumber protected from rain.) What is the closest airport? Check for the average RH or call your county extension agent and ask him/her, as this number would be used for evaluating crops and soil moisture. I suspect that your friend's moisture measurement is not accurate.

From Professor Gene Wengert, forum technical advisor:
The sunlight and the resultant local heating will lower the RH and the EMC for the air right around the exposed lumber surface. For this reason, most folks will use a sheet of plywood or other similar material to cover the tops of the piles and prevent the excessive drying and shrinkage on the top layer of lumber.

From the original questioner:
I checked the moisture on the end of the board after cutting it off. Gene, if you would give me an address, I'd put a small piece in a plastic bag, then in paper, and mail it to you.

From Professor Gene Wengert, forum technical advisor:
The average MC of the piece you sent me was 10.8% MC. I cut it into several pieces and one was 11.1% MC, but the rest were 10.6 and 10.7% MC.

From the original questioner:
Thanks, Gene - guess you are right about my moisture tester.