Kiln dried lumber cracking

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Forum topic by John Choponis posted 01-12-2011 07:40 PM 2928 views 0 times favorited 10 replies Add to Favorites Watch
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John Choponis

120 posts in 2892 days

01-12-2011 07:40 PM

Last fall I had some 3” white and red pine blanks cut for making tables. All of the lumber was stickered and cover over side for a year. I then took it to a saw mill that has a kiln and had it dried. I picked it up two weeks ago an re stickered it in my shop, I have in floor heat that stays at 75deg. After about a week i notice that some of the blanks were cracking, just looked at the entire pile and every blank has cracked beyond use.

I done this several times before and never had this issue.

Any ideas, suggestions?

10 replies so far

View Colin 's profile


93 posts in 3010 days

#1 posted 01-12-2011 09:10 PM

That’s a tough one. How low of a moisture content did they dry it to? What’s the relative humidity in your shop?
Here is a chart I have used.

58-64 RH – 11% MC
52-58 RH – 10%MC
46-52 RH – 9% MC
39 -46 RH – 8% MC
32 -39 RH – 7% MC
25 -32 RH – 6% MC
19-25 RH – 5% MC

So if you had it dried to 8%, you would want between 39 and 46% humidity. Adding heat drives the RH down. So at 75 degrees, you may have over-dried the air. But then you said you have done the exact same thing before without problems so ???


View John Choponis's profile

John Choponis

120 posts in 2892 days

#2 posted 01-13-2011 12:52 AM

They told me they blanks were dried to 6-7%. I just checked my shop an i am at 40 %RH.

I bought some fresh cut white ash back in November and put it in the shop at that time, it has not cracked a bit.

View Bill Davis's profile

Bill Davis

226 posts in 4123 days

#3 posted 01-13-2011 01:00 AM

As Colin says it’s a tough one but IMO mostly because of the lack of the kind of information that would help make a more scientifically supported answer. What I can say is this:
1. Cracks, splits, checks etc in wood are caused by tension built up in the wood that is stronger than the wood fibers can withstand without separating.
2. Tension is caused in wood by rapid moisture sorption (absorption and desorption) such that the outer shell of the piece changes moisture content (MC) and attempts to shrink or swell more than the core which hasn’t yet followed.
3. ”...more than 90% of problems with wood involve moisture.” (Hoadley)

That being said I would conclude that your wood (with its initial MC) was placed where the relative humidity (RH) was enough different from the woods equilibrium relative humidity (ERH) that it tried to change dimensions too fast.

To prevent the same from happening you need to know the woods MC and then place it in an environment with a RH not too drastically different from the ERH so you can control the moisture sorption to a reasonably slow rate. It would likely help if the kiln where it was dried was instructed to dry it to the MC close to the ERH of your location. That way there would be minimal sorption when stickered in your shop.

View reggiek's profile


2240 posts in 3469 days

#4 posted 01-13-2011 01:10 AM

Bill explained it quite well….another part of the equation is how the wood is cut – the amount of sapwood and core wood…and whether quartersawn….etc…. Another source of stress is whether there was alot of branching and twisting in the tree. All of these items relate to the internal stress and fiber tension as Bill mentions.

-- Woodworking.....My small slice of heaven!

View Colin 's profile


93 posts in 3010 days

#5 posted 01-13-2011 09:14 AM

Bill, I agree with what you say but as the OP said, his wood was dried to 6-7% and his RH in the shop is 40%... according to these numbers, his shop was at the optimal RH to store his wood. It would have added a little moisture but not enough to where it should have had much effect. Besides, adding moisture does not cause splitting in my experience, it is usually from the surface of the wood drying faster than the center in other words storing the wood at too low a level of RH.


View TopamaxSurvivor's profile


18389 posts in 3875 days

#6 posted 01-13-2011 10:33 AM

Did they dry it by itself or run it with other lumber?

-- Bob in WW ~ "some old things are lovely, warm still with life ... of the forgotten men who made them." - D.H. Lawrence

View Tony's profile


986 posts in 4229 days

#7 posted 01-13-2011 10:52 AM

You do not state the MC of the wood before it entered the kiln. I am also assuming that the lumber came from more than one tree (if from one tree, it may have been a bad tree: poor lignin content).

For 12/4 (3”) stock I would normally air dry this down to 18% before considering moving it into the shop or to the kiln.

I am lucky, because of the prolonged winters here (Finland) with temperatures down to -30 C (-22F), the water is drawn out of the wood (freeze dried) within 18 months to 2 years, but normally I would expect in a more temperate zone, with good air circulation (important) that 3 to 4 years to get the wood down to 18% (12/4 – 3” stock).

Another factor you have to take into account is when the trees were felled. Here they are always felled in the depth of winter, when there is minimum MC in the tree and the growth cycle is dormant.

Kiln drying wet wood, especially too fast, which is the case these days (time is money) increases the stresses in the wood because of the rapid drying process, by the time the core is at 8%, the outer sides may be at 2-3% for some time, eventually the MC will stabilize, with the outer surfaces increasing in MC to the equilibrium of the RH of the environment.

In summary: They were probably dried too fast, with too high MC. If you can in the future, cut and store rough lumber outside ready for future use in .years to come, some of the lumber I have stored is 10 years old, keep it dry and well ventilated, it will probably good to use long after you have departed this earth.

-- Tony - All things are possible, just some things are more difficult than others! - SKYPE: Heron2005 (

View Nomad62's profile


726 posts in 3157 days

#8 posted 01-13-2011 11:10 PM

It sounds to me as if you did the best you could with what you had. I would suspect the tree was of poor quality for sawing. There are a couple of questions, tho. How long did the kiln operator keep it in the kiln… did they cook it hot trying to dry it quickly? Did they steam it at the end of the cycle to relieve stress? Did they use a cycle, or just stick it in the kiln? Lastly, maybe the tree had been dropped hard, and caused some stress cracking, or ring shake, that ruined it before you even cut it.

-- Power tools put us ahead of the monkeys

View Colin 's profile


93 posts in 3010 days

#9 posted 01-14-2011 01:56 AM

If the wood was dried too fast, would it take over two weeks for the checking to start?


View Scsmith42's profile


125 posts in 2876 days

#10 posted 01-27-2011 05:20 AM

I operate a sawmill and a kiln, and from what you describe the problems are related to drying the wood too quickly when it was green.

Most drying related defects occur when the wood is dried from green down to 35%, but they don’t become visible until the wood is below 25%.

There is no way to tell for sure, but most likely the wood dried too quickly when it was air drying. If the temps were too high or the air flow through your stacks too great, then this could cause a problem.

I recall someone that once transported 8/4 green oak, stickered, on a trailer for a few hundred miles in the middle of the summer. The boards were orientated side to side, so that the air flow went right through the stack. Every board was destroyed due to the high air flow (over 5000 fpm instead of the recommended 300 fpm).

I seriously doubt that it was anything that occured after you brought it home. Also, presuming that the wood was below 25% MC when it went into the kiln, there is probably not anything that your kiln operator did to cause the problem (unless the wood became saturated with water before being kiln dried).

-- Scott, North Carolina,

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