Drying Ash Flooring
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Kiln-drying is essential to kill bugs and achieve the proper low moisture content for stability in service. May 11, 2005
Question
I have air dried ash at 12-14%. Am I asking for trouble if I mill it into flooring first and then kiln dry it?
Forum Responses
(Sawing and Drying Forum)
From contributor J:
If it is air dried that much, I don't think it needs to see a kiln. I would go ahead and work it into flooring, and then just give it plenty of time to acclimate inside the house it's going in. That is exactly what I did with my white oak flooring, and it worked out fine.
From Gene Wengert, forum technical advisor:
Yes, you are asking for trouble if you want the floor to look excellent. First, the wood will dry when it is in the house to about 6% MC. This drying will mean that it shrinks. Depending on the grain, the shrinkage will vary from 1% to 2% in width and thickness. This means that joints that were flat when you milled at 14% MC will not be straight. How picky are you? Can you tolerate 1/32" gaps? You may not have every piece at the same MC, which also means variations in shrinkage. With ash, you do need to heat it to 130 F or hotter to kill any insects (powder post beetles). Do not use it without this treatment! It is difficult to acclimate the wood (as suggested above) in a house, as the house is not hot and the RH is not under the target value. Therefore, it will take a very long time and may not be effective if it is not really dry in the heated house (which means very cold outside and no humidification in the house). It is silly to skip the proper drying step to save a few dollars unless you do not want a premium floor.
From contributor J:
I realize there is an overt fascination with kiln drying, but think of all the wood floors that were laid and look great 100 years later and never saw a kiln. Think about this... what do you think the moisture content of pre-fab kiln-dried flooring is when you buy it off the shelf? 6%? Not even close. If it was, why would they acclimate flooring for at least a few weeks in the room it will be laid in before nailing it down? Because it needs to come down from 12% to 6% or 8%. Before I did my floor, I talked to a couple different professional flooring guys, and they both told me exactly that - if you already have it air-dried down to near 12%, kiln drying is a waste.
In regard to gaps, it depends on the width of your flooring. If you get into plank flooring that is 7" wide, slight gaps are unavoidable if there is any seasonal humidity change at all - maybe 1/32"... Even if it is laid at the 6%, those wide boards will expand enough that they will leave gaps when they contract back down. But if you are doing strip flooring (2-3") and acclimate it, there is no difference between that and kiln dried. 7% is 7%, regardless of how it got there. As far as the t&g fitting perfectly, that's why they invented floor sanders. Any unfinished floor will have slight height variations, which is why they need to be sanded after they are nailed down. This is due to miniscule variations in tongue or groove height, nothing to do with moisture or how it was dried. Regarding insects, if they are present when drying is complete, there are other ways to kill them aside from kiln drying. They make some wonderful fumigant tablets. The best ones are restricted, but if you know a farmer with a RUP license...
From Gene Wengert, forum technical advisor:
You seem to have missed the point that this is ash. Show me an old floor that is ash that was not kiln dried? You apparently did not read what I wrote earlier that stated that acclimatization in a house would take a long time. I did not say it was never done or was impossible. I can show you 100 year old hardwood floors that do indeed have insect holes and that are not very flat. For each old floor that you see today, there are hundreds or even thousands that have been destroyed because they did not serve well.
Flooring has been kiln dried for many years - nearly 100. In fact, the oldest kilns that were around in the 1960s (that is, the kilns were over 30 years old then) were almost all flooring kilns.
Your comment about fumigant tablets is absolutely irresponsible with respect to wood. Such a procedure is unsafe and is illegal for wood floors.
When you buy prefab flooring off the shelf, it will always be very close to 6 to 8% MC. The NOFMA has a standard that is adhered to; so does the AWI. A flooring company could not afford to produce 12% MC floors due to warping and size change.
Any professional "flooring guy" that says air-dried to 12% MC for strip flooring is okay and further kiln drying to 6-7% MC is a waste is not correct. The insect risk alone is reason to kiln dry. The size change is further reason to avoid such a practice.
Have you ever heard of pre-finished flooring? There is no way that you can sand a pre-finished floor. Pre-finished floors are very smooth and the tongue and groove is not off. The same is true for a quality floor that is unfinished. The t&g are within several 0.001". It is news to me that sanders were invented to sand floors... I thought floors were invented to give sanders something to do. Are you serious about why sanders were invented?
From contributor C:
Sanding brand new flooring after installation is to insure smooth surface for finishing and for comfort. Most pieces that are not perfectly level usually are that way because of the subfloor being less than perfectly flat, improper installation or less than perfect milling. If you notice, a great deal of pre-finished flooring has the slight v groove at end and side joints to hide these less than perfectly level conditions. There are times and places to save money; air drying ash flooring is not one of them
From the original questioner:
Thanks for the response. I'll kiln dry it first - I was just trying to save time and mill this first while kiln was full with other wood.
From contributor T:
Take the time to do it right. We put ash flooring in our new log home, and it is beautiful. We took the extra step of sealing the back of each piece before installing. Possibly unnecessary, but we thought it might reduce moisture absorbed from under the floor.
From contributor N:
Did you seal the t&g as well? I have been told that this causes the floor to squeak.
From contributor J:
I admit, I've never made ash flooring, but aside from the insect vulnerability, I've always found ash to act much like oak in regard to how it dries and works. I've kiln dried most of my oak flooring, but my only point was that it can be done by air drying, and the results can be just as good. Sure it takes more time, and you have to be a little more careful, but I just hate to see people say you absolutely can't do it that way, because I've done it, and it works... but again, that was white oak.
From Gene Wengert, forum technical advisor:
Ash and oak do not dry the same. Oak may take 30 days to dry, while ash should be dried in 7 to 10 days when kiln drying. Ash can be dried quickly and should be for best color; oak must be dried slowly to prevent checking. Oak honeycombs; ash does not. Ash stains and has a risk of powderpost insects; oak does not generally stain (except sapwood at times) and white oak especially has little risk of insects.
From contributor T:
We did not do the edges, just the back. I had not heard that, though. I guess we got lucky.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor A:
Yes, you should kiln dry this before milling. I have been a professional floorman for over over years and have seen the effects that moisture can have on wood floors. First it will acclimate to less than 12% and that takes more than a few weeks in the house. If you are in a very dry climate it can go as low as 5%. In Oklahoma it varies from 6 to 9 % depending on the season. There will be seasonal movement if you have humidity fluctuation. Secondly, if you lay flooring with too high of a moisture content over a moisture barrier (not optional) and sand it flat, the top of the board will dry out faster than the bottom of the board and the result will be cupping. Even if you are laying 1.5" flooring. Wide plank will cup even more noticeably. If you re-sand, it will crown when it finally dries out completely.
As for sanding, even the best milled flooring will fit the tongues tightly, but the boards are never milled perfectly to the same thickness. This is called overwood and must be sanded down to the lowest point if you want a flat floor. Even engineered pre-finish has this problem, hence the v groove, although the tolerance is much tighter as this flooring is run through a thickness sander prior to finishing.
An actual anecdote that we experienced was a $5 million home with 4" walnut herringbone, kiln dried before milling and summer in Oklahoma with 90% RH. Herringbone can only be laid when the length is divisible by the width. Otherwise your pattern will go crazy. Try it yourself. I had to build a plastic tent over this wood and run a dehumidifier for two months before the moisture content fell enough to shrink the boards back to their original size so we could lay it. Don't skip the kiln drying.
