Troubleshooting Wood Flooring Shrinkage

Moisture change is always the culprit when wood shrinks. Here, pros a case of gapped-out wood flooring. July 28, 2006

Question
I am looking for info on kempas, which some customer friends recently installed as flooring. The flooring was pre-finished and has shrunk over a period of 6 months or so since installation. The gaps between the 3 5/8" planks are now up to 1/8" wide. I suspect that the lumber was improperly dried to begin with, partly due to the very slow way in which the shrinkage gradually appeared. This home is humidity controlled, so I doubt that extreme ambient dryness is the problem. Does anyone here have experience in the drying of kempas or related woods? Does my analysis make sense to you? The wood was stored inside the home for several weeks prior to installation, so I'd think that any moisture picked up in shipping would have had ample opportunity to dissipate. I am thinking that the wood may have been dried too quickly and case hardened, causing the remaining moisture to seep out very slowly. Or could kempas be a wood which suffers exceptionally large dimensional swings due to moisture changes? I'd like to discover the root of the problem so that its solution can be addressed successfully.

Forum Responses
(Sawing and Drying Forum)
From contributor H:
Is there radiant heat in the floor? Storing the flooring on site is no good if it is all stored in one pile in one place. Had a similar problem with Jatoba pre-finished and radiant heat. One client stored his floor in one room in one pile for a month. It shrunk over the winter and tightened up in summer. Another client spread his flooring out in open bundles throughout out the home for a few weeks before installation. It never moved noticeably. Is there vapor barrier under the floor? Is it installed over wafer/osb/type subfloor?

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From the original questioner:
There is no radiant heat – it's a standard gas furnace. The subfloor has no vapor barrier but it is over a finished and heated basement area, not crawl space. The subfloor is plywood.

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From Professor Gene Wengert, forum technical advisor:
It is not casehardening, as casehardening does not affect moisture movement. Absolutely. Positively. Wood only shrinks over a time period for one reason and that reason is that it is losing moisture. So, the wood was too wet at the time of installation for the home's RH. It seems likely that the wood was not properly dried. It is easy to check. Measure the width of the flooring strips now. The amount they are smaller than the original size is then used to calculate the percent shrinkage. For each 1% size change, add about 4% MC to the present MC value. (This is a rough estimate; we would make more precise calculations if we were going to court.) So, with 1/8" shrinkage and 3-5/8" widths, that is 3%, so the wood was 12% MC wetter than it is now and now it is likely around 7% MC, so that gives me 19% MC at the time of manufacture. If this is indeed true, then it is more than just bad drying; it is likely no kiln drying at all.

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The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
It is also possible that the wood was properly dried, milled, and finished. It could have easily gained significant mc somewhere between the orient and where it was finally installed. This is why it is the responsibility of the installer to confirm that the MC of the flooring is acceptable before nailing it down.