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Old 07-17-2009, 04:16 PM  
dayton
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Nestor,

Thanks a MILLION for the informative, very thorough explanation. I truly learned a lot about what's going on and what to do about it. I've taken some vacation time to spend solely on catching this stuff where it is so your links and suggestions are INVALUABLE to me, as I'm a new home owner with very little experience, esp. dealing with stuff like this.
Much, much thanks....you have made my day by taking the time to set me straight on the whole thing.



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Old 07-17-2009, 06:31 PM  
Nestor_Kelebay
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Dayton:

Thanks for the fan mail.

I skimmed over that post and I should explain that statement:

"It is only that range of moisture content (from about 16 to 30 percent) where the moisture is coming from the wood cell walls that results in any dimensional change (swelling or shrinking) of the wood (primarily across it's grain).

The fiber saturation point is typically around a 30 percent moisture content. Below that ALL of the water that evaporates from the wood comes from the cell walls. If you put the wood in an oven and bake it, you can drive it down to a 0 percent moisture content.

It's just that under typical indoor temperatures and relative humidities, wood will typically maintain a moisture content of approximately 16 percent.

So, if you dry wood below a 16 percent moisture content, the moisture will come out of the cell walls and the wood will therefore shrink. It's just that 16 percent is typically what it will dry out to under normal conditions because that's the stabilized moisture content under comfortable indoor living conditions.

Here's an excellent paper on wood shrinkage:

Minimizing Wood Shrinkage Problems

It's written by Dr. Stephen Smulski who used to teach at the University of Massachusettes in their "Building Materials and Wood Technology" program. It's basically university for carpenters. If you have a question about plywood or Oriented Strand Board or house wraps or engineered wood joists or a variety of other wood building related topics, you'd do well to check here first. It's a good source of reliable information:

UMass Amherst: Building Materials and Wood Technology » Publications

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Old 07-18-2009, 08:29 PM  
WallyPop
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I'm sealing the edges of 4'x8' plywood roof sheathing prior to applying a drip edge on my flat roof deck. It's called AnchorSeal, variations of which are applied to the freshly cut ends of valuable logging woods to prevent such moisture checking as you've experienced.

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