Mold on Southern Pine

The wood doctor explains why Southern Pine supposedly dried to 18% moisture content would still support mold after being stacked inside a dried-in house under construction. January 25, 2013

I buy KD 5/4x6 SYP deck boards from a local pine mill. They are kiln dried, but only down to 18% target MC. I know pine is mostly about volume and mass production.

I buy these deck boards and then mill them with our molder into T and G V-groove paneling or car siding as it used to be called (sometimes into flooring). I warn all my customers about the potential pitfalls of SYP, shrinkage, warping, and etc. I tell them it needs to be acclimated as long as possible.

I sold a customer about 4000 sf and we stacked it in the newly constructed house, in the dry. After acclimated for a couple of weeks with the heat on they began installing or nailing it up. In the center of the wood they are now finding a lot of mold on the surface of the wood. What is going on with the mold? How or what is happening?

Forum Responses
(Sawing and Drying Forum)
From contributor R:
Eighteen percent seems fairly high for paneling or flooring. When you put it in the house to acclimate are you stickering the wood or stacking tight? If you are doing the latter that is where the problem is coming from. Have you thought about having the wood you buy re-dried to a lower MC?

From Gene Wengert, forum technical advisor:
If the wood is 18% MC, does this mean the entire piece is 18% MC? Let's assume that it means that the average MC of the piece is 18% MC. We know that the core is wetter than 18% MC and the shell is drier; we know that the ends are a bit drier and the center is wetter. We know that not every piece is checked for MC, so you really need to confirm the MC yourself.

Finally, we know that not every piece is 18% MC, but there is at least a 5% MC spread from driest to wettest. Maybe the kiln people thought the load averaged 18% MC, you the narrower stock would be drier and maybe the east end of the second track is drier, etc. So, some bundles will be quite wet when the operators are drying based on the average. Again, you need to check the MC yourself, probably using the pin type meter so you can get a core reading.

We also know that moisture in excess of 22% MC is required for mold growth. Now, when you plane the wood you are actually increasing the MC as you are removing the driest part of the lumber. Does all this help?