Surface Mold on Framing Lumber

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French If the wood will be dry in service, mold on the surface is strictly a cosmetic concern. Here's more info on washing and drying for the sake of appearances. April 20, 2011

Question
I have some air dried pine that I would like to use for framing a porch around my house. The MC is 11 to 20%. There is mold that has been growing on the lumber since we cut it and I did nothing to prevent the mold growth. As we are framing the porch roof it looks as if this mold is dying. Is there any harm in just leaving the mold on the lumber and closing it up? Should I spray with bleach to kill the mold?

Forum Responses
(Sawing and Drying Forum)
From contributor A:
All wood has mold spores on it. If the moisture gets above 30% it will start to mold. If you spray bleach on it and let it dry it will be clean looking but by the time the bleach dried there will be mold spores on the wood again waiting for it to get wet again. Since the wood is dry and it will remain dry and be covered I would not worry about it. If it will be seen I would bleach it for appearance more than anything. Too strong a bleach solution is as bad as damage caused by the mold.


From contributor E:
A similar question was in this month’s Journal of Light Construction. The response from an environmental consultant was to wash it with soap and water then soak with bleach for 15 min, then rinse. After drying, sand and paint with a fungi-static coating.

From Gene Wengert, forum technical advisor:
Why not use bleach? It has been suggested in the past.

From the original questioner:
Thanks for all the responses. I sprayed the lumber with a bleach solution after we framed the porch. I used a vented soffit board and it looks like it should work.

From Gene Wengert, forum technical advisor:
Mold spores that can eventually form active fungal mold on wood are in the air everywhere. We need four elements for active fungal growth on wood - moisture (for wood, this is over 92%RH), oxygen, temperature (50 to 120 F) and food. For mold, the food is not the wood but is dirt, microorganisms, etc on the wood (and the sugars right under the bark).

Do you want to use some chemical on the wood that will last forever? If not, then you can use bleach to clean the wood and this, especially on pine, will keep the wood free of fungal mold unless it gets really wet afterwards. Do you expect the wood framing to get wet? If so, then mold is not the issue, but decay. You should use pressure treated pine for wet locations. If it will not be wet, then eliminate the present mold and build. You can remove the mold with power washing. Mold is only on the surface as there is no food inside the wood. Rough lumber, when planed, eliminates to mold.

Why eliminate the mold on dry wood? I suspect that someone is concerned that the spores will be released. Active mold can also generate some odors that are not pleasurable. But if the wood will be dry, there are not too many issues that I can imagine. Incidentally, pressure treated wood does not prevent mold growth, as the chemical does not poison the food for mold. So, why put something on wood to control mold that will not control mold? There is a lot of wood with mold on it used for construction today. Overall, Contributor E’s response is the best, except that the final coating is not necessary or helpful.