GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Mold on new construction lumber

pnwbuilder | Posted in General Questions on

I’ve started framing my new house a week ago. Purchased all materials from a single supplier. Some of the dimensional lumber has quite a bit of mold on it and to my surprise a factory wrapped stack of plywood sheathing has what appears to be moldy patches as well. The plywood was dry throughout the stack, and moldy patches are only noticable on one side of each sheet which makes me believe that it got that mold at the factory and not during storage. Is this something I should be concerned about?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    BILL WICHERS | | #1

    Are you sure it’s mold and not just staining? I’ve seen grayish stains on new lumber before.


  2. pnwbuilder | | #2

    On dimensional lumber it looks like mold, but I am not 100% sure about plywood. Why would mold grow on one side of it, but not on the other that is in contact with it while it is still stacked? I've attached a few pictures

  3. Zdesign | | #3

    A water and bleach combination in a garden sprayer will take that right off. I am running into the same issue this year with all the excessive rain. There are many products that you can buy at the box stores but the bleach and water combo is by far the best.

    1. pnwbuilder | | #4

      I've used bleach to cleanup mold on an interior wall during bathroom remodel in the past and it does great job at cleaning it. I can't say how it looks until the next remodel, but as strange as it sounds, bleach seems to be going to do more harm than good. Take a look at this discussion:

      1. Expert Member
        BILL WICHERS | | #5

        Bleach solution won't really do any harm unless you make it overly strong (too much bleach diluted with too little water). You can also use hydrogen peroxide which is a less persistent mold killing agent.


      2. Expert Member
        MALCOLM TAYLOR | | #6

        I think was they were getting at is that if the same conditions persist, the sheathing will re-grow the mold bit faster if it has been treated with bleach. If the plywood doesn't get left in conditions the would allow mold to regrow, the one-time bleach treatment should be effective.

      3. pnwbuilder | | #7

        I didn't mean that bleach would do harm to the wood from structural perspective, but the study in the article that I've linked to seems to imply that for some reason the mold came back stronger on the wood that was treated with bleach.

        As far as the stains on the plywood that I have go, I wander if those are caused by beetle killed wood used in the production of that plywood. I've sanded the surface a bit to make sure that it's not paint or dirt and it looks like it is not, the staining goes dipper into the wood.

        1. Expert Member
          BILL WICHERS | | #8

          Ah. Mold does seem to like more heavily processed wood. Maybe a bleach-etched surface is easier for the mold to grow on. I have no idea.

          I have lots of beetle killed trees on my property. The beetles eat the living part of the tree, just under the bark. If you chip off the bark you can see squiggly little paths the larvae have chewed all over the surface of the tree. Once enough of the living part is eaten, the tree dies. The inner part of the tree is mostly untouched. I doubt you'd see anything different with beetle kill wood compared to regular wood since the surfacing at the mill would have taken off any of the outermost part of the boards.

          There are very few living ash trees around my area due to the emerald ash borer. We have something going at some of the pines too. Most of the Birch this far south has been killed off too (although in northern MI winters are cold enough to kill the birch borer so the trees survive). It's sad really. Ash trees are pretty trees.

          The state has a law against moving firewood, which transports a lot of the boring insects, but people do it anyway.

          1. pnwbuilder | | #9

            Thanks for the info Bill. Beetle killed trees is something that I've heard about in the past, but never new all the details. Apparently these little bugs are carriers of a fungus that spreads into the wood and gives it the bluish stain appearance. I don't know how deep into the wood the fungus can penetrate, but it is killed during the plywood manufacturing process, so there is no harm structurally and it won't start growing again. I am still waiting for the final word from the plywood manufacturer on whether it's mold or something else.

  4. GBA Editor
    Brian Pontolilo | | #10


    Whether or not your try to clean up the staining with bleach (some people also use Borax to stop mold growth), I don't think you have a problem here. As your house gets dried in and the lumber dries and stays dry, the mold will not be able to continue to spread. But if you are really concerned about this, you could also return the lumber. I have returned lumber and other building materials that I didn't like for various reasons many times.

    1. pnwbuilder | | #12

      Those are good points Brian. I am going to spray bottom 2' of the plywood sheathing with Boracare anyway, so I might as well spray the stained areas while I am at it. Returning the plywood is certainly an option, but the supplier is going to charge restocking fee and it will introduce a delay that I would rather avoid if this is not a real concern.

  5. Peter Yost | | #11

    Be interesting if you could measure the moisture content of your plywood and framing. Mold growth typically gets a hold at about 19% moisture content. But mold growth is a surface phenomenon; it's all about the water activity right at the surface.

    To get rid of the mold, you ultimately need to get rid of the moisture. To get rid of the mold growth you can see, it's just like cleaning any surface: mild soap solution and then let it dry. Trying to kill the mold growth rather than just cleaning it off is a real uphill battle; if the surface remains wet the mold will likely grow back even with a mild bleach.

    This remains a mold classic reference:


  6. pnwbuilder | | #13

    I don't have a moisture meter, but I'll see if I can borrow one. I doubt though that the moisture content is an issue at this point, since these patches appear only on one side of the plywood sheets, but not on the other, even though the non "moldy" sides were in direct contact with the "moldy" ones.

    Thank you for the link, it was a very good reading.

  7. Expert Member
    BILL WICHERS | | #14

    >"Apparently these little bugs are carriers of a fungus that spreads into the wood and gives it the bluish stain appearance. "

    That's a different problem. I'm familiar with all of the tree-killing fungi, but I know about "Oak wilt". Oak wilt kills red oak trees, and is a fungus that gets sucked up the living parts of the tree (xylem and floem, basically the tree's blood vessels), and plugs things up, killing the tree. With Red Oak trees, as soon as you see any signs of fungal infection, the tree is too far gone to save and all you can do is try to limit the spread of the disease. With white oaks, if you start treatment IMMEDIATELY when you notice the first signs of the disease you can often times save the tree.

    Oak trees are unusual in that their roots grow together underground so the fungus can spread from tree to tree through the underground root system. The way this is limited when trying to stop the spread of the disease is to cut into the ground with a trenching machine to break the root-to-root connections between trees. Another thing to do is NEVER cut the trees during the growing season, ALWAYS wait until late fall when the sap stops flowing. The exposed sap attracts insects that carry the disease.

    As far as I knew, the fungus doesn't penetrate into the interior wood of the tree (which isn't alive), but there are many kinds out there so I can't be sure if that's always the case.


  8. Colin63 | | #15

    Mold needs stagnant air, moisture and darkness to thrive. Eliminate one variable and it won't grow. Ie the moisture will be gone once it moves from the lumber yard to your house

  9. Expert Member
    Peter Engle | | #16

    The stains on the plywood look similar to Blue Stain to me.

    The FAQ is for SYP lumber, but I think similar effects happen with certain infections in other softwoods as well. Like everyone else says, clean the surface, dry the wood, and don't worry too much about it. For extra insurance, go ahead and spray everything affected with borate to suppress any future growth.

    1. pnwbuilder | | #17


      Thank you for the link. As they suggested in the FAQ for SYP I've tried to sand it off, and it does go dipper into the plywood layer. At this point I am pretty sure it's not mold, but I'll spray it with boracare when I'll be treating bottom 2' before installing insulation.

  10. Jon_R | | #18

    I'd spray it with "Bora-Care with Mold-Care".

    1. pnwbuilder | | #19

      Mold-Care is pretty nasty stuff from what I've heard, I am not sure if I would want that on the interior surface of the sheathing.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |