two boards in crawl space These two boards are in a dry crawl space. Whatever is on them is not present on any other boards. Is this mold? Or something else?

  • 2
    It is probably those two boards were used somewhere else before being used there. Too neat for mold/fungus/water stains.
    – crip659
    Commented Sep 30, 2023 at 17:09
  • @crip659 I agree it's likely from those boards being used elsewhere before being installed in their current location. But it could still be mold from a previous location. That said, and it's real hard to tell from the photo, I think it may be concrete residue. Commented Oct 1, 2023 at 9:03
  • 27
    It's a cardboard box. It appears to contain puzzles and games.
    – Valorum
    Commented Oct 1, 2023 at 18:39

4 Answers 4


I've seen similar patterns where apparently some of the boards were first used in a concrete "form", then the form was disassembled (after the concrete was poured and hardened) and the lumber reused.

  • The person who built my garage back in the 50's used a reeeeaallllly long 2x4 (I've never seen one this long anywhere else) as the screed for the concrete slab, then used it to brace the wonky truss system they built.
    – Z4-tier
    Commented Oct 2, 2023 at 13:01
  • 1
    TL;DR: It's nothing to worry about.
    – FreeMan
    Commented Oct 2, 2023 at 17:24

I'd guess you have two boards that are pressure treated (or possibly paint-on treated) with a copper-containing mixture sharing the diagonal sheathing job with a bunch of un-treated boards. Hard to think that water or mold would be so contained to just two boards, not running onto their neighbors, and that they would happen to appear cuprous green.

  • I agree that there is almost no chance that those 2 slats would look like that from mold that occurred after installation. It is possible that the boards have mold from a previous installation, but I can't tell from the photo. The 2x4 looks a bit suspect, but maybe that's concrete residue? Really hard to tell from the single photo, which is blurry for the 2x4. If the 2x4 isn't holding any weight, I would maybe consider replacing it because it would be so simple and inexpensive to replace. But I have a hunch that's concrete residue on it. Commented Oct 1, 2023 at 9:00

It looks a little too blue-green for this, but boards left in the sun will eventually turn grey due to UV weathering.


It looks like you can visually inspect the cut end of the 2x4. If so, does it show the same discoloration? If there is no discoloration on the cut end, then, as many others have said, it is probably residual concrete or mold from a previous installation.

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