Are the black spots on my OSB mold? If so, should I be concerned? We had a problem with vent but has been corrected. Thanks.

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Image is of worse area in attic.

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  • Hard to tell just from picture, but black mold not nice. Would shave off a few pieces of OSB(not black spots) and have it tested or looked at, to be sure. If mold, looks like it is just starting
    – crip659
    Commented Feb 6, 2021 at 1:48
  • spray some bleach, apply Kilz such as this and problem solved. Can't post pics on comments, so here is a link to a potential product: homedepot.com/p/… Commented Feb 6, 2021 at 3:36
  • No one can really answer that unless you get some better pictures. It could be blackish mud or sludge. Cut apart sewar lines at your house and move that stuff around. Pretty much the same blackish tint.
    – DMoore
    Commented Feb 6, 2021 at 4:37
  • I wouldn't spray pure bleach, start with 25 percent diluted with water. And there are no death certificates with mold as the cause of death. Commented Feb 6, 2021 at 5:38
  • 2
    You'll get a lot of ominous warnings in hushed voices about "black mold" from people regurgitating internet wisdom. The truth is that mold spores are literally everywhere, and many harmless types are dark in color. If you've resolved the moisture problem you've resolved this issue. Period. I wouldn't even bother with cleanup in an attic.
    – isherwood
    Commented Feb 8, 2021 at 15:22

1 Answer 1


That is mold. Assuming this is what it looks like after it's been sufficiently dried so more mold won't develop beyond what we're seeing here, that is surface mold isolated within a wall cavity, and you've got nothing to worry about.

Typically, if it is surface mold or the decayed portion of the wall system is small enough to be safe and acceptable, you would leave it alone, as molds become dormant below 20% WMC (your framing should be between 12% and 16% once fully dried and ready to be closed back up), as long as the following criteria are met:

  • If the sheathing was never saturated (or above 28% WMC) or if the saturation was limited (in a non structural wall);
  • If the moisture problem has been remedied (dried and not going to get wet again); i
  • If it will not be disturbed, so as to spread spores into living space; and,
  • If the decay is surface only (has not transmitted through to the other side of the sheathing is a good indicator of compromised sheathing).

Best practice would be to replace the sheathing in the event any of the above are not met. Cost of doing the repair vs cost of not doing the repair will drive the decision, except in the case of damage to a critical building component in which case the cost of not doing the repair is also a question of safety/liability.

  • Attic is 500 sq ft, has mold in one half of it, with about three each 3 ft x 1 ft dark spots, & many MANY spots similar to photo. Most of area is crawl space and difficult to get at. No raised, fuzzy mold, no wet sheathing. Had 3 'mold removal' companies in and confirmed that it was definitely mold. Cost ranged from $6000 CAD to 'do it yourself.' Opted to try to take care of the problem first, not with bleach but a recommended mold remover. Sprayed yesterday and will recheck it in a day. Dehumidifier is set at 45, & have bathroom fan running. Any other comments or advice is appreciated.
    – donna
    Commented Feb 8, 2021 at 15:08
  • No change to my answer... if you could put your finger through it with ease, you would want to replace. Otherwise not a problem as described above. Looks like that's roof sheathing right by your eave? I don't remember if you said. Anyway, that's fine - localized, primarily surface, non structural etc etc. Most would leave it be, your attic isn't occupied space, but you can always get mold specific funguides if it worries you, or just like a stain on your close you can spray it with bleach (wear a respirator with filters for bleach) and the black will get bleached to unnoticeable. Commented Feb 8, 2021 at 19:56
  • Your attic isn't useable space is it, and you're not regularly and frequently going in and out normally right? I would honestly not worry about it myself. Be very wary of mold remediation companies, like pest management, if they don't guarantee the problem won't return, I would have a hard time distinguishing them from thieves. Mold is one of those things that people get scared about. In your case, as long as your attic is not gapingly open to your living space, there is no way for it to get to you and you are clearly describing surface mold that is not a threat to your health. Commented Feb 9, 2021 at 6:11
  • There is some roof sheathing on the peaks, i.e. the areas where I can almost stand up in. It is not anywhere else. Does that make any difference? Wood is solid in attic, i.e. cannot punch hole through it. I never go into the attic. I thought that the Mold repair management companies over-charged. The most reasonable, best-lowest offer would still have been $500 per hour. Thanks for passing on your knowledge. I will re-spray, keep area ventilated.
    – donna
    Commented Feb 9, 2021 at 15:22
  • No change to my advice. Technically, you don't even need to deal with it, but what you're doing is best practice. It won't spread, it won't continue to degrade your roof sheathing, this is just surface mold. You are approaching it correctly. Many would choose to ignore it altogether. (I like you would spray it as directed by the bottle, and then I might take a paint pen and trace the perimeter and check on it every year. If it spreads a little, you know your attic ventilation needs some additional attention)... Shameless self promotion: if you like my answer please check it as accepted Commented Feb 9, 2021 at 15:56

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