My house is one year new construction. I didn't use the dehumidifier at the basement until one month ago I realized the basement was damp and there were some mold on the wood frame on the basement ceiling(not serious, already cleaned). I also noticed there are many black spots on the plywood on my basement ceiling and the plywood behind the electrical panel, they don't look like mold to me, but I am not sure. Can someone please tell me what are these black spots? I am concerned they might be mold.

basement ceiling

plywood behind electrical panel

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  • Looks like mold, don't think someone is going round with a black marker...
    – Solar Mike
    Aug 11, 2019 at 8:10
  • 2
    I think it is ink or paint that resulted from a misfire of the printer in the osb factory notice how it stops at the seam between two sheets
    – Kris
    Aug 11, 2019 at 20:42
  • In the mills I work (have worked in) they use stamps for grade marking. We do use a spray for blows or voids that exceed specs but this is a different die that disappears as most sheets do have some small voids that are marked but if only a few the strength of the product is not compromised. The reason you see more on 1 particular sheet than another is it was processes at a different time the adhesive and the mix vary slightly from batch to batch and even sheet to sheet within a batch again this is not unusual. I do know about matrix printing And this is not close two large of an area.
    – Ed Beal
    Aug 12, 2019 at 14:33
  • Do you think they are mold or some printings/splatters, Ed?
    – user41662
    Aug 12, 2019 at 15:04
  • I'm with Kris, I think it is ink. A bunch of little spheres is almost certainly a misting of droplets that hit the sheet. Aug 12, 2019 at 15:44

3 Answers 3


enter image description here As you can see the factory prints information on each sheet during the manufacturing process.

Occasionally there is an error and ink is splattered during the time the sheet is passing through the conveyor getting trimmed,stamped,edges painted,etc...

Looking at your first picture we see the pattern of black spots stops at the joint between two sheets. That assured me that this is not a case of mold growing.

  • There are many these spots all over the basement ceiling, here and there. I just checked them all. Most patterns stop at the joint. Looks like the ink splatter. What surprises me is these errors happened so often in my current, whereas I didn't see them in my old house.
    – user41662
    Aug 11, 2019 at 22:00
  • There may have been hundreds of sheets getting the splatter before the problem was noticed
    – Kris
    Aug 11, 2019 at 23:23
  • I don't remember whether or not these black spots existed at the time we moved in. The 2nd picture is the back of electrical panel which is at the obvious place, it seems there were no such spots at the beginning or else I would have noticed. Still not very sure if these are mold or not. I feel nothing when touching them. But Ed's answer also sounds reasonable. Is there any method i can use to tell if these are ink or mold?
    – user41662
    Aug 12, 2019 at 4:39
  • inspectionnews.net/home_inspection/…. You could call a mold remediation specialist to verify just be sure the company is reputable with good bbb rating etc
    – Kris
    Aug 12, 2019 at 13:02
  • 1
    One additional thought (since this just popped to the top of the list) - in addition to it neatly stopping at the edges of the OSB, it also hasn't "grown" onto the joists, either. Mold won't care about these edges and will spread wherever it feels like.
    – FreeMan
    Oct 27 at 18:06

Well that’s not plywood but it is OSB , high moisture in the air can allow mold like you have. A simple 3% hydrogen peroxide and water will stop it from getting worse. I currently work in a mill and the wood stored outside prior to finishing and a biocide spray looks just like this. The only way to prevent it from returning or getting worse is to remove the moisture as you have found your dehumidifier will remove the moisture, I have also found sealing the floor with epoxy paint really helped reduce the moisture but I did still need the dehumidifier just did not need to empty it as often.


This is for sure black mold on the osb. After dealing with a lot of coughing and trouble breathing, i found out from the doctor that i have mold micro toxins in my blood. Then when i inspected the house i saw these same black spots in the basement and attic. I tested them with a mold test kit and it turned out to be a black green mold. After hiring someone to fix this all across my house for $4000, my coughing went away and breathing was back to normal.

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