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I recently bought an older house (~2000) and I noticed that pretty much all the joists in the basement ceiling have this discoloration. I was wondering if any of you have experienced this/can confirm that it is mold (and whether or not I should panic). It's only on the bottom of the joists as far as I can tell..

enter image description here

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  • Mold usually spreads, so just on the bottom and going up the sides, it might be more dirt/ink stains(imgur not working). Mold needs to be tested, pictures don't tell. If just mold, spraying some bleach on it should work, plus drying/lowering the humidity.
    – crip659
    Feb 8 at 1:38
  • A 20 year old house is not "older". According to my house (built in the 1890s), it's still a toddler.
    – FreeMan
    Feb 8 at 12:11
  • thst looks like ink on the edge
    – jsotola
    Feb 8 at 18:26

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I don't see anything there that concerns me. It looks like the joists were stored wet for a bit, maybe on concrete. What I would do:

  • Give it a wipe with a white cloth. Does it come off readily? Is it powdery? Does it have a distinct musty odor?
  • Watch it for changes. If it's active mold you'll see it grow. If it is mold, but it's from the construction phase, just wipe it off with a household cleaner and forget it.
  • Examine your home for leaks and be aware of the relative humidity. Home management means knowing whether you're outside reasonable limits. You may want to invest in a dehumidifier*.
  • If you're really concerned, have it tested.

* This is the one and only device for which I buy warranties. These things have almost invariably failed on me after one or two seasons, and I really hate thinking of them as disposable.

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    Based on the FPL's Wood Handbook (page 4-4), 80% relative humidity is about the threshold to start worrying (it correlates to about 16% equilibrium moisture content). 85% relative humidity (with an equilibrium moisture content of about 18%) would be my do-something threshold. Hygrometers are cheaper than dirt on Amazon. Alaska has the highest RH of US states, where it's actually not intuitive at all--dew point is intuitive.
    – popham
    Feb 8 at 4:30

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