I was in my attic and took this picture.

No visible water and nothing damp to the touch. 8' from a small window. Not above a bathroom. I don't think it's from poor ventilation.

If it is mold how can I find the ventilation issues?


It's on this side:

enter image description here

  • 7
    Is this new or just the first time you looked? The pattern almost looks like a foot/boot print, so might have been there forever.
    – crip659
    Commented Dec 8, 2022 at 16:01
  • What ventilation is present? Finding "issues" starts with finding the ventilation. Those windows are not functional. You don't need visible water for mold.
    – isherwood
    Commented Dec 9, 2022 at 14:02

3 Answers 3


Probably chalk dust (blue, or possibly green depending on the camera, both common colors used in builder's chalk-lines) from the time of construction. Very "slightly smeared boot tread" with a lighter less-smeared one beyond it, turned 90 degrees or so. Someone refilled their line, spilled a bit, and walked in it, and on this board.

detail of boot-prints

You could be paranoid and support the mold-testing industry by taking a sample, or you could wipe it clean and only worry about doing that if it ever reappears.

  • 1
    Just as an FYI, mold tests are not terribly reliable because mold is technically everywhere. I've seven seen professional tests vary wild in results for the same location. I would expect the DIY tests to be even worse.
    – Machavity
    Commented Dec 9, 2022 at 14:13

That pattern isn't characteristic of mold; see below. Mold likes to develop where there is an infiltration source and drying time is the slowest. If you have an excessively humid attic then you would see this stuff on every single rafter.

enter image description here

If it were mold then rest assured that it's old and likely inactive; white coloring means it has passed it's prime. Spray some bleach on it and scrub it off with a brush. Wear a respirator for good measure.

This is what an active mold problem looks like. Albeit, this one might be subsiding given the amount of white color.

enter image description here

Your "mold" seems to be a footprint. Take a look at all those footprints on the rafter behind it.

enter image description here

  • I know we can't post products, but can you tell me what type of device i can measure humidity with? (When guy came to quote my garage floor he put a meter on the floor and told me there was no moisture below the slab). Is there something similar?
    – Marinaio
    Commented Dec 9, 2022 at 18:21
  • @Marinaio If you're checking the humidity in the air then an analog or digital hygrometer would work. If you have a dehumidifier then you can run it in the attic for a few minutes and see its humidity reading. If you want to check the moisture content of your rafters then get a moisture meter; this is likely how your slab was tested.
    – MonkeyZeus
    Commented Dec 9, 2022 at 18:48
  • 1
    @Marinaio I do not endorse nor recommend any of those linked products. They simply showed up first in my google search.
    – MonkeyZeus
    Commented Dec 9, 2022 at 18:52
  • @Marinaio Any luck with the moisture testing or figuring out if that was merely a footprint?
    – MonkeyZeus
    Commented Dec 16, 2022 at 15:41
  • Not yet. Want to stop by H F Tools to look for a moisture device. I looked again and I don't see anything of the examples others posted so I may have over reacted.
    – Marinaio
    Commented Dec 17, 2022 at 20:49

Mold is caused by:

  1. Spores
  2. Food
  3. Moisture

Spores are almost everywhere so there is little you can do about it. We build our homes out of "mold food" like wood. So there is little you can do about that.

The remaining thing is moisture. Fix the moisture problem and the mold will stop growing.

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