My house has two floors. These blemishes are on the first floor ceiling, along the southern wall. The floor joists run east-west (left to right in the first picture). The blemishes do not pass beyond the first floor joint. There is no indication of water damage on the second floor, as far as I can see.

However! The majority of these blemishes are directly underneath the room that has the horrible fiberglass smell from my prior posts.

I was going to drill into one to see what is going on inside there - but my stud finder indicates there is a stud in each area. I am wondering if these spots are from previous water damage - where someone perhaps put spackle over the area, sanded, and then painted it.

It does seem clear to me that someone spackled it at some point in time. I cannot explain why the stud finder things there is something there.

Perhaps I just have to cut open a hole near one of them and stick an endoscope up there.

enter image description here enter image description here

Edit: mouse confirmed. I used a hole saw to cut a hole in the ceiling and stuck an endoscope up there.

mouse poop

It gets even worse: it looks like mice have access to the entire subfloor, due to a power cable (?! there's no power over there) and holes in the wood:

hole in the wood oversized hole for wire

  • 1
    Most stud finders will sometimes find other stuff than studs, but do not tell what it is. Joists should be at 16 inch from each other, with an empty space that people use to run pipes/cables/ducts though.
    – crip659
    Mar 29, 2022 at 21:42
  • Looks a lot like early signs of mildew (especially the ceiling marks), which means moisture (past and/or present). Mar 30, 2022 at 1:59
  • I linked to what I think is the prior post you're referring to. If that's not it, please edit to include a link instead of making people guess. Additionally, we can't tell from this picture which way East to West is. Would you kindly indicate for us.
    – FreeMan
    Mar 30, 2022 at 12:47
  • Thank you, @FreeMan, that's the right question. In the very top picture, that left most corner is west - from left to right in the picture is from west to east. I guess I am going to create a followup question.
    – negacao
    Mar 30, 2022 at 12:56
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    I edited that left-to-right info into the question itself so it doesn't get lost.
    – FreeMan
    Mar 30, 2022 at 13:12

1 Answer 1


Based on some of your previous posts, rodent urine from a nesting spot seems most likely.

Damp materials can affect stud finders oddly, depending on the studfinder. I've reverted to the "use a very strong magnet that can find screw/nailheads even under a lot of joint compound" as being generally more reliable than the other sorts I've used.

  • It is indeed mice.
    – negacao
    Mar 30, 2022 at 12:56

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