Moisture. Could be from ac or roof but no telling from picture so it's into the attic for a look see.

I went into the attic and there is no water leak but a small chemical residue. See the photos.

I touched the darkened floor/ceiling and it has become very soft. It also burned my hand slightly like an acid. I am thinking of getting some baking soda and put on it to neutralize the acid. Don't know if should spray slight water to activate it too. Then think of repair.

Did a little test with a slight amount of baking soda and water spray and it is acid. It bubbled up. See the link to the short video please.
https://drive.google.com/file/d/1Kdv...ew?usp=sharing

That metal vent and pvc pipe fooled me into thinking condensation or leak. Now that you found acid I'll have to dust off my ouija board and ask how acid made it's way to your attic.

I hope to find solution. I am not sure how acid got there but now that it is there, how do I go about cleaning/neutralizing it and fixing the ceiling? I thought of gathering the insulation in a couple of trash bags, use baking soda and spray water to neutralize the acid for several hours, then vacuum the area and if I can find, put fresh insulation and if I can't use the old insulation. Then once this is done figure out a way to fix the ceiling and the damaged boards.

Are you sure you dont have some animal living up there, and that the spot is urine, rather than acid. Makes no sense for acid to be there, and something definitely has disturbed that insulation.

Bats?

Thank you for replies. OK maybe an animal or acid or whatever? How to I go about fixing it? Thank you.

Have to know the source/what it is and cleaning is only a temporary fix given the damage. Perhaps hire a licensed home inspector and it shouldn’t cost very much, while repairing the damage might be.

You have to take care of the source of whatever is causing it before you even think, about doing a repair.

OK. I will investigate the source of damage but can you explain in details or direct me to video or reading material that explains how to go about the repair? Is this something that I can tackle myself? Are the parts needed readily available from local hardware stores? Thank you.

I am watching this video which is very informative

https://youtu.be/BUcT5LHmA7w?si=aYK63Zf2oP_IwZuD

My question is since it is near the edge of the ceiling and on the outside of the Master bedroom is a big drop to the hall downstairs, what are the challenges involved? I will try to show this in pictures.

I used some baking soda and water to take care of acid (or whatever it was) then vacuum it clean. I think as a temporary repair to strengthen the backing (attic side) till finance allows would be to use Self Adhesive Drywall Repair Patch with some joint compound perhaps.

If the cause was bats or something like that they are no longer there and the roof does not seem to have any leaks, bone dry.

To repair the front of the drywall perhaps the method this guy uses in this video might do the trick at least temporarily especially with the reinforced backing as described above.

https://youtu.be/oApdUrRGVeo?si=PW4If2oDfr-IPyX2

I appreciate your helpful comments and suggestion but please try to keep any criticism to minimum.

Thanks.