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I recently discovered that water was dripping from the ceiling light fixture in a first floor bathroom. It turned it was due to a toilet leak (faulty wax ring) in a bathroom directly above.

The toilet leak has been fixed, and in fact there was no evidence of water in the upstairs bathroom at all - it must have leaked directly through the floor around the toilet flange/pipe.

I removed the light fixture in the downstairs bathroom that the water was dripping through and discovered a patch of mold. It's only the size of the fixture (about a foot in diameter) and aside from the visible mold there doesn't appear to be significant or visible water damage. The sheetrock is relatively hard and dry, all things considered. This bathroom is otherwise dry and in good shape - I had no idea this mold was lurking behind the fixture.

Here's an image:

Mold

Cleaning off the mold on the surface of the ceiling should be pretty simple. But is there anything else I must do to protect myself and my home as a result of this leak?

What about the space between the ceiling and the second floor? What I'm afraid of is what I can't see on the other side of that sheetrock but I don't know if that's something to be concerned with or not. Is it sufficient to clean away the mold, ensure the ceiling is dry and call it done?

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Looking at the pattern of the mold here, it seems plausible that this is just surface growth that occurred between the fixture and ceiling. Drilling a hole to inspect in the ceiling is a good idea, but if the leak is fixed I wouldn't expect there to be deep damage. Widespread mold growth is typically supported by ongoing leaks, not just one event. –  Shimon Rura Jan 9 '12 at 3:52
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4 Answers

There will be the potential for the mould to reappear as (as you have correctly surmised) there might be still some water in the ceiling space.

If you can, lift the floorboards in the upstairs room. This will allow you to see the extent of any damage and also get the air into help dry the patch out. If the area affected is not too wet then leaving the boards up for a while will help. If it is really wet then you might have to cut away the damaged board and repair the ceiling.

There are other questions here on how to patch a hole in the ceiling.

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I can't easily get under the floor in the upstairs room (it's tiled) so my only options are to break through the ceiling space or just clean the ceiling surface. I'd much rather do the latter, I just want to ensure that I'm not leaving behind a long-term problem between the ceiling space that would be harder to fix later than it may be to fix now. Thanks for your answer! –  kurt Jan 5 '12 at 18:07
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@Kurt - you could cut a small hole (the size of your finger) which might be enough to feel whether the other side is very wet or not. This would be easier to fill - especially if you cut it near the light fitting (turn off the power) where it would be obscured –  ChrisF Jan 5 '12 at 20:42
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I've had to repair a similar issue in the past... I think it's worth opening up some sort of hole (above or below, whatever works best for you) and setting up a fan to completely dry out the inside before you close it up.

Also, because the drywall has gotten wet, you might run into some issues with any paint you put on it peeling up (I had that issue with plaster, so it might be a different scenario for you).

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Definitely this type of wall needs to be repaired as early as possible. If the damaged wall is non-porous then you can simply wipe the mold away by using a wet cloth. You can also use a mold killing product if you like such as bleach, vinegar, borax and more to remove the mold. However if the wall is porous, like an unpainted drywall, then you need to do one thing and that is cut away the wall where the mold is growing. This is because mold grows inside the material, instead of just on the surface, and if it is not cut away then it will not be removed completely. The main cause of mold is water leakage and so leaks should be repaired.

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If the plaster has completely dried out, you could just try painting a damp retardant paint such as Ronseal One Coat Anti Damp Paint.

This should prevent the stain coming back through the paint, which is waht would likely happen if you just used emulsion.

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