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If a house had mildew (I'm assuming it's mildew, it's black and slowly spreading) on the ceiling of the top floor of my house, what could cause this and how serious could it potentially be?

The building is a 3 story (minus the roof space) house in the UK. It was built in 1997, and the same family lives there today that moved in 16 years ago. If you want pictures or have questions, leave a comment and I will update the question. I don't know a lot about DIY/home improvement which is why I am asking here.

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A picture always helps. Is this mildew on top of the roofing material? I am not understanding your question clearly. –  shirlock homes Jan 14 '13 at 12:24
    
"on the roof of the top floor..." do you mean the ceiling? –  Niall C. Jan 14 '13 at 14:09
    
That's embarrassing (had a total mental block), yes I mean the ceiling, I will add pictures later –  JMK Jan 14 '13 at 16:49

2 Answers 2

It probably means that you have a hole in the roof that was large enough to drip enough water to stain the ceiling (which doesn't take much, really) but small enough so that the amount of water dripping through is contained by the ceiling and does not leak through inside the room. I would try to get the coordinates of the stain on the roof and get on the roof, project it there, try to find the hole and patch it.

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leaks are fun! it may be leaking from somewhere else, running down and dripping from the location you think the leak is. –  fungku Feb 13 '13 at 21:22

Mold and mildew are always serious as they present a health hazard and over long periods of time, also structural problems (rot) due to the root causes. Both are usually present due to excess moisture - this can be humidity in the environment from things like showers or clothes dryers, or from a more obvious water source like a plumbing or roof leak.

It might be worth getting a humidity monitor (or a thermometer that also gives you humidity) - if this is high then you need to find the source. It might be as simple as ensuring that your clothes dryer is properly vented outdoors or adding a extraction fan to your bathrooms. If you live in a really humid environment then it might be necessary to run your A/C or add extra air flow via fans.

If the excess humidity is due to a leak then you need to resolve the root problem.

After the root problem is resolved you should be able to remove the visible mildew with a weak solution of water and bleach.

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