So it happened several months ago in summer, and now, in december, during the winter you decide to do something about it?
If you didn't inform your landlord right when it first appeared then HE is the one who deserves compensation.
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I would call my landlord first to see what he thinks about that and how can you solve that problem.
That's something that was hopefully done 6 months ago! In case it was then start reducing the rent by 30% starting next month. If you didn't report it, well, I hope you have good liability insurance.
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Because mould is not a nice and healthy thing to have at home, and he should get someone to paint that and to find the leak in the roof...
Painting it over will not solve the problem. The insulation on the roof needs to be removed and most likely thrown out. Since the water might get in at another location that means the whole flat roof needs to be exposed. All the walls and ceilings need to be dried out thoroughly. Hopefully it's all bricks and concrete, otherwise the building materials have to be replaced. Then everything has to be treated with fungicides so the shit will not come back. Once everything has dried out completely the insulation and roofing can be put back in place. Good luck, it's december...
Depending on how bad it is the only choice you might have is to move out immediately.