Just been to a friend's place in a block of six flats and his is on the top floor under a flat roof. Built 1960-1970 approx.

His ceilings in two of the large rooms appear to have blotches of damp stain on them. They are not by the windows and not next to the kitchen or bathroom where condensation might be expected. I have noticed that the ceiling artex (yuk!) is split, along the lines of what I assume to be the slabs as they were cast when the building was erected. The blotches seem to be almost entirely on one of the five sections and not significantly on any of the others.

The managing agents claim it is condensation, but unless that section of concrete isn't insulated as well as the others, I find that hard to believe.

The roof is in a good state of repair and a leak appears unlikely.

I am not familiar with the construction of this type of building which prevents me from suggesting what the problem might be. Does anyone here have experience of these structures and what might be causing the damp spots?

I'm sure you've asked all the obvious questions, but how long have the patches been there? I'd have thought it must be some sort of leak as you say it seems to be in the one section, and you don't reckon its an obvious condensation situation. Let's face it, a pin prick in a flat roof will cause an issue at some stage.

I'd have thought that the ceiling would be plasterboard battened off the slab - with or without some insulation somewhere. Is there any signs of the staining following joints? You mention the splits - do you reckon they're board sheet size or following a larger pattern of one section of the entire roof slab? Also you say, that the patches are nearly all in one area - are the other ones close by and tracking along the joints or other end of the flat?

Sorry mate, not being much help here but you may be lucky and get someone reading this who's done a bit of NCP car park construction!

Julian, thanks for your comments. It may be a leak, or residual moisture from a previous leak.

The ceiling is solid concrete behind the Artex. There isn't any plasterboard. I assume that the solid concrete is insulated somewhere between ceiling and roof covering, otherwise it would be exceedingly cold and mouldy. Thats why I wanted to understand the structure before trying to work out exactly what the problem might be.

When I did a quick flick round the net (as I imagine you've done!), the suggestion is that damp linked with the resulting corrosion in the reinforcement bars is often an issue, and that some will have insulation and some won't - but even those with insulation will be pretty poor.

So, assuming its a badly insulated structure whichever way you look at it, if there is no leak, then could it have anything to do with the fact we've had the coldest winter on record? That ceiling could be even colder than usual, maybe your mate was dragging a calor gas heater around trying to keep warm, and it therefore may be a condensation issue?

Probably be best to do a test with a bit of undercoat or zinsser and see if it comes back?

Again, sorry mate, probably not helping much, but I like head scratchers like this!

Best of luck

Thanks Julian, testing is probably not a bad idea, although a spray can is more likely as I am lazy.