Leaking flat roof - some of underneath of concrete surface has crumbled off

[MattB83]

We've got an old outhouse attached to the house. It's got a flat roof with a fairly old covering - when we arrived it was apparent there was a small leak for a while and I patched it up and it seemed fine. We rarely use the shed but I went in tonight and everything in it was covered in dust - the floor has some water on it and it's not clear where it's come from but I think the rood is leaking again. The top layer of cement has flaked off on the right-hand side and if I rub it, small bits will come off, you can see in one pic it's nice and 'skimmed' and in the other it's not.

Obviously the roof needs to be repaired to stop the leak, but, once dry, can the concrete be re-skimmed?

Note: The pics look like water is pouring in - it isn't, this is bits of cement dust caught in spider webs and reflecting the light! The pic makes the roof look like it's sagging but I'm fairly sure it's not having looked at it carefully.

 

[robinbanks]

It's RAAC! Run!

On a serious note, I'd say it just some surface spalling caused by damp from a leak, possibly also in combination with freeze/thaw cycles in the past (the water freezes and jacks the surface of the concrete apart as it expands leading to a fine dust) . Fix the leak, let it dry out (could take months), wire brush the surface dust off and paint with a breathable paint if you want to just bind it together. Alternatively, batten and plasterboard it

For the roof covering, you might find it cost effective to order a pre-sized EPDM roofing kit intended for sheds like SkyGuard, and just recover the roof