moisture damage to ceiling

[sburty]

Hi I recently bought my first house and ive noticed that the ceiling in the kitchen sagged slightly in one place. The ceiling is plaster board and skimmed flat with plaster. the other day i saw that in the centre of the sag there was a wet patch about the size of a finger nail. When i touched it the plasterboard was damp and crumbly. I removed/ cut away some of the board and the joists were damp along the bottom. Looking up there appears to be no leak as the sides of the joist are bone dry.the bathroom is above the kitchen and under the bath is also bone dry Can anyone spread some light on where the moisture could be coming from and what i should do next? the plaster board is foil backed and the ceiling was put up 1 year ago i believe.

 

[JohnD]

there appears to be no leak

Oh yes there is. You just haven't found it yet. The water can run along on top of plasterboard, especially if foil backed, until it finds a hole to go through.

Start by looking at the waste trap of the bath and shower.

It will be easier if you take up the bathroom floor.

 

[theoldun]

As John D has said you definitely have a birds beak some where. With foil backed plasterboard it could run from one side of room to the other before finding a low spot.
Lifting floor boards in bathroom can be a pain. We would drop the ceiling if we could not find it.
oldun

 

[steve1978]

drop the ceiling to find it as your gonna have to do this anyway as it will never be right once its got wet and a far easier and cheaper job to do than pulling up half your bathroom floor

Steve

 

[sburty]

I will drop the ceiling as Ive smashed a whole in it already trying to find the leak. a friend said that it could be coming in from an outside wall. The house has a poorly fitted conservatory without flashing, it just has damp proof course type material stuck to the wall, its not even cut in.