Causes of damp?

Hi,

I've taken some tiles off my bathroom wall and have noticed that the wall underneath looks quite damp. You can actually see tracks where water has been running. Although I've never noticed any drips as such in the kitchen below.... but I've not decorated/investigated that one yet. The room in front of that does have a problem with wall paper peeling off so I suspect its affecting the whole of the wall.

The wall is a cavity wall. External face is accrington brick - which I believe is like an engineering brick and is impervious to water. The internal wall is just standard brick (not common concrete) with a render/plaster finish. The cavity has been retrofitted with insulation - blown polystyrene I believe.

The internal wall has been patched in places and I think damp/water is running down between the old render and the internal brick and then when it meets the new patched rendering its moving to the inside on the join - does that make sense? Question is how does the damp get across to the inside wall in the first place?

Looking at the outside wall:
- the pointing is failing in a few areas. The pointing is mastic based. Why do you use mastic on engineering brick and not just sand and cement? Is that to prevent water getting through?
- the barge boards may not be providing the optimum seal between the brick wall and slate roof tiles.

I'm assuming I need to solve the source of the damp before patching the wall up? If I used a membrane to stop the damp coming through the bathroom wall it would simply move it into the room below.

How do I identify where the damp is coming from so that I can get the right repair undertaken?

thanks in advance for any help

Probably not much help but here goes.

Water may be getting into the cavity, dripping through the roof itself and with all the polystyrene filling the void, the water will not get away. It would normally evaporate away before getting to the inside.

Has your roof got sarking felt under the roof tiles. If so, this can channel any ingress of water down to the cavity. It makes it hard to locate where any water is getting in through the roof. Check the roof slates for any obvious signs of missing/broken tiles and look all the way up to the ridge if sarking felt was used.

thanks for the tip. I guess I need to have a good look in the loft.

The roof is slate, but the previous owner must have had problems and had it lined with foam. Not sure what that is called, but it looks to me like they've squirted expanding foam on the underside of all the slates.

I believe this insulates the roof as well as stopping any slates falling off and any leaks. The down side is its difficult to see where any water may/may not be tracking from... which worries me if that is the source.

The foam will be acting the same as the sarking felt in that it will be channeling the water down to the cavity. It really is not that easy to find the leak. It normally is vertically up to the ridge (it doesn't normally travel sideways, if your lucky.)

One other thing, just make sure your gutters are not overflowing at the point/area you are having problems.