Tile cladding

Grumpyyeti

Hi. I'm new to this forum but saw this thread which although a few years old may throw some light on a problem I'm having with water damage to a ceiling. I'm going to have to get a professional involved but any thoughts could help me in the discussion. The house is a mid 1970's semi with brick ground floor and tile hung upper floor back and front and two storey of brick on the side, gable wall. An original single storey flat roof extension to the rear extends the kitchen beyond the upper storey. The flat roof was replaced when I added a further extension to the kitchen a few years back. The lounge has patio doors at the rear so I'm assuming that there is a substantial RSJ across the width of the back of the house supporting the upper floors and bridging the kitchen part way along its length. The back of the house faces due west, the direction of the prevailing winds, and is very exposed as there are no houses close by to the west and south.

My problem is every so often, and there can be years between this occurring, I get a patch of mould and water damage appearing on the kitchen ceiling with the paint peeling off and water finding its way under the paint on the walls, always in the same place. It's not a large area but when it dries out the paint is damaged and needs redecoration. I've tried various things, including spraying the whole gable end of the house with Water Stop, trying to cure this - desperation will drive you to try anything no matter how remote the chance of success. And because it is only an occasional thing there's no way of knowing whether each attempted repair has been successful until the problem comes back to show it wasn't. The photo is as things look at the moment; the wall looks OK but actually the paint has lifted under the surface.

The side of the house is brick with a cavity which more than 50 years ago was insulated with rockwool insulation. There are two pipes which pass through the wall in the area where the internal damage occurs. I've checked and sealed around both pipes and also checked the holes where the insulation was blown in. This wall has also been treated with Water Stop. The pipes are the foul drainage from the toilet and the waste water from the bath/sink. I've removed the panel behind the toilet and there is no sign of damp from either pipe and logically if either of these were the problem I would have expected it to be a constant issue not one that comes and goes with long intervals in between.

If I understand correctly houses of this type in the 1970's were built with a single skin of block work on the upper floor covered with a waterproof/breathable membrane and finished with tiles on battens, so there is no cavity. It seems far-fetched but could the membrane become less effective over the past 50 years and allow any rain getting past the tiles work its way down to the top of the ceiling board? I know it sounds crazy but this has really got me stumped so any thoughts would be very welcome. Oh, and I'd better mention that about three years ago the whole house was re-roofed. Thanks in advance for any thoughts or advice.