Damp on bathroom wall

We have damp on our bathroom wall which we only discovered when we removed wallpaper, re-plastered and redecorated.
Our first thought was that we hadn't let the plaster dry for long enough or that the new paint was absorbing moisture from the steam, but then we noti ced it was worse after it rained (and, boy, did it rain in late winter/spri ng).
We've had two roofers check our tiles and a builder look at the wall. The r oofers found nothing wrong and the builder suggested that the double-skin b ricks in our 1920s house had no cavity above ground-floor level because of a recess for pebble dashing.
I've just been out to look at what happens in heavy rain (getting soaked in the process) and noticed the lie of the tiles, with the lowest tile being at a less sharp angle to the others, leaving a line just above the brick pl inth. See picture at http://twitpic.com/e83zlm
There was a drip coming from this line but I wonder whether this is also tr ickling water into the wall. The main area of damp is about 2-3ft away from the plinth, although there is a small area of damp at the very top of the plinth on the inside.
My theory now is that the water is trickling in but because the plinth is d ouble-skin with cavity, the water is seeping into the wall nearby. I know t hat with flat roofs the damp can often be feet away from the leak.
Any views on this idea, please, and any ideas on how to remedy this?
Will
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On 13/07/2014 12:45, William Bramhill wrote:

I'm guessing the wall faces the prevailing weather (S/SW). It was normal round here to pebbledash that aspect but IME it was not always done properly so that patches were "live" from the outset. A hairline crack is enough to allow a lot of rainwater in which then becomes trapped behind the render and appears indoors *somewhere* on the internal wall. By tapping with a screwdriver handle the bad bits will sound hollow.
Since your damp coincides with rainfall, I imagine we can discount the theory that you are running a laundry or engaging in other high humidity activities :-)
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On Sun, 13 Jul 2014 04:45:22 -0700 (PDT), William Bramhill wrote:

Can't see the pitch of the roof, but a GF's bungalow had quite a shallow pitch, a cracked tile let in a trickle of water, at the shallower bit the felt underneath had gradually sagged a bit under the weight until a pool formed and had then degraded to the point of leaking. I used silicone sealant to glue the tile together until we could get a new one, but she sold the place before the tuit landed.

Well, in the GF's case I noticed a trickle from a 13A socket. Knowing that electricity and water mix distressingly well...! The leak was directly above the socket, so I was lucky there.
--
Peter.
The gods will stay away
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Dear Phil, Stuart and Peter
Many thanks for getting back to me so quickly. I hadn't considered a hairline crack in a tile.
Here are more pictures of the side of the house, with the bathroom being nearest to the camera on the first floor.
There are also three shots of the damp on the inside, with the blank wall being the worst affected but the window wall is also showing signs after today's heavy rain.
http://twitpic.com/e84x0m http://twitpic.com/e84x1s http://twitpic.com/e84x2q http://twitpic.com/e84x48 http://twitpic.com/e84x61
Regards
Will
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Condensation and subsequent mould in my wife's flat did not have the appearance of the internal photo. Either lateral penetration as said above or water finding a route through the roof tiles and puddling where the roofing felt lifts over the gutter board and thence into the end wall internal brickwork.
And, totally off topic... noting your CV from the photo site, can you kindly arrange for the Sunday Times Review compositor to shift the sudoku puzzles slightly higher such that the paper fold does not interfere with the bottom line? Many thanks:-)
--
Tim Lamb

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Noted, Tim. Will speak to the puzzles guy tomorrow!
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Excellent.
I have been meaning to write but, somehow, it did not seem appropriate to whinge on the letters page.
--
Tim Lamb

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