What's causing this damp patch?

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I live in a basement flat and we've recently started getting a damp patch appearing on a wall at the front bay window, which gets worse when it rains. The rain doesn't actually hit the exterior wall since it's covered by the above flat's window sill, but there's a damp patch in the same place on the exterior wall, and no where else.

There's no water pipes nearby but the part of the patch is around the hole for the virgin tv cable, though I've sealed that and there doesn't seem to be an improvement. it's also just in the middle of the wall, not at the bottom or the top. Didn't have this problem last winter and nothing has changed so can't think what could cause it.

Does anyone have any ideas what could cause this?

Thanks
 
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Have you asked the tenant above if they have damp?
 
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The people upstairs have no signs of damp, and yeah the external wall is rendered. Seems possible that the damp could be travelling through the render but I don't understand why the damp patch is only appearing in the middle of the wall?

The whole exterior of the house was re-painted last year but on closer inspection looks like there's a few exposed bits under the damp patch so I've given the whole area a new coat of waterproof paint, hopefully that will help.
 
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Its only a possibility David, but is that render loose? You can often tell by tapping it, listening for a hollow sound.
My theory......water runs down the window, under the sill and finds its way behind the render. The water can't get out so it makes its presence on the inside wall. Do check for sealing compound under the sill.
John :)
 
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I was going to suggest what he just suggested. ^
 
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I think you might be on to something.. I had a poke under the windowsill and some render just fell off revealing a pretty deep hole into the wall, below where the damp patch is so I've now filled it. hopefully that will fix it, thanks for the help
 

ree

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"Waterproof paint" will only cause further difficulties, it seals in the damp or moisture - hence the use of masonry paints or, now a days, the application of pre-colored synthetic renders.
 
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By waterproof paint I mean waterproof breathable masonry paint (sandtex)
 

ree

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Thanks for that information.

Why dont you post pics of the inside and the outside of the damp patch area wall(s)?
 
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