at the corner of our house, at the bottom of the stairs, on one wall we have a wet patch. There...

There is no black mould, but white stuff is accumulating (salts?). A surveyor friend reckons the render is in good condition. The waste pipe from the bathroom goes down the wall at this point but we can't see a sign of a leak. There is a small hole in the wall at second storey level. Also we think it's cavity wall but that no insulation has been put in. Please help

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