My house is under erm...medical attention at the moment, and amongst quite a few jobs is a damp patch under the stairs, it is on an internal wall, but is the otherside of an external wall, and has about a 15-20inch black patch - I know this to be damp - just havent a clue how to treat it or sort it, any tips etc would be great, thankyou in advance!
If your house has cavity walls then the chances are that something is bridging the cavity where the damp patch is.
If you have solid walls look for obvious sources of water on the outside.
If neither of these see if there is a water pipe buried in the wall.
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