Large Damp Patch on Ground Floor of 1900's Property Coming from Outside? Damp can be a major issue in the home. Find answers to questions or post your own here.

[cmelikian]

We have a 1900's house and on the ground floor there is a large damp path which has re-appeared. We bought the house last September and found the wall soaking wet, light switch internals rusted etc. We had a damp company come in to take away the plaster, dry the area out then re-plaster with the special plaster and the damp has now come back.
The details of this are:-

• Tenants don't open the windows much *but* the issue was there when we bought it and the property was supposedly empty for months
• There is a commercial car garage next door and their wall is barely a centimetre away from our wall all along the whole house and to the height of our roof.
• The garage has a brick missing in the wall about 0.5m from where our damp patch is and you can see the wall is dry around there so seems unlikely the wall is wet from outside?
• We repaired the roof and guttering between us and the garage in September so there's no possibility of water entering from there but it may have gone down from there before the repair.
• The damp patch is about 1.5m above the floor and another patch is in the corner. at the same height.
• The area is colder and wetter (obviously!) than the other areas of the same wall
• Despite the massive downpours over the last month or so the patch hasn't got significantly worse.

So after all that my thoughts are that it could be either:-

• That the wall wasn't dry externally when we did the work so the patch is existing water coming through. When it has finished coming through the wall will be dry and stay dry now the guttering is sorted.
• There's more water coming in externally from somewhere?
• It's all condensation from the tenants but then why was the wall so wet when we bought it? Other walls we treated in the property have been fine so far.

Anyone have any thoughts?