Strange Damp problem on internal walls

[penterp]

Having recently aquired a property for improvement there were signs of damp on a few internal plastered walls. The plaster actually felt damp to the touch and was off scale with a damp meter in places.
The recommendations were to remove plaster to 1M high and inject chemical DPC.
Having removed the plaster there is no sign of damp in any of the brick work which is dry as a bone.
Why should only the plaster be damp. There were no signs of damp on any of the outside walls.
Any help and comments would be appreciated.

 

[JohnD]

Maybe the previous occupiers used to drape wet washing around the ome or over radiators.

Open the windows.

You can also look for a water leak in pipes or drains, possibly under the floor.

You do not mention if it is a flat or a house, how old it is, what the floors are made of, which rooms show damp.

 

[penterp]

The property is a detached house.
Ground floor rooms affected.
Floors are "bitumen" on concrete.

 

[Andrewmudge]

I expect it's rising damp continually brought to the surface (plaster) because the rooms are warmer than the centre of the wall and it's continually evaporating. Exposé walls below floor and inject dpc then use a dpc to keep the patching in of the floor of the walls so as the damp in the floor cannot pass back to the walls.