What is the cause?
This Victorian terraced property has solid brick walls. It has severe damp at ground floor level, but no issues on the first floor.
The difference is that the external ground floor is painted.
This traps moisture which then evaporates inwards due to warmer internal temperatures.
In addition, gypsum plaster has been skimmed over the original lime plaster internally.
Lime is vapour-permeable whereas gypsum is not. They are not compatible materials if moisture is passing through the wall.
So how do we resolve this?
This issue is one of lateral damp (through the wall) not rising damp (from the ground soil). There is already a horizontal chemical damp proof course (DPC) which appears to be doing its intended job.
As such, we would advise the following internal remedial works:
- All plaster finishes in the affected area should be hacked off, not just up to 1000mm above the finished floor level (FFL)
- The renovation plaster must be moisture and salts resistant. (Salts are the generic term for contaminants in the soil and thus clay bricks i.e. sulphates, potassium, nitrates plus carbonic acid (CO2 and H20) from rainfall)
- Once you block lateral moisture from the inside surface of the wall, you can safely skim with gypsum or an alternative finishing plaster.
Externally, we would recommend:
- Removing paint with a chemical paint stripper
- Re-pointing where necessary with proper gauged mortar 1:4 etc
- Injecting with chemical DPC to the vertical junction of the rear elevation and yard boundary wall.
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