Have you ever wondered what those damp patches are that suddenly appear on walls, typically on and around chimney breasts?
More often than not they are actually hygroscopic salts!
These unsightly stains are caused due the burning of fossil fuels typically used in open fires found in older buildings; hence the location of the patches. Over time, the smoke/soot from the burning of coal, wood etc sticks to the surface of the walls and leaves behind invisible salts.
The patches normally only appear during spells of high humidity, because the salts draw moisture from the air and you are left with "staining" or "damp patches".
Once humidity levels decrease, the stains tend to disappear!
The treatment required is to remove the contaminated plasterwork and install ULTRA 3 Mesh Membrane directly onto the substrate (brick, block etc). Then, plasterwork and wall finishes can be reinstated.
It is extremely important to provide a barrier (ULTRA 3 Mesh Membrane) to prevent the salts from leaching back into the new plasterwork.