Damp on chimney breast of Victorian end terrace house

This common condition is called ammonia sulphate contamination, yes it kicks off damp meters because it is a salt . The chimney breast must be stripped back to the bricks and waterproof rendered . If you search the internet, you may find articles I’ve written about this . The cause is seldom known to contractors, sadly .

Hygroscopic salts in the masonry is common on a chimney breast with burning fuels over the years such as coal wood etc. Damp patches can appear and disappear they will come and go as the air changes. Even soot in the chimney causes damp if the chimney is not in use obsolete. 100% salt contamination

The previous answer is only partially right. Yes it is due to salt contamination in the plaster. Not only will the salts themselves affect a resistance moisture meter but the salts may also be hygroscopic and absorb moisture from the air making the surface damp. With minor contamination in localised areas, this can sometimes be treated as a cosmetic issue. The application of a salt neutraliser (commonly Acetic Acid) and a sealer like Zinsser Stain block will sometimes alleviate the problem sufficiently to allow re-decoration. If the problem is too severe, we would normally fix a cavity membrane and dab fix plasterboard. If there is sufficient space, it can be fitted over the existing plaster.

Salt affected plaster. Hack off and replaster using Thistle Dri-Coat or Tilcon limelite , easily available from larger builders merchants.