Plastered fireplace attracting damp - advice needed

[melody]

The back wall of our dining room (interior wall) used to have a fireplace which was removed by the previous owners.



A damp patch where the hearth used to be has been there since last year. We sketched an outline in pencil and the damp hasn't spread or moved but the wall is gradually falling apart.


It looks like a gypsum skim which is now crumbling. But I have no idea whether to just remove the damp plaster and redo with lime or if there could be an underlying issue with the old chimney. We had the chimney and flashing checked so don't think it's a leak. There is no french drain at the front of the house but again it seems odd that only the damp would be there.

 

[melody]

Would love some advice on what to do as we are struggling to get someone to come and help us without knowing where to start. If you know of anyone in Sussex area too we are very open to recommendations!

 

[stuart45]

Google hygroscopic salts in chimneys. It's a common problem. Gypsum tends to hold moisture and fail when constantly damp.

 

[Cubist]

It's more than likely that the damp you're seeing is caused by salts absorbed and trapped in the plaster and/or the underlying masonry during use of the fireplace in the past. Sadly it is sometimes necessary to remove all the affected plaster before applying a new coating of a lime based plaster. Before doing that though you may want to try treating the damp affected areas with a salt neutraliser and then making any essential repairs. There are any number of proprietary products available from hardware stores for this use many of which are reasonably inexpensive, but it has been recently suggested in this forum that a solution of white vinegar, water and a few drops of dish soap can also work. The latter is dirt cheap and has a variety of alternate uses, as I can personally attest, but have never used to treat your issue. If nothing else the vinegar solution is great for killing the weeds around pavers and also for cleaning glass and some metal ware. May be worth a try.

 

[Stevers]

The failed gypsum plaster (over a cement render) on my basement chimney breast looked exactly like that. Removing back to the brick/stone and replacing with NHL render and plaster has sorted it. A modest area like that is well within the scope of DIY, and a skim of old fashioned Polyfilla (basically a chalk powder with binder) can be used to make anyone the plasterer they aren't. The NHL render/plaster will draw the moisture out and that may result in salting that can be brushed off. Once it's dry enough the salting will stop as there won't be enough moisture for the salts to be transported in solution. Breathable paints will be a must, but not being a purist I use Contract Matt Emulsion (cheap) or F&B Estate Emulsion or equivalent (non-vinyl) Flat Matt Emulsion (lovely colours) - in my experience no suitable finish will be washable in any normal sense - it's just the price you pay.

 

[Lacan07]

Its likely salts (and perhaps remnant of soot) rising to the surface. In humid conditions it will suck up the moisture.

Nothing to worry about but ideally you'd have the wall lime plastered with cow sh*t incorporated into the base coat.