Damp Inspection/Brick Salts Recommendations, East Midlands

[PLadsun]

I have a late-C18 (I think) terraced cottage, of solid single-brick construction, solid floor downstairs and a tiled roof with a very small roof void. It has been let for some years after a fairly involved renovation - the current tenants are not ventilating the property adequately nor keeping the house consistently warm, and a result, it’s fairly humid and black mould has appeared in the bedroom. I reluctantly agreed to a damp survey via the managing agent, and the results were exactly as expected (lower the ground level outside the entirely unaffected single-storey extension, strip walls of all plaster, another injected DPC, replaster with fleece-lined waterproof boarding etc).

After a lot of toing and froing about heating and opening windows (the damp report did at least blame the mould on condensation) and the use of a large dehumidifier, the mould appears to be in retreat - however, there are still some areas of concern. The most problematic of these is the chimney breast, which has been an issue since the refurbishment - there are now large fluffy salt deposits all around the downstairs fireplace, and I’m not sure how to solve it. It’s gypsum plaster over red brick, the chimney is cowled and isn’t used, roof and lead work is sound - but the adjoining property isn’t part of the original terrace and appears to have its own wall built up to mine, rather than shared, so there is potential for moisture to be there.

I would expect the salt deposits to become less obvious once the house has dried out somewhat, but they’ve been an ongoing issue. Can anyone suggest a course of action, or someone knowledgable in the East Midlands area to inspect?

 

[Stevers]

Gypsum plaster over a salt contaminated chimney breast (from combustion) is likely to be the issue. The problem with gypsum is that with salt contamination it can become hygroscopic (will attract moisture).

I had exactly this problem with a 'damp proofed' basement chimney breast. The render was disrupted and the galvanised plaster beads had rusted. Any steel that was placed on the mantle of the stone fireplace surround would also rust. The gypsum, cement and most of the cement slurry coat was cleaned off, the brickwork repaired and the whole replastered in NHL render/plaster. After an expected initial flowering of salts, it has now settled down to appear completely dry, functioning as something of a lung/safety valve to the rest of the wall which remains tanked and gypsum plastered. Nothing we put on the mantle of that replaced fire surround comes to any harm now. Thinking back to how horrid it was, I really couldn't be happier with the way it's turned out!

 

[aockenden]

Gypsum plaster over a salt contaminated chimney breast (from combustion) is likely to be the issue. The problem with gypsum is that with salt contamination it can become hygroscopic (will attract moisture).

I had exactly this problem with a 'damp proofed' basement chimney breast. The render was disrupted and the galvanised plaster beads had rusted. Any steel that was placed on the mantle of the stone fireplace surround would also rust. The gypsum, cement and most of the cement slurry coat was cleaned off, the brickwork repaired and the whole replastered in NHL render/plaster. After an expected initial flowering of salts, it has now settled down to appear completely dry, functioning as something of a lung/safety valve to the rest of the wall which remains tanked and gypsum plastered. Nothing we put on the mantle of that replaced fire surround comes to any harm now. Thinking back to how horrid it was, I really couldn't be happier with the way it's turned out!

How long did that initial period of flowering salts last after the lime redo? I will need to do the same to mine at some point and interested how long I'll have to wait.

 

[PLadsun]

Thanks, the gypsum plaster dates to at least the mid-‘60s and was redone because it was in poor condition, rather than because of any damp or efflorescence. I was hoping to get away without stripping it all and replacing with lime given it’s occupied.

Are the salt neutralising solutions a waste of time?

 

[Stevers]

How long did that initial period of flowering salts last

From memory the worst of it was over after about a month. Since it just brushes off without damaging the finishes it's more of a nuisance than any sort of problem.