Another Damp Patch

[Roger111]

Hi All

Me again seeking the usual superb advice I see on here. Grade 2 listed 3 story mid terrace, 1840's, welsh slate.. I have a chimney breast that runs down what was the back wall of house, but is now in the middle of an outrigger ie two story extension tacked onto back of house in 1890 ish.

In the ground floor kitchen I have a damp patch that wnt dry after 4 weeks since reskimmed in gypsum. When I bought house all kitchen in back was gypsum having been done at least 40 years ago and this needed a skim. Chimney above all way to second floor roof is bone dry, wall below is bone dry. I took off air vent where fireplace was, bone dry, even dust soot at bottom so no water getting in there.

Im just confused why i have this single 3ft by 4 ft damp patch?

Any advice greatfully recieved

Best

[Feltwell]

Pictures always help, of the surroundings.

Obvious question, no pipes anywhere in the vicinity?

Anything that could make this patch cold compared to surrounding dry areas? I'm thinking cold that will make condensation form on it's surface.

[Roger111]

Ill get some picks up.

No pipes that i know about. Ive had heating off for a while but rest of plaster has dried? can imagine why it could be cold as it has same exposure as rest of wall. I cant see it being due to water down chimney as everything else is dry above and below?

Thanks

[a twig]

Could be tracking in from higher up, I had an annoying damp patch and it turned out the failed pointing was about 6ft up and 3ft across, and the damp was just where it was finally making its way to the surface

Failing that any weird finishes back there? Stain blocking paint been used in the past etc?

[Roger111]

AH

All good points. Ill take another look

Thanks again

[Surveyorman]

Chimney breasts are always high risk areas that suffer from retained moisture in one form or another due to their solid mass construction and rarely have an effective damp proof course within them (although given the age of your property an original damp proof course would most likely have not been present in the first instance.
It is however very important to be aware that chimney breasts will inevitably be contaminated by aggressive ammonium salts which are a by-product of the historic burning of fossil fuels and are commonly found in chimney flues and chimney breasts.
Nitrate and chloride salts are also found where ammonium salts are present.
Ammonium salts are generally hygroscopic (having the ability to attract moisture from the atmosphere) - This very often accounts for mystery damp patches that appear (and then disappear) especially at times where humidity levels are high within the property. I witness this frequently and is always around the same time of year (autumn/winter).

As an aside: Under no circumstances should gypsum based plasters be used as backing coats as these are notorious for their moisture and salt retention capabilities.

[Roger111]

Hi

I can confirm its gypsum. Lime was ripped off many moons ago im told. Worth removing and redoing in line for problem area? Or alternatively would thick stain guard work?

Best

[MatthewC]

I can confirm its gypsum. Lime was ripped off many moons ago im told. Worth removing and redoing in line for problem area? Or alternatively would thick stain guard work?

Yes, lime is what I would do, after stripping the gypsum and leaving it for a bit to start drying out. I bet the gypsum feels cold to touch, as it has water behind it. I would guess stain guard is aimed at soot more than a simple damp patch - anything modern is only going to address the symptom (a damp patch) as opposed to the cause (the retention of moisture which has found its way in).

[Roger111]

Thanks

SInce Keeping the heating on and allowing to dry out the issue seems to have gone. Ill keep an eye on this. Rest of house is lime plaster and seems ok. Thanks all for the advice!