Where we survey and repair many chimneys we come across lots of different situations and hear so many different stories of mis-diagnosed repairs.

This one just shows how confusing it can be for the layman……….Old listed building which has had many damp issues over the years, and it would seem the damp diagnosis is poor to say the least. Anyway the owner of this building contacted me because they have water running down the walls when it rains, but not all the time?! The chimneys has been re-built by a local builder after he diagnosed penetrating dampness, and said a chimney re-build would fix it.

During the works and after the works the same problem continued just like it used to, unfortunately he didn’t want to help anymore as he had been paid and he’s off to the next chimney repair.

During my initial phone call it was discussed how when diagnosing dampness issues it’s like a jigsaw puzzle fit all the pieces together and voila there you have it…….the most important piece of the jigsaw puzzle is the testing of the samples of plaster / mortar, this is done following the laboratory methodology in BRE Digest 245. This basically means the plaster sample is analysed in our laboratory for how much moisture present, if there is any in the sample, and if there are any hygroscopic salts present and the amount, and type.

The survey

The property was occupied and is a small school with many little children all producing lot’s of moisture. Ventilation of the property isn’t ideal, and the relative humidity at the time of inspection was 75% @ 19.5 ºc.

The chimney wall is brick with a painted finish along with the front wall where there are also very similar visual issues, the wall at lower level was panelled potentially masking any dampness issues.

The visual appearance is patchy stains and on close inspection it appears that there is visible damp. The weather was dry during the inspection in also for a number of days before the inspection.


By inspecting the wall with our calibrated Flir T600 thermal imaging camera we can check for any hidden anomalies and also calculated dew points for condensation, to see if this is a possible cause. This is done by meterlink with our Flir MR77, which measures the internal atmosphere. Condensation wasn’t occurring and all surfaces were significantly above dew point……no risk at that specific time.


Where we specialise in these issues the cause is apparent to me, but we still have to go thorough the process of elimination to ensure correct diagnosis.

The problem is hygroscopic/ deliquescent salts derived from long term burning of fossil fuels in the chimney; these salts have migrated from the chimney to the adjacent areas. Hygroscopic salts absorb water from the air; a good example is that of sodium chloride, common salt, which under humid conditions crystals become ‘sticky ‘clump together. However, there is an extreme of hygroscopic-deliquescent-where the salts absorb so much water that affectively they dissolve in themselves and thus we have a solution. The point at which deliquescent occurs depends on the particular salt. For example common salt is hygroscopic at 75% relative humidity but increased this to 85% plus and keep it there then it becomes deliquescent and therefore liquid. Once in liquid form the salts become mobile and can move away from their point of origin.

Difference salts become deliquescent at different humidities. Some like calcium chloride and many of the calcium salts deliquesce at very low humidities: others such as sodium chloride become deliquescent at much higher humidities. Whatever the case there are occasionally conditions where the wall or material becomes physically damp solely due to the contamination by such salts.

This shows why the wall in question was presumed to be wet because of penetrating damp when it rains, however before it rains it can become quite muggy and this often slight adjustment  (higher) in relative humidity is enough to tip the balance.

The below shows a time lapse of hygroscopic salts appearing.


Posted by Complete Preservation


  1. Very interesting.
    I have some very serious damp issues at our rental property. We have had a fair few damp specialist contractors and roofers, neither can find the cause and offer a solution. Could you possibly help us? The property in Frome Somerset.
    Many thanks Nige


    1. Hi,
      This is something we can give you advice on. Please contact me on 01225 769215, we can then discuss the issues and the possible survey.


  2. Hello Complete Preservation.
    I have a problem that I have had specialists try and repair, this is costing me a fortune which I can’t afford. I have found 3 of your posts on chimney damp and they all are slightly different issues, but I like your approach and demonstration of each. Please can I have a phone consultation, and then book a survey please?


    1. Hi Dawn, chimney damp can be very complex as there can be a number of issues. It is impossible to tell exactly what is going on without taking samples, but people still charge for surveys without really knowing the cause. I’d be happy to speak to you regarding your issue 01225 769215


  3. Oliver Foster June 21, 2018 at 15:57

    Hi, I read this article with great interest as I have been trying in vain to remedy a problem in my 1830ish stone built terrace cottage.

    There was a blocked up chimney in the fireplace which has now had a cowl added however the stone floor surrounding the chimney and the base of the walls continue to get very damp in wet weather.

    I have had the fireplace opened up and the chimney is now open straight through.

    The original base of the fire and the surrounding fireplace and chimney appear dry if very sooty and with a build up in place of salts.

    Please could I ask, would removing the plaster and cement backing to the chimney breast wall potentially help stop the trapping and bridging effect which seems to occur between the chimney breast and the stone floor?

    The stone floor is original.


    1. Hi Oliver, sorry for the delay. I can’t really give advice on something like this without carrying out a survey, and sampling….sorry


  4. Oliver Foster July 9, 2018 at 14:27

    No problem, thanks for the reply anyway and for the advice on the site


  5. HI. I also have a damp problem on my internal living room room where an old coal fire used to be. The fire was removed and boarded over and plastered. An air vent was put in to try an illeviate any damp issues. This did not work! I now have a tide mark of damp from the skirting board and around that whole area with the paint becoming flakey with a powdery effect. Im very worried this is rising damp! I am in Northumberland so too far for you to travel. Can you give me any help at all? Thank you.


    1. Hi Jane, please send me your number by email to enquiries@completepreservation.co.uk and we can discuss


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