Damp on internal granite stone wall

8 Nov 2012
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United Kingdom

I have recently moved in to a 150 year old granite stone terraced cottage and have been advised by a damp specialist that I have rising damp on an internal solid granite wall.

The wall is 60cm thick and made with solid granite. It used to be the external wall but an extension was built 30 years ago, thereby making it an internal wall with the living room on one side and kitchen on the other. Th wall is currently plastered and painted. There are no current signs of damp on the walls and they are not wet to touch but the damp readings were high on both sides of the wall. The living room has never been heated well, the fireplace was closed off and the kitchen does not have a suitably sized radiator. The property is fully double glazed.

The damp specialist advised that I have rising damp as the wrong plaster has been applied to the wall. He advised the plaster draws in condensation which then migrates into the wall. He has advised I have a chemical DPC (injected) and then two applications of tanking slurry in order to resolve the problem.

As the wall is internal, and has been for 30 years, I am unsure whether this is the correct diagnosis so wondered if anybody might be able to confirm:

1) Is it possible for plaster on an internal wall to draw moisture out of the air and cause "rising damp" in solid stone walls?

2) If i remove the plaster, air the wall and heat the room properly over winter is it likely the damp levels might be reduced?

3) If the diagnosis is correct, is an injected DPC and tanked slurry appropriate for solid stone walls?

Any insight you can offer will be hugely appreciated.

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Let me first start by explaining how you diagnose rising damp... You need to positively identify 3 things...

1. Is there moisture present at depth in the masonry? The only way of checking this on site is take a drilled sample of granite and carry out a calcium carbide test. Or you might have a gravimetric test done at a lab.
2. Is there a moisture profile consistent with rising damp. E.g, wetter at the base of the wall but decreasing gradually with height. Again, you would have to take several calcium carbide tests to establish the moisture profile.
3. Was a salts analysis test carried out which identified nitrate salts present in the apex of the damp profile?

If the surveyor failed to carry out any of these tests then he doesn't know what he is talking about. I won't wait for your response because I'm comfortable even now that we've already identified that he made a diagnosis based on guesswork.

Now to the specific problems relating to a 600mm thick granite wall...
Due to the age of the property it was probably constructed using a lime mortar. There are two facts to consider here; lime mortar has a large pore structure and a low mechanical strength when compared to modern Portland cement. This is good for two reasons:
1. Good porosity = good vapour diffusion (the wall dries quicker)
2. Low mechanical strength is good because strong mortar restrains the granite rather than accommodating the movement. This can lead to stress fractures within the granite.
The wall should only ever be repointed in a lime mortar or a weak 9:1 mix of sand and cement. Mechanically, the latter is more suited but I'd go with the lime mortar for its improved vapour permeability.
Any inappropriate pointing up or inappropriate plasterwork may well trap damp in the wall. Plasterwork and pointing should be in Lime, anything else should be removed. If you so have a rising damp problem then it's due to then it's not likely to be in the granite. The pore structure is too dense to accommodate a rising damp complex but it could rise up the walls through the mortar perps and this is why wall ventilation is so important. The wall is likely constructed on nothing more than rammed earth and obviously has no dpc present but neither point should cause concern, I only make the point so you can see there there is a direct pathway for moisture transfer from ground up through the mortar perps.
Unfortunately the fact that there is now an extension on the other side of the wall means that you could have reduced wall ventilation by 50%. My view is that the wall is no longer exposed to the elements so this loss of ventilation should be offset by reduced exposure to the elements.
Your surveyor is bending a known fact to his advantage with regard to the plaster. Where walls have suffered from rising damp the salts migrate to the wall surface and contaminate the plaster. These are hygroscopic salts and as such absorb moisture from the atmosphere back into the wall setting up a vicious cycle. Salt contaminated plaster needs replacing but then unless he carried out a salts analysis test he wouldn't know whether or not the plaster was contaminated.
Removing the plaster and allowing the wall to dry for 6 months will improve the situation markedly, in fact the wall would be best left exposed as a feature wall. Is this possible?
DPC injection and a tanked slurry is completely inappropriate for this situation, don't waste your money! Hope this helps. Joe
Many thanks Joe - a really concise and easy to understand response.

No salt test was done. From memory, the damp readings were high from top to bottom of the wall so i suspect your diagnosis is correct.

We've just had a radiator installed into the room and hope, within the next month, to have a log burner installed too. Would it be foolish to see how heating the room over winter affects the damp readings (i'll buy a decent moisture reader) before hacking off the plaster from the wall?

Either way, you'll allayed a lot of fear so thank you so much!
does anyone drape wet washing around the house?
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Hi JohnD

I think the previous owner(s) used to hang wasing around the house. There's evidence of condensation spots on some walls in the house which have most probably been caused by drying clothes. Both structural surveyor and damp surveyor advised it was also likely.

We've only been in the property for 5 weeks and i'm a stickler for trying to avoid drying anything indoors. We've got UPVC double glazing and the fireplace was blocked and unvented (now remedied) so there's not been sufficient air flow through the property either.

I'd give it as few months of normal heating and ventilation before re-assessing it.

I wanted to give an update to the situation now that a few months have passed since the first diagnosis of rising damp. The room has now been heated for two months. I've bought a moisture reader and have removed some sections of plaster and mortar in order to investigate further. The following is now the situation:

1) Damp meter readings are high at the base of the wall and decrease to an acceptable level around 1 - 1.50 metres up the wall. There is no visible evidence of 'rising damp'.

2) The granite/rubble/mud wall is covered in a light, slightly crumbly mortar (maybe lime?) ranging from 1 - 2 inches thick. Covering the mortar is one application of lime plaster, followed by numberous other layers of gypsum plaster.

3) The granite/mud/rubble wall is evidently damp beneath the mortar and plaster. The mud between the granite/rubble is damp to touch. I found an old, small piece of damp wood between two pieces of stone which crumbled as soon as i touched it.

4) The floor is a concrete base covered in an old tanking agent. It appears to be largely ok but there are a number of cracks with moisture coming through. The floor was covered with a new damp proof membrane and laminate flooring.

I have talked with the chartered surveyor who performed my structural survey (not the surveyor from the damp treatment company) prior to moving in to the property. He advised a chemical DPC would not be appropriate due to the structure of the wall but that tanking and replastering the walls would be a suitable modern means of treating the issue. Both sides of the wall would need treating.

Is anybody able to shed any light on whether I may have a serious damp issue which could be resolved through tanking and replastering or merely walls which could be stripped back to the granite/rubble/mud wall and left to breath for a period of time? Could the tanked floor be responsible for moving the damp issue to the walls?

Please let me know if you'd like any photos of additional info.

Thanks in advance!

Ignore the recom. to inject a chemical DPC.

Remove skirting and any other wood trim.knock off,
back to masonry, on both sides of the wall to a height of 1m.

Render in two coats ( 3:1 sand and lime ) and a remedial skim finish. Stop just above the FFL. This will/should last 20 yrs before renewal might be req'd.

Remove any wood ie. wood plugs, from the random rubble wall.After removing some render and noting the condition of the surfaces, would it be reasonable to leave the stone exposed?

If your solid floor doesn't have a membrane under the concrete then you enter a whole new ball park of work.

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