103V: damp surveys limits under ground shower

Lower ground-floor section of a heritage property with damp in an under ground shower. Buyers want to tank it, but is that the sensible approach, given that it is already damp proofed?

Surveyor Tips:

  • Some damp issues are so small that they are best dealt with through annual maintenance.
    • But make sure clients understand that and why it is the best approach to damp proofing.

Root cause

Condensation and penetrating damp.



Here are some comparative images taken at the time of the original survey in February 2020, and recently.

There was a large quantity of visible calcium sulphate salts in 2020. Calcium sulphate is a key ingredient in cement and other building materials. If diluted in water, salts tend to move to the surface and crystalise. These can be removed with sandpaper and decorated. Calcium sulphate salt crystals are most commonly associated with vapour and condensation.


You followed my main recommendations and made significant improvements.

I believe the salt crystals that recently reappeared are because of the low temperatures compounded by residual moisture trapped under the paint. I suspect that part of the problem is the paint itself, I should have recommended anti-condensation paint last year.


The surface of bathroom walls were marginally damp in 2020.


By contrast the damp meter readings were significantly lower by 2021.

I tested the surface of a sample of internal walls with a Protimeter damp meter in conductance mode. These meters measure electrical conductance of salts in water, a proxy for damp. Readings below 20WME are considered dry. The range is 8WME to 99WME. See surveyor.tips/dampmeter. The surface of all walls measured were dry in 2021.


In 2020 there were crystallised salts on the surface across much of the bathroom ceiling.

It is notable that none of the dampness is accompanied by tea bag like stains, which would normally be the case if there was penetrating damp. 


Part of the risk comes from thermal bridging, that is areas of heat loss. Blue is about 5°C colder than yellow.

Thermal bridging can cause localise condensation under the surface of the paint, causing salts to blister the paint. Heat loss can also cause interstitial condensation. That is condensation that take place within the building material.


In 2020 there were salts on the inner corridor.

There was a vast improvement within one year, at minimal cost. I would estimate a reduction in surface salts of 99%. I doubt structural damp proofing would have been as successful, see surveyor.tips/damp-treatment-fails/, roughly 80% of damp properties in London have misdiagnosed damp issues and failed damp proofing treatment.


The property has been damp-proofed, as evident by the horizontal disruption up to plaster about 1.3M.

While replacing original absorbent plaster with damp proofers impermeable slurry can improve the appearance of treated walls; 1) typically it needs replacing every 10 years unless the root cause is dealt with, i.e. excess vapour in this case, and 2) it reduces the absorption of vapour and therefore the buffering effect of normal absorbent plaster.


There was extensive mould in the bedroom in 2020 but not in 2021.

Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from insufficient ventilation, poor air circulation and a cold surface. See surveyor.tips/mould. Mould is inhibited by nitrates found in soil, picked up by groundwater or rainwater passing through soil. The presence of mould eliminates any water passing through soil as the root cause and points toward condensation from excess vapour.


The secondary cause of high relative humidity is low temperature relative to the source of humidity.


Heat loss is the reason for condensation and mould under the lintels in the bedroom.

It would’ve been better to have installed some form of insolation. However, a simple solution is to use anti-condensation paint such as DryZone by Safeguard supplied by the former head of the Property Care Association, PCA. I successfully use it in my bathroom.


There is still evidence of salts crystallising under the lintels in 2021.

However the level of crystallisation is considerably less than it was in 2020. Anti-condensation paint is much thicker than normal paint with greater volume to absorb vapour. This makes it far less likely that salts will crystallise underneath the paint.


I left a datalogger. It doesn’t measure surface relative humidity. Surface RH is typically 7% higher for 1°C less.

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