102V: PCA contractor misdiagnosed rising damp, dry-cellar

The vendor was outraged to be asked to reduce the sales price by £6,000 following a damp proofers report.

RICS surveyor reported: “Due to the external render bridging the damp proof course and that the property is in an area with naturally high groundwater, the risk of rising groundwater damp by means of capillary action via the bridging components exists. in this event, it would probably be the best course of action to commission a PCA-approved damp contractor to specify a repair specification.

The RICS surveyor’s rising damp diagnosis, with no evidence other than a small semi-circular patch of dampness by the entrance door.

Tapco, a contractor specialising in rising damp proofing, quoted for work; “High levels of moisture consistent with rising damp detected

High levels of moisture exceeding 20% were recorded to random lower areas of the ground floor walls (Shown on our floorplan). The pattern of these readings are consistent with rising damp, which in our opinion is being caused by the existing damp-proof course having partially broken down or being bridged. This is allowing moisture from the ground to be transmitting through the interconnecting pores of the wall by capillary action.

Rising damp is identifiable as having high levels of dampness at the base of the wall, with a sharp cut off at the top, where it becomes immediately dry. Visible signs were also evident in the form of water staining. For your information, masonry with an average pore size of 0.01mm gives a theoretical height rise of 1.5 metres above the source of moisture (the ground), therefore when damp above this height is found, it is deemed to be from another source.”

The quote appears to be about £10,000 although, it isn’t clear.

Root causes

The root cause vapour causing condensation, through lack of bathroom extractor fan etc.

Surveyor Tips


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The main area of concern is the rear bedroom wall. There is a dusting of mould on the wall, by the passive vent.

Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from insufficient ventilation, poor air circulation and a cold surface. Mould is inhibited by nitrates found in groundwater, eliminating rising damp; surveyor.tips/mould.

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I tested the walls near to the foundations and they were completely dry (which is unusual in cellars).

This further eliminates rising damp, as groundwater rises from under the building.

I imagine the walls were dryer than in previous surveys, following use of a dehumidifier. However, I find it completely unacceptable that rising damp contractors, such as Tapco don’t check cellars, or comment on “failed” past chemical treatment (other than the generic statement; “We were not made aware of any previous guaranteed damp treatment”). 

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I also measured the subfloor humidity.

49% RH is relatively low humidity for cellar. Another sign that the property is not and never has suffered from rising damp.

I also doubt there is a leak from the drains otherwise the cellar would be humid.

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I’ll check timber and found no significant signs of rot or woodworm.

As a matter of good maintenance, you should remove any item blocking the free flow of air in the subfloor void. So I recommend moving the wooden box which I believe was full of earth, spare tiles and anything the else that obstructs airflow.

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The damp meter reading was low along the bedroom walls.

I tested the surface on the inside at the base of all external walls every metre, chimney breasts and 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. Walls measured were largely dry on the surface except where mentioned in this report.

Most external walls in the flat have dry lining.

Dry lining involves separating the external wall from the internal plaster. It is rare that these walls are damp on the surface, kif it is it would not be because of rising damp.

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I recommended that you placed a datalogger close to the damp wall.

The datalogger was place a bit too close to the radiator, so we moved it into the corner. 


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One simple solution to the ventilation challenges is to open windows on the safety lock.

Windows can be locked safely in position, yet still provide sufficient air movement. Unlike passive vents, windows are less likely to suffer from mould, and if they do it is easy to wipe away without noticing it, as part of the normal cleaning process.

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I tested all external walls. There was no dampness on any of the external walls at the time of my survey.

I also tested walls in radio frequency mode. Water reflects radio waves at a set frequency similar to mobile phone shields. Meters can’t differentiate moisture from other dense matter such as metal and concrete. They help trace damp in a normal, homogeneous wall.

Readings below 300 REL indicate that a wall is dry below the surface, 999 REL is the limit. These meters are for scanning, mapping and profiling, see surveyor.tips/profile. 

The only high damp meter reading in the property is on the corridor by the back door.

This wall it’s not dry lined and therefore in contact with the cold external wall, increasing the is risk of condensation. It can take 10 months for a saturated damp wall to dry out, even with good ventilation. However, there is very little moisture left, this will reduce dry time.

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A short distance away from the back wall, the meter reading is low.

If this wall was genuinely suffering from rising damp then the meter reading would be consistently high across the wall, like the moisture on a wall above flood water.

Other matters

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Water flows onto the wall from the shower tap whenever it is turned on.

This should be easy for a plumber to fix and cost little more than the callout charge. I understand the bathroom is being updated anyway. The floor appears to be a solid floor, therefore there is no risk of timber rotting below it.

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I noted flowing from the top floor flat boiler overflow pipe. The owner should repair it.

Rising damp risk assessment

Elevation is: 40M above sea level.                 The flood risk is: no risk.

Sub-soil rocks are: Near low productive aquifer with intergranular flow. Bedrock geology: between Lambeth Group – Clay, silt and sand, and Harwich Formation – Sand and gravel. .
Signs of groundwater: I can see 1M+ below ground level, there are no signs of groundwater.

Therefore the risk of rising damp is a remote possibility, see a good explanation by Dr Robyn Pender of Historic England  https://youtu.be/Jo8oF9ubvtI

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The vertical horizontal holes around all external  walls shows, the flat has already been treated for rising damp.

Despite the low risk of rising damp, there has been an attempt at chemical treatment against rising damp. Rising damp treatment is normally a sign of insufficient ventilation. The walls have been drilled into by about 200mm and injected with damp proofing chemicals. In theory the chemicals are absorbed into the brick pores reducing the bricks ability to absorb moisture. Walls are replaced with damp proofers slurry. This can cause problems with absorption imbalance, resulting in increased condensation on other walls or vapour becoming absorbed and trapped behind the slurry. There was evidence of surface condensation in areas probably not treated with replacement plaster but no evidence of trapped vapour. The solution is through ventilation and humidity control.

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