102S: damaged from damp proofing

Damp proofed tenanted garden flat with mould and condensation everywhere, including tiles falling off the wall.

We had been having numerous damp problems at a rental property and wanted to discover the root of the problems in order to address them so we booked a report with Damp Surveys Ltd. We were staggered at the level of detail in Simon’s report, which left no stone unturned. Simon was a delight to deal with and we found the cost surprisingly reasonable, particularly once we received the very lengthy report with numerous photos and suggestions. We wouldn’t hesitate to recommend Simon to anyone considering looking for expert advice on all damp issues.

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

Root causes

The root cause vapour resulting in condensation and mould, made worse by damp proofing treatment. Penetrating damp potentially made worse by damp proofing treatment.

Surveyor Tips

  1. There is often a ridge where damp proofers have replaced normal absorbent plaster with impermeable damp proofer’s slurry. You often find high damp meter readings in radio frequency mode just below the ridge, and in both conductance and radio frequency mode on and about 10cm above the ridge.
  2. This is cause by condensation becoming trapped between the permeable and impermeable layers – a sign that;
    • there is ongoing damp and that rising damp was almost certainly never the problem,
    • condensation was problem the root cause of damp.
  3. The solution are:
    • Remove the root cause and allow the wall to dry out.
    • Once dry the plaster can probably be sanded down and decorated.
    • Since the wall is no longer homogenous, i.e. there is difference in absorption rates, consider removing the plaster to ceiling and dry-lining if internal using a solvent based adhesive (in case there are hygroscopic salts), or applying thermal insulating plasterboard if an external wall, to reduce the risk of condensation through heat loss.


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The concern that initiated your instructing a damp survey, was loose tiles in the kitchen.

From careful examination I’ve come to the conclusion that the loose tiles result from the poor tiling, not from dampness, at least not current moisture.

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I tested the wall with a Protimeter damp meter in two different modes.

I tested the surface on 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.

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The alternative method uses radio waves to detect density, including moisture, about 7cm deep within the wall.

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. 

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Vapour and cold walls could be aggravating factors. There is no externally ducted kitchen extractor fan.

Ventilation is most effective when air is extracted close to the vapour source; bathroom, kitchen, drying clothes and occupied rooms. The internal ventilation does not meet Building Regulation 2010 Part F requirements. This is best achieved with mechanical extractor fans. 

  1. Bathroom 15 l/s with a 30-minute overrun.
  2. Kitchen 30 l/s adjacent to hob; or 60 l/s elsewhere in kitchen.
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The bathroom extractor fan was sealed up by the tenant, concerned about the noise when windy outside.

With the tenant’s agreement and understanding, I remove the tape and measured the extraction rate. The fan meets the building regulation requirements or 15 l/s. However it doesn’t stay on after lights are switched off. Given the challenges and risks consider a continuous flow fan, such as the Elta Mori, see recommendations.

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Close to the bathroom, there is an under-stair cupboard, with mould growing across the external flank wall.

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. The secondary cause of high relative humidity is low temperature relative to the source of humidity, see surveyor.tips/humidity.

Mould is inhibited by nitrates found in groundwater. The presence of mould at the bottom of a wall eliminates rising damp as the root cause and points toward condensation from unvented excess vapour.

There was a high damp meter reading, consistent with condensation.

There are also slightly brown / yellow marks (more easily seen on the previous image), these stains are a sign of penetrating damp, that is from rainfall.

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Outside we see paint peeling from the wall. This causes moisture to penetrate, while reducing evaporation.

Note vertical drill holes above the word “from”. These drill holes would have resulted from chemical damp proofing treatment. It is difficult to assess the level of damage caused by the damp proofing treatment.

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Looking further up from the neighbour’s side, we can see some cracks to the render

Render cracks can draw a surprising amount of water into a wall, through “capillary” action. These cracks should be raked out and filled, either with acrylic caulk for small cracks, or a cement-based render similar to the current render for large cracks, and painted.

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Some of the render at the base of the wall appears to have been damaged by damp proofers.

The drill marks for injecting chemicals. The render has been damaged, risking rainwater penetrating. The neighbour’s refuse bags splash rain onto the wall and reduce evaporation. The damp proofer’s slurry inside hides penetrating damp increasing the risk of rot in the sub-floor, the greatest concern with misdiagnosed rising damp treatment. The sub-floor humidity was found to be 55%RH, which is dry, so no likelihood of rot. A wet brick has half the insulating properties of a dry brick, increasing the risk of condensation.

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The corridor immediately inside, displays the typical effects of misdiagnosed rising damp and slurry treatment.

The line of damage is along the top of damp proofer impermeable slurry. The remainder of the wall is made of the original absorbent plaster. Vapour becomes trapped between the interface between the old absorbent plaster and new impermeable layer. Moisture can’t evaporate through the impermeable slurry, so it concentrates at the base of the absorbent plaster. It illustrates that the root cause of dampness is insufficient ventilation and heat loss.

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There is the similar effect closer to the cold external door.

Here we can see vapour becoming trapped behind a coat of gloss paint.

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In the front bay, mould is growing on the carpet. There is evidence of condensation in the bay.

The tenant collects moisture in a little dehumidifier. Consider providing a larger standard size humidifier.

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There is some significant plaster damage to the right side of the front bay (as seen from the street).

Without damaging the wallpaper it was impossible to determine if there was discolouration coming through from penetrating damp. Therefore it was difficult to estimate the cost of rectification without damaging the wall paper. 

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The wall was damp, but not as damp as it might be imagined given the apparent damage.

I often find that damaged plaster looks worse than it is. That said there is a likelihood of needing a skim coat of plaster. Allow the wall time to dry out.

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Although condensation is probably a factor, it is possible that there is some penetrating damp from rainwater.

The windows have been replaced with double glazing (without trickle vents), the sides have slightly come away from the render and ideally should have new mastic sealant.

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Looking up we can see algal stain from rainwater escaping from a gutter, or downpipe. This could be old.

Looking carefully at the render we can see a large vertical crack in the centre of this image. Rainwater can be drawn in and run down the inside of the render, only to appear anywhere on the internal wall below.

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There is some exposed timber and a crack around the top of the gutter.

The timber doesn’t look too rotten. However you should look and film the rainwater goods  during a rainstorm.

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Looking from above, see can see a slipped tile that should be refitted.

There are no signs on the ceiling of dampness, suggesting that the tile is not causing internal dampness.

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There are also render cracks lower down on the wall that should be raked out and filled as previously reported.
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And similarly to the rear of the property.

Rising damp risk assessment

Elevation is: 29M above sea level.                 The flood risk is: no risk.

Sub-soil rocks are: Near low productive aquifer with intergranular flow. Superficial deposits: Taplow Gravel Member – Sand and gravel. Bedrock geology: London Clay Formation – Clay and silt. .
Signs of groundwater: 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

Wall damaged by rising damp proofers, risks penetrating damp and condensation through heat loss.

The building has been damp proofed against rising damp, the dots are a sign of chemical damp proofing, hear what Dr Robyn Pender of Historic England says about it.

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 and trapped vapour. The solution is through ventilation, heat and other forms of humidity control.

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