102Q: suspect penetrating damp – first & second floor flat

Landlord concerned that there was penetrating damp coming through gaps in render.

Root causes

The root cause was vapour through insufficient ventilation, causing condensation and mould.

Surveyor Tips:

  1. No signs of penetrating damp, would see external cracks and internal tea bag stain like discolouration.
  2. Hygroscopic salts in top floor chimney breast.
  3. Fascinating example of old chainsaw retrofitted DPC – the work of Peter McDonald I believe.


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There is dampness in two corners in the rear bedroom. Colourless semi-circular damp is classic condensation.

There is no tea stain like discolouration, which is the tell-tale sign of penetrating damp, therefore penetrating damp is unlikely. Semi-circular dampness at a wall base is consistent with condensation. Corners lose heat rapidly, as there is greater external surface for heat loss. 

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Looking outside of the corner there appears to be metal beading used to form straight render corners.

A small section of paint has flaked off, I don’t believe the water is penetrating, but it should be repainted, because the paint will retain some dampness resulting additional heat loss. And in theory there could be a render crack behind the flanking paint.

The relative heat loss at the time of the survey was only about 3°C.

However, when the property is being lived in, there is likely to be a greater relative heat loss, especially behind furniture.

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A metal detector identified some metal deep in the wall. This could be the remains of electrical wiring or piping.

Metal courses rapid heat loss and could account for the water damage to the plaster.

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Using a damp meter in radio-frequency mode I was able to identify damp deep in the wall at the base.

I tested walls with a Protimeter 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.

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By contrast the top of the wall is dry, both on the surface and at depth.

This tells me that the wall is dry enough to replaster.

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I tested the timber to make sure that there was no rot in the skirting boards.

Although the timber was slightly humid it was less than the 28 WME necessary for rot to start growing.


You left the datalogger, to measure the relative humidity fluctuations overtime.
Graph from the data-logger

The data doesn’t tell us a great deal because the property has been empty with limited heating. I will email an Excel spreadsheet which you can add data to visualise changes over time, once the property is tenanted, see section later on monitoring humidity.


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The bathroom vent is extracting air at 6.2 l/s, with no overrun.

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. 

 See surveyor.tips/vent_regs specifically P39 and P19:

  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.

I recommend to landlords that they install a continuous flow fan see recommendations, as I have done myself.

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There is mould growing in the kitchen, caused by insufficient ventilation and poor airflow into cold corners.

Consider adding anti-condensation paint or  insulation into this difficult to heat corner.


Flaking paintwork, could line-up with a crack in the render, resulting in ingress. But I doubt it.

Flaking masonry paint should be removed and repainted. Any cracked render should be raked out and filled. I tend to fill small cracks with external acrylic caulk, and larger cracks with a render similar to your current render, which I believe is cement-based. 

There is a crack to the windowsill and cracks in the render above and below the window.

These cracks should be filled, but I don’t believe that they are the causing any internal dampness.

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The chimney breast has some dark patches that look like hygroscopic salts.

Hygroscopic salts come from the historic burning of coal in a fireplace below. They cause condensation at normal levels of humidity, see later in the report.

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There is a  high damp meter reading deep in the wall.

The dark paint obscures the presence of hygroscopic salts. These salts are benign, and not a source of moisture. So there’s no need to do anything. If the patches concern you, a solution is to cover with a solvent-based, primer-sealer such as Zinsser Cover Stain.

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If you’re thinking long-term then you may consider installing a modern Wi-Fi thermostat.

The benefit is that you can set up multiple temperature settings, without allowing the temperature to drop below the dew point. In addition, as a landlord you can monitor the temperature making sure the temperature never drops to low. Nest (Amazon) allows you to monitor humidity. Other Wi-Fi thermostats include Hive – British Gas and Honeywell. They allow you to site the meter near to the cold wall. However, you need Wi-Fi for it to work.

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Consider just installing 30 kitchen extractor fan on the wall where there is already an air brick.

Kitchen extractor fans on adjacent walls should have her 60 l/s speed, and ideally be humidity activated.

Damp proofing against rising damp

One of the most interesting damp proofing techniques, seen in front of the ground floor.

I suspect this retrofitted DPC was installed by cutting out a course of bricks with a chainsaw. Possibly the work of Peter McDonald (now retired, formerly CEO of SafeGuard, PCA trainer – when he worked for or ran South London Damp Proofing – as was, as I recall).

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