Lovely Victorian London terraced house, probably Clapham, owned by a RICS Chartered Surveyor, had damp proofing from Kenwood, which describes itself “The Leading Damp Proofing, Woodworm & Dry Rot Specialists”. They use chemicals and impermeable slurry to hide, rather than deal with the root causes of damp causing damp to eventually break through, especially when light sockets are fitted.
Surveyor Tips:
- There were a number of techniques used to identify the root cause of damp;
- Getting the client to buy and install dataloggers in advance of the survey.
- Leaving an inexpensive sub-floor measuring device (hygrometer probe eBay £5) and asking the client to report back.
- Chatting to the neighbour and asking her to take a photo of the other side of a damp wall (during Covid lockdown access was limited).
- Generally working closely with the client, opening up access to the sub-floor and getting them to move dataloggers around.
- The owner was unaware that the modern kitchen extractor fan actually worked well, so they did not use it.
Root cause
Suspect damage to neighbour rainwater pipe (demonstrated by localised high hygrometer probe readings), leak under kitchen sink, condensation and interstitial condensation.
Illustration
The point of ingress can be hidden, and walls absorb moisture at different rates than they would had the original plaster been retained.
Sockets are prone to dampness as a section of wall is cut out, a metal box installed and covered in absorbent plaster, all of which exacerbate the effects of condensation and absorption, see surveyor.tips/sockets.
The RSJ loses heat to the subfloor causing condensation either above or below the ground floor, or probably both.
Again the vapour could be from below timber floor or above it.
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 8 to 99WME. See surveyor.tips/dampmeter. Walls measured were largely dry on the surface except where mentioned in this report.
See the datalogger later in this report. I note the hygrometer left under the floor near the lounge front window, “the reading has generally varied between 85% – 95%”.
There were some surface calcium sulphate salt crystals, but no evidence of hygroscopic salt.
They were high damp meter readings.
Looking through a thermal imaging camera where blue/green is about 5˚C colder than yellow/orange, we see that the base of the wall is cold.
It’s difficult to tell but I suspect their chimney breast is still in place, and that there is a cold flue running up to the back addition roof. It is likely that there is only a single skin of wall between your opened up chimney breast and their cold chimney flue.
I felt all the way around the toilet before and after flushing and looked everywhere but so no signs of a leak. The wall was damp. I asked the neighbours and they told me that they had no damp issues on their side of the party wall. The wall is dry lined, but probably touching the slate floor.
The stain has been painted over with stain block, again hiding the root cause and damp. However it looks like water is condensing on the cold pipe and possibly floor and absorbed behind the skirting. Cutting back the plaster so that there is no contact should stop this.
I found no evidence of wastewater leaks.
The stopcock on the street did not turn the pressure off, nullifying the test. See https://surveyor.tips/leak-tracing/ for further testing. But given the new mains pipe, I doubt this is the root cause.
I don’t think that this is the root cause of damp.
It is possible that this was the original root cause that resulted in misdiagnosed rising damp.
I left a hygrometer probe under the floor near the front window. You noted readings between 85% – 95%. This suggests high sub-floor humidity near to the front drain, a tell-tale sign of water leaking from the front rainwater drain. This will have an impact and should be thoroughly checked for leaks or better still bypassed – see datalogger to end.
I’ve never heard of a slate damp proof course breaking down although it is much used as a justification for damp-proofing treatment. The issue with DPC is more that any water bridging them, or forming on top, has can be absorbed downwards, so becomes absorbed upwards.
Damp proof membranes are recommended. However, the downside of a DPM is that it has the effect of pushing humidity away from the solid floor onto other floors and walls. There is no evidence that this is causing particular problems in this property.
Note the new looking green timber joists, a sign that they have been tantalised presumably after dry rot treatment possibly following damaged drain to the front.
This would reduce the chance of a leak occurring in the subfloor void.
Rainwater could be entering from this under ground vent. This could be the root cause of high sub-floor humidity.
In my opinion there is insufficient sub-floor ventilation. Without ventilation, even the smallest volume of water will redistribute itself and cause condensation to form on the damp proof course.
Dataloggers
Datalogger conclusions:
1) There is a consistently high sub-floor humidity (top dotted line). This suggest that:
a) water is likely to be condensing on the sub-floor DPC and being absorbed upwards,
b) that the front rainwater drain or underground vent is letting in water,
c) there is insufficient airflow (although secondary to ingress).
2) Massive heat loss in the kitchen causes high %RH, likely to result in condensation.
3) The dew point difference, i.e. vapour difference between the kitchen and lounge, suggests from night-time heat loss is causing absorption of water into kitchen walls, overloaded walls are likely to result in condensation. It suggests the lounge walls are less able to absorb moisture, probably because of the damp proofers’ slurry.
Other matters
Nothing major, but you should check annually and fill cracks and gaps remove flaking paint and repaint timber to reduce the risk of Phellinus contiguus or window rot. Window rot is very common in old properties, it is a slow growing white rot. None of the timbers are structurally significant. The rot can be cut out, sanded, filled and painted for protection.