102F: damp-proofing, despite undrained down-pipe and hygroscopic salts

This is a Victorian terraced house on a hill. The owner had it damp proofed with a thixotropic silane siloxane cream and dry lining. However, since the property did not suffer from rising damp, but undrained down-pipe, poor ventilation and hygroscopic salts, damp continued. The buyer brought in one of the “free” contractors to quote. As usual they misdiagnosed the problem as rising damp (why wouldn’t they that’s how they profit), read more…

Surveyor Tips:

  1. Check gutters, down-pipes and flow of rainwater from a property before damp proofing.
  2. Damp proofing hides the symptoms, you could end up with rot in the sub-floor (not the case here – examples to follow).
  3. Hygroscopic salts can cause rising damp like symptoms, they normally result from burning of coal in chimneys, see later this month.
  4. Think about damp proofing as a short term solution to hide symptoms, that will need addressing again when you sell, unless you address the root cause.
  5. A house on a hill is highly unlikely to ever suffer from rising damp, which is from groundwater.
  6. on’t expect a damp proofing company to recommend treatment for condensation, when they make virtually no money from it.


A picture containing indoor, dirty
Description automatically generated
There is dampness in the cupboard in the front reception, with mould growing on the skirting board.

Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from a source of dampness or vapour, insufficient ventilation (in this case, within the cupboard), poor air circulation and a cold surface. See surveyor.tips/mould.

A picture containing indoor
Description automatically generated
There was a high damp meter reading on the left-hand wall.

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.

A picture containing text, hand
Description automatically generated
Other walls were dry on the surface in the front room. However the front walls have been dry lined.

Dry lining is a form of damp-proofing typically with timber batons fixed on the wall and plasterboard is attached. There’s nothing wrong with this approach, but like chemical damp-proofing, it is hiding the problem rather than dealing with the root cause of dampness.

A picture containing person
Description automatically generated
About a metre above the floor, there is a high damp meter reading on this wall, on the left side of the front bay.

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. 

A picture containing rock
Description automatically generated
Looking outside we can see that the drain rain from the bay roof is blocked with shingles.

The property is over 100 years old. I suspect that the drain is damaged undern ground, although you could clear the drain and use a CCTV camera to establish if there is damage. It would be easier and lower risk to simply, direct rainwater away from the property with a horizontal length of pipe and elbow fixed to the downpipe, see you recommendations.

The greatest risk from hiding damp is water running into the subfloor void, causing timber joists to rot.

I measured the relative humidity in the subfloor void. The humidity measured 80%RH which is below the 85%RH threshold of what is considered excessively humid. Rot produces water as the by-product of respiration. High humidity is a sign that there is rot. I therefore doubt there is any significant rot in the subfloor void. Again, it would be wise to remove the source of water by attaching a horizontal pipe (with slight fall away from the house).

A picture containing text, wall, indoor
Description automatically generated
There high meter readings in radio-frequency mode up to about 1.5M on the rear left-hand wall.

The wall surface is dry, but there is dampness deep within this wall. There was originally a chimney breast on this wall. Hygroscopic salts, such as calcium nitrate can be released from the historic burning of wood, coal or manure briquettes. Calcium nitrate causes vapour to condense at normal levels of relative humidity (~50%RH), it is benign, but can leave colourless damp spot, see end of report. Salts can move through wet plaster when chimneys are altered. Hygroscopic salts are a symptom of replastering, not a source of water.

A picture containing text, wall, indoor
Description automatically generated
To see whether salts were on the surface, I boiled a kettle by the wall. There were no visible salt damp patches.

I believe hygroscopic salts are trapped behind the new plaster and will not cause any ongoing concern. It is possible that the wall has been successfully damp proofed.

Testing the wall outside we found high damp meter readings about 1M above the outside level.

This is a sign that the are salts in the brick up the wall. You can see slightly damp patches of brickwork on the outside. This a phenonium, not a problem.

A picture containing building, outdoor, device, meter
Description automatically generated
The wall near ground level is dry.

If there was rising damp as claimed, then the base of the wall would be damper nearer the ground, than higher up the wall, as rising damp is groundwater. Groundwater is rare in London, as it is pumped out. It is rarer still on the side of a hill.

A picture on a wall
Description automatically generated with low confidence
There are signs of dampness between the two first floor front windows stretching towards the electrical socket.

The dampness is very light coloured. Typically when there is penetrating damp that is Rainwater that penetrates from the outside it brings a tea bag stain light discolouration through. So it is possible that this is condensation. 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 owner tells me that filter plaster crack and painted the wall.

A pile of dirt on the side of a building
Description automatically generated with low confidence
A little bit of pointing is loose and there are some gaps in the mastic surrounding the windows.

Gaps should be filled especially around windows, see recommendations.

A picture containing wall, indoor, person
Description automatically generated
There is a high damp meter reading on the ceiling, on the right-hand side when looking from the street.

There was a slight discolouration from dampness in the corner, with no discolouration.

A picture containing stone, cement
Description automatically generated
Looking in the loft above we see you early signs of timber decay, likely to be a wet white rot.

The section of timber was damp, the decay was not structurally significant, but the source of water should be stopped.

A picture containing stone
Description automatically generated
Looking outside we see that the roof looks in order on both sides. There are no obvious causes for concern.

A section of slate has been built into the party wall, three courses down from the top, this should stop rain water seeping from the top of the party wall.

A picture containing brick, stone
Description automatically generated
I note there is a distortion of the gutter and potential for water to splash in from the gutter, under the roof.

Climate change is likely to increase the intensity of rainstorms. I recommend looking to see what happens during a rainstorm, and together with your neighbour, ask a specialist roofer or installer of gutters, to make recommendations such as installing a deep flow gutter. 

A picture containing outdoor, person, feeding
Description automatically generated
There is a marginally high damp meter reading underneath the chimney breast.

This is quite common. Ask your RICS homebuyer surveyor if he has views on whether there is ample support for the chimney breast, that has been removed to the rear of the property.

A picture containing tower, building, outdoor, tall
Description automatically generated
There are no cowls stopping water from entering  the obsolete chimney stacks, this is very common.

The actual cost of installing a cowl is around £100. However scaffolding could cost many, many times this. The issue is marginal and not worth spending money on scaffolding unless and until a roofer has to make changes to the roof, and therefore scaffolding is in place.

A picture containing indoor, wooden, wood
Description automatically generated
There is some ventilation in the roof. 

There are signs of mould on timbers in the roof but this could be old.

A picture containing engine
Description automatically generated
The bathroom vent is marginally slow and the overrun is short.

A quick clean and an adjustment to the overrun timer is an easy job for a homeowner. Just take off the plastic covering in the centre of the fan, unscrew the grill, Google the manufacturers details seen on the inside and look up how to adjust the settings. Paragraph

Leave a Comment

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.