103F: Neighbours leak and hygroscopic salts

This Edwardian semi-detached house on a hill has had damp proofing, yet damp persists. The key concern was dampness to the rear of the property, which appeared worse on the party wall side. There were many, complex overlapping issues.

Surveyor Tips:

  • They key findings was damp coming form the neighbour’s side, which would not have been picked up without assistance and access from the neighbour, with whom the client had good relations.
  • It is very important for neighbours to avoid disputes, as it makes it so much harder to identify issues, costs escalate when often the problem is easy to fix.

Root cause

Leak on the neighbours side, almost certainly coming from obsolete heating system pipes buried in the floor of a rear extension. The issues were made worse by insufficient ventilation and heat distribution. There were hygroscopic salts on the first floor, around two separate chimneys.


The house has been “damp proofed” with chemicals this has the effect of reducing the absorption of moisture in some walls.

There are a lot of the separate damp issues. It’s easiest to explain from the top of the house working down.

As is common in Victorian properties, there are hygroscopic salt in the chimney breast, here on the first floor.

Hygroscopic salts originate mainly from burning the timber. Calcium nitrate is the most common of these salts. It causes condensation at ~50% RH. See later for more details.

There are also hygroscopic salts in the rear bedroom chimney breast, most notably in the ceiling above.

There are what looks like slight dampness and mould in the top floor rear bedroom eaves.

Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excess humidity results from insufficient ventilation, poor air circulation and cold surfaces. See surveyor.tips/mould.

There is mould growing around the front window.

It looks like condensation has formed on the single glaze window and is causing slight window rot to the timber below it.

There is penetrating damp from a leaking roof above the bay.

I understand that you had the leak repaired. Note the discolouration like a tea bag stain. This is a sign that water has passed through brick or over timber, bring the colour with it.

I measured the front wall with a damp meter, much but not all of it was dry. It takes many months to dry.

I tested internal walls with a Protimeter damp meter in conductance mode. These meters measure electrical conductance of salts in water, a proxy for damp.

There is damaged skirting board, I recall that this was in the bathroom.

The damage appears to be woodworm, probably would weevil.

Wood weevil occurs with Coniophora puteana, or cellar rot, a brown wet fungus, see surveyor.tips/rot. Rot requires water to grow. Remove the water and both will stop destroying timber. I recommend that the skirting board is removed and replaced.

There is a wooden step to the back door.

The step is loose and damp inside.

I looked inside and found rot. It looks like Coniophora puteana, or cellar rot.

I understand that the floor is a solid floor and that therefore and there is no such structural implications to the rot. See later in the report.

Outside the front of the rainwater drain looked blocked.

I tested it with a jug of water and found that water does not flow away.

A blocked drain could cause water to flow into the sub-floor resulting in high humidity, which in turn can cause moisture to condense on the damp proof course and for rot to grow on timber. I measured the sub-floor humidity, it was 75%RH, which is normal, suggesting no condensation or rot in the sub-floor void. 

The drain should be unblocked or bypassed as soon as possible.

There is no ducting from the kitchen extractor fan.

Humidity will inevitably form in the poorly heated rear of the building.

The thermal image highlights much cooler rear to the building.

Blue is roughly 5° colder than yellow. I suspect there is poor insulation and that the heating is switched off at night.

I found dampness throughout the rear ground floor utility area despite the passive vent.

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. 

There is a lot of metal in the rear area. I suspect RSJs are supporting the opening.

The rear section of the ground floor is an extension into what would have been an outbuilding, built without insolation in mind. Note the dribble marks a sign of condensation. The colour is odd, although it does not look like a leak or penetrating damp.

There are calcium sulphate salts on the wall surface.

Calcium sulphate is a key ingredient in cement and other building materials. If diluted in water salts tend to move to surfaces. These can be removed with sandpaper and decorated. 

Your neighbour kindly let me look in her kitchen. There is more dampness on the other side of the wall.

The wall is stained which suggests that water it’s coming through brick, such as from a leak. They have more salts in a smaller area, suggesting that the leak is from their side. I understand there used to be a boiler here. It is possible that the pipe to the boiler is leaking or there could be residual moisture. They should test for a mains leak.

I did not notice a drop in your pressure, but the should be double checked with the neighbour’s side.

See https://surveyor.tips/leak-tracing/

There were no obvious signs of this roof failure to your neighbours side.

There was no obvious signs of damage to your roof.

I noted in your neighbour’s kitchen there was a high damp meter reading along the external wall.

At first I wondered whether water was leaking across the solid floor.

Looking outside we can see the distinctive damp look of  hygroscopic salts.

For this reason I think the root cause of damp on the neighbour’s external wall is hygroscopic salt and the damp on the party wall is probably from the old boiler pipe. I think condensation taking place in the rear of your ground floor, accentuating damp on your side.


Thank you for sending a csv file from the datalogger.

There is nothing remarkable. Humidity appears consistently below 65%RH and temperature consistently around 15°C. 

That said, the profile, extend and dribble marks in the utility room, looks like condensation is a component. It is possible that condensation is taking place during a cold snap, or is historic.

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