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Victorian lower ground floor flat with damp proofing on all walls


Steps to identifying the source or sources of damp. Water in residential properties comes from one or more of four distinct sources:

  1. Mainswater; typically called a leak, but it could include mainswater, heating system, drains, washing floors, accidental spillage and built-in moisture (from construction).
  2. Rainwater; referred to as penetrating damp or ingress.
  3. Groundwater; the water under the water table, that causes rising damp, as water cannot fall, or be easily drained away. Groundwater is commonly pumped out of big cities.
  4. Condensation; from excess vapour meeting a cold surface, this includes interstitial condensation and hygroscopic salts, although dampness associated with these salts are in equilibrium and therefore not a source, per se. Hygroscopic salts are claimed, without evidence, by the damp proofing industry, to be a means of identifying rising damp.


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I tested for a mains leak by looking for reverse flow on the kitchen tap. There was no reverse flow.

There appeared to be no mainswater leak. The drains should have a CCTV drain survey.

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The thermal lens picks up an odd heating system pipework partially embedded, but no obvious signs of a leak.

The radiator needs topping up roughly every six months, this could a sign of a leak. However, the floor was not wet as you would expect from a leak from a heating system laid on the concrete screed.


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The section of aluminium shelving above the window is causing rain to bounce off it and penetrating inside.

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A slight brown stain can be seen above a horizontal line.

The brown discolouration like a teabag stain is a sign of water flowing through building material. In this case it is caused by rainwater.

The horizontal line results from treatment for rising damp including the replacement of absorbent plaster with impermeable slurry up to about 1M/1.3M.

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Render cracks, such as to the front, could cause internal dampness – but not the profile or extent of dampness.

Render expands in the sun and contracts at night. This often causes cracking. Cracks suck in water like capillaries, water cannot evaporate out through the render so penetrates inside. Sometimes the crack merely dampens the outside brick causing heat loss, increasing risk of condensation.

Cracks should be filled with a filler that can expand and contract, such as an external acrylic caulk. Render needs to be check annually. Cracks can reappear even after filling.

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Dampness was detectable up to almost (but not quite ceiling level) by the front bay.

The lack of stain could be from damp proofer’s slurry blocking rain, or from condensation.


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According to the British Geological Survey map, there is no risk of groundwater anywhere near your property.

  • Rising damp is the “upwards flow of moisture through a permeable wall structure, the moisture being derived from groundwater”.
  • Groundwater is subterranean water below the water-table, a “capillary fridge” is detectable up to about 1M above the water-table. The capillary fringe is unsaturated and therefore only causes a small amount of absorption in bricks in contact with it.
  • Groundwater behaves differently to rainwater in soil, as it is at or below the water-table therefore there is no gravitational force. Water can’t disburse. By contrast rainwater can and should be drained away from a wall.

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Surface water flood risk can be mistaken for groundwater. But even the risk of surface water is low here.

Water flows down the hill, along Woodcombe Crescent and Devonshire Road, but does not appear to affect the building itself. If necessary, I could drill down by about 1M, outside below floor level to test for water. I would need freeholder permission and a drain map.


Almost every property will suffer from condensation to some extent. Modern living with hot showers, increased occupancy, double glazed windows, central heating, drying clothes internally and working from home during Covid, increases the vapour load and risk of condensation.

Lower ground floor properties have increased risk of condensation from heat lost to the ground, reduced external airflow, increased external humidity towards the ground and safety fears reducing the time that windows are left open.

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The bathroom fan extracts at 3.3 l/s, without an overrun. It should extract at 15 l/s with a 30 min overrun.

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I could not get the kitchen extractor fan to work – I may not be pushing the right buttons.

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Typically browns stain are caused by movement of water such as a leak or rainwater, rather than condensation.

Condensation forming on cold tiles can act in a similar way to a leak, resulting in a brown stain, or there may be metal causing rust.

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The brown around the sockets is caused by rust. The salts are calcium sulphate, AKA gypsum found in plaster.

I tested and found some nitrates in the salts. Calcium nitrate is a common hygroscopic salt. However, when humidity exceeds about 50%RH, surface hygroscopic salts look wet, not crystalline as is the case here. The profile is distinctive and different to your walls.

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Dampness is unusually high up on an internal wall, as determined by a Protimeter radio frequency damp meter.

The damp meter’s measurement limit is 999REL, below 300REL is consider dry. 999REL indicates water absorbed at depth. High readings illustrate the extent and profile of dampness, and are used to help pinpoint the source of moisture surveyor.tips/profile.

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The neighbour’s side of your bedroom party wall was dry at the base of the wall (however it might be dry lined).

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There is mould around the kitchen.

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.

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The colourless crystalised surface salts around the window are only consistent with condensation.


  1. The replacement of normal, absorbent, homogenous plaster with slurry hides damp, without dealing with the root cause. Profiling damp is an important tool for identifying the root cause. Slurry makes identification of the root cause much harder.
  2. Penetrating damp is clearly affecting the building to some degree. Leaving it vacant, without background ventilation or a dehumidifier would have led to condensation. I think the bathroom and kitchen extractor fans are insufficient.
  3. There could be another source of moisture such as from the heating system or drains (these being the only sources I was unable to check for leaks or look up online).


  1. Replace the extractor fan with one that runs above 15 l/s (after assembly, not just claimed on the package) set up with a 30-minute overrun. Probably better still, especially if the property is vacant for long periods, install a continuous flow fans such as Elta Mori dMEV.
  2. The render cracks should be filled with an external acrylic caulk.
  3. Remove the aluminium self above the window (unless there is a good reason for it/them to be there).
  4. I would be prepared to drill ~1M underground to test for groundwater, with freeholder permission and an accurate plan of the drains (Thames Water search).
  5. I suggest you get a CCTV drain survey of all drains.
  6. Consider having your heating system checked for leaks. Alternatively, a good way to identify leaks is to pour inexpensive perfumed into the radiator (when the system is off). Let the system sun, use smell to locate any leaks.
  7. Use two dataloggers continuously to review humidity.
  8. Consider installing an externally ducted kitchen extractor fan with rigid ducting, either above the hob, or light switch operated. Hoover HGM600X (£149) has a Which! Best buy and use an extractor whenever cooking.
  9. Consider using a dehumidifier, especially if the property is ever left vacant, see Which! Guide; https://www.which.co.uk/reviews/dehumidifiers/article/how-to-buy-the-best-dehumidifier such as Inventor EVA II Pro 20L R290; £179 ideally buy one with a continuous function, draining it into a sink or drain.
  10. TIP: property management. Observe and photo or film around the property during a rainstorm to see where water flows, making sure water flows/drains away from the building.