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104Z: White rot following suspect mains leak

This lower ground floor Victorian conversion rental has had white rot, high sub-floor humidity remains high. There is a slight leak in he mainswater.

Surveyor Tips:

  • Measure the sub-floor relative humidity.
  • Measure the pressure on the mains-water using a pressure tested.

Root cause

Leak from mains water.


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I visited the property a few years ago to inspect rot found under the floorboards.

I determined that there was white rot which is a slow-growing rot. The rotten floorboards remain intact albeit damaged. I recommended improving the subfloor ventilation and internal ventilation. The air vent has been improved, but not sufficiently, see actions.

Update As you will see from the hygrometer probe place in the sub-floor near to the rear vent, the reading is 99%RH. This tells me 1) that rot is probable still growing, 2) the vent is still not sufficient, 3) a possible leak in the sub-floor. The spread of water looks like a leak.

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Mains water pressure undetected slight leak – here comparing a pressure meter 20 minute later.

Mains water pressure tests don’t form part of a standard damp survey as they take about 30 – 40 minutes each test, it can be a challenge to find an effective stopcock and leak free external tap and tests regularly result in false positives or negative. With the involvement of all occupants in the building I tested the building as a whole, and then tested the lower ground floor flat twice. The gauge was filmed in time-lapse over ~20 minutes. In each case there was a marginal loss of pressure, see the above deflection during the final test. This deflection probably equates to in the order of a drop of water every 10 seconds.

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There is dampness up the walls around the rear reception, such as seen above – damp shown on a damp meter.

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Looking through a thermal lens where yellow is about 5°C warmer than blue, we see an unusual spread of heat.

The spread of heat suggests that there is heat loss. Water from a leak, can cause heat to spread out over a floor. The tenants told me that they have never had to top up the heating system. The heating system should be a closed look. The tenants say they have not topped it up. The leak most likely in the cold water probably somewhere near the rear reception.

Examples from other properties visited recently

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Here is an example leak, looking vertically up, from under a leaking central heating pipe.

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Here is a thermal image of an underfloor heating system where heat is not being spread out by water.

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The result of the leak in this property includes salts seen about 1.3M from the ground.

The disruption of paintwork is caused by calcium sulphate salts. Calcium sulphate is a key ingredient in cement and other building materials. If diluted in water salts tend to move to the surface. These can be removed with sandpaper and decorated.
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The thermal image helps visualise where the dampness is.

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The brown stain comes from water in contact with timber or brickwork.

The damp seems to be in a band about 1 – 1.3M. This probably results from original plaster being replaced by damp proofers impermeable slurry blocking moisture below this band.

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Moisture can be seen above the tile in the kitchen.

This wall is the other side of the damp wall, seen above. It appears to be caused by a combination of dampness from a suspect leak and condensation from the kitchen.

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Mould is growing behind a kitchen unit.

Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from insufficient ventilation, in this case local or trapped humidity, poor air circulation and a cold surface. See surveyor.tips/mould.

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Looking under the unit we can see a pipe, probably a cold-water pipe. Cold water comes in at 8°C in winter.

The pipe felt moist, not wet. I believe that the dampness here is caused by water condensation on the cold-water pipe, but there could be a leak, see recommendations.


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There is a continuous flow fan in the bathroom which meets building regs requirements of 15 l/s.

The bathroom fan is working well.

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There is no externally ducted kitchen extractor fan.

I recommend that a kitchen extractor fan is installed. In the meantime ask tenants to cook with tops on pots and pans.

Drying clothes

The tenants are drying clothes indoors during the winter it’s recommended that thirdly start the process of drying outdoors or invented room such is the bathroom.

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Trickle vents were closed. I opened them during my visit.


  1. Call a leak detective or specialist plumber to track and fix the suspect leak. I recommend using a gas leak detection service. Then pressure check to make sure there are no other leaks. It is possible that the central heating system is leaking, this may require a heating engineer.
  2. Install a kitchen extractor fan.
  3. Remove the brick and Airvent to the rear under the rear door and replace with a Wicks plastic air brick, or similar.
  4. Cover exposed cold-water pipes, such as under the kitchen hob, with insulation.
  5. Ask tenants to:
  • dry clothes or a vented room (such as the bathroom with door closed and vent on),
  • cook with tops on pots and pans to reduce production of vapour from cooking,
  • keep heat balanced across the property and across the day and night so that the wall temperature doesn’t drop below the dew point, typically 10°C in winter,
  • keep bathroom door closed,
  • keep flow around external walls, by minimising clutter,
  • keep some ventilation in occupied rooms, such keeping trickle vents open.


This report is provided to help the owner of the property as a courtesy, without payment or benefit from recommended actions and therefore without accepting any responsibly.

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