102L: mouldy second floor flat, suspected rain water from roof terrace

This one bed purpose built Victorian second floor flat has dampness in the bedroom under a roof terrace. It is suspected of leaking. The Landlord has made many improvements to ventilation, but dampness and mould continues.

Root causes

The root cause was poor vapour, that is high relative humidity through poor insulation to rear of property. This is compounded by poor heat balance and vapour from tenants drying clothes in bedroom and unvented kitchen.

Surveyor Tips:

  1. Where ever possible look at a damp wall from below, side ways and above.
  2. Knock on neighbour’s doors, or pre-arrange before survey.
  3. Leave inexpensive dataloggers with tenants and show them how to send data back – they want to be involved, rarely are aware of the vapour they are ass and are often happy to be part of the solution.
  4. Defective pointing is rarely causes internal damp.
  5. Look for changes in both the property, neighbours and lifestyles and consider how these could impact on the property.


Despite ventilation improvements to the bathroom, dampness persists on the bedroom rear wall.

There is a question as to whether the damp is penetrating, that is rainwater getting through the brickwork, or formed from internal damp through condensation. If it were penetrating damp, you would expect to see discolouration like a teabag stain. There is very little staining. The small stain could be caused by condensation within the building (interstitial condensation). There was condensation during the survey, such as a sheen on the rear wall and on windows. Penetrating damp can’t be excluded until condensation is removed.

Mould has grown where a storage unit was trapping humidity against the wall, behind a small square box.

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. This demonstrates that internal vapour is at least part of the issue.

A thermal imaging camera illustrates the challenge, yellow is about 5 ̊C colder than green.

From looking over the parapet wall, from the top floor flat, it looks like the rear of this building was rebuilt potentially with brickwork with poor insulating properties.
There also appears to be a steel joist (“RSJ”) supporting the roof terrace. RSJs are prone to losing heat rapidly.

Many of the window surrounds are made of embedded metal.

Metal can cause condensation, sometimes deep within the wall. This can result in rust, leaving a stain on the external wall. There are calcium sulphate salts on the surface. 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.

Carbon dioxide results from respiration, not from excess humidity. I measured the CO2 in the front bedroom.

High CO2 is not dangerous but makes a place feel “stuffy”. It is a sign that there is insufficient fresh air coming into the property. The CO2 meter reading of 1,036ppm is high. The normal level is around 500ppm. High levels of CO2 suggests that windows are not being opened enough for the level of occupancy.
Even if the bathroom and kitchen extractor fans exceeded building regulations, they could not account for such a high level of CO2. The tenants should be encouraged to monitor CO2 and open windows when the level exceeds 800ppm.

I left a datalogger to monitor humidity changes over time.

The tenant has been kindly sending back data. I asked the tenants not to change the way that they are living so that we get a baseline of data.


I encourage homeowners with property showing signs of damp or mould to monitor the relative humidity, so that they can identify the source of vapour and or timing of vapour production. The data can be used to improve ventilation, increase heat balance, insulation and improve airflow through reduced clutter to see the benefits.

Monitor humidity and temperature against wall or ceiling, such as ORIA Wireless Thermometer Hygrometer (20m (2 Packs)) – see surveyor.tips/datalogger.

Looking at the data logger we can see that heat loss (insulation) is the main cause of high humidity.

There is a large temperature fluctuation, the temperature is lost almost as rapidly as it is gained i.e. insolation is poor. The dew point rises rapidly with heat increases, this tells me that there is a buffering affect, i.e. the walls are absorbing and releasing moisture. There are no humidity (dew point) spikes that fall outside the heat spikes, this suggest the bathroom extractor is working well and not the problem. I also suspect that the tenants stopped drying clothes on the radiator in the bedroom while the data logger was recording humidity.

Measuring the temperature of the wall, by the data logger at 12.7°C.

The calculated relative humidity of the ceiling during the survey was 92%RH which is ideal relative humidity for mould to grow.

The thermal image highlights the area of heat loss (thermal bridge).
The location of the thermostat by the boiler and nearby radiator, means that it is being unduly influenced.

There is a 10°C temperature difference between the thermostat and cold rear wall by the data logger. Consider a modern thermostat located away from the radiator perhaps in the back bedroom, with multiple temperature setting so that there is always a minimum temperature above the dew point.

Clothes are drying on the radiator.

It’s understandable that tenants try clothes on radiators. Consider providing a vented dryer or asking tenants to hang clothes in the bathroom with the door closed and vent on.

The kitchen extractor fan it’s not ducted out.

Ventilation is most effective when air is extracted close to the vapour source; bathroom, kitchen, drying clothes and occupied rooms. The internal ventilation does not meet Building Regulation 2010 Part F requirements. This is best achieved with mechanical extractor fans. See surveyor.tips/vent_regs specifically P39 and P19:

  1. Bathroom 15 l/s with a 30-minute overrun (provided).
  2. Kitchen 30 l/s adjacent to hob; or 60 l/s elsewhere in kitchen.
There is condensation taking place at the top of the bedroom cupboard.

These dribble marks are consistent with condensation.

Looking at the section of wall behind the cupboard, we see plant growth on the back of the wall.

Plant growth suggests that there is some water penetrating the external skin of bricks. However, internal dampness does not look like penetrating damp. Damp bricks can cause additional heat loss.

Neighbouring property

The neighbouring property has the exact same problem.

There was mould and condensation around the back wall and window surrounds. However, unlike your property, the only external wall in the neighbouring property is the rear wall.

There is a large patch of calcium sulphate salts from condensation in the middle of the wall.

The patch of calcium sulphate salts was dry.

They have an additional challenge in that they have no bathroom extractor fan.

Given the lack of bathroom extractor fan the tenants of the neighbouring property tell me they purge the air, daily (otherwise condensation and mould would be considerably worse. Note also that this property doesn’t have the cold flank wall (and I suspect don’t use their bedroom for drying clothes).

The level of carbon dioxide was lower, supporting this claim.

The tenants tell me that they leave the bedroom windows open for some time every morning. Ventilation in a rented property cannot rely on tenants keeping windows open.

From top floor flat

It is possible that moisture is penetrating from the terrace above.

However, it could not explain all the internal damp. Therefore I recommend managing vapour production and insulation loss first, before considering external factors.

I looked over the wall at the back of the building.

While the pointing isn’t perfect, I don’t believe that the pointing is the root cause of damp. There is no clear sign of penetrating damp, and plenty of evidence of condensation.

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