103H: Chimney, roof leak and poor ventilation

This semi-detached 1920s house has mould and condensation on most external walls, there is also penetrating damp, typified by browns stains to the rear bedroom under the chimney breasts.

Surveyor Tips:

  • Another case of overlapping damp issues. Be careful to look for the brown stains, a tell tale sign of penetrating damp.
  • Make sure to have 3M long selfie sticks (monopole – Amazon etc.) – I use two strapped together. It saves the time and cost of using drones. Put the camera on video mode and review on a big screen screenshooting images for the client.
  • Easy bathroom ventilator fixed during the survey – happy client.

Root causes

  1. Excess vapour causing condensation and mould resulting from insufficient ventilation and cold bridges.
  2. Penetrating damp from roof and or damaged flaunching on chimney breast.


There are two sources of damp. A teabag stain like discolouration on the ceiling is a sign of penetrating damp.

The brown colour is caused by water soaking through bricks or over timber. 

Using a damp meter in radio-frequency mode, the meter reading was at its highest level of dampness.

I 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.

Above in the loft I found the brick chimney stack was damp.

There are signs of water on the timber.

The sarking material was full of rubble just below the hip, although not much water lands on the hip. Most roofing issues are low down, where rain is funnelled and accumulated.

A few defects can be seen on the outside.

The white arrow points to where the rubble from the sarking material appears to originate.

There are no obvious signs of damp there and it’s unlikely that much water could penetrate through the hip. 

  • By contrast the blue arrow points to damage flaunching – the most likely source of ingress.
  • The red arrow points to are damaged tile. 
The flaunching is cracked, the pointing it’s damaged and there is some plant growth.

I think the top of the chimney stack is the most likely source of ingress. However, other elements should be checked once there is direct access to the roof.

Looking more closely at the broken tile, it is also possible, but unlikely source of ingress.


The extractor fan only lasted for five minutes I moved the jumper so that it should now last for 30 minutes.

The extractor fan was running at 9.7 l/s. I cleaned it up so that it now runs at 13.1 l/s, better than most UK extractor fans, but not as fast as building regs requirements of 15 l/s.

Mould is growing above the extractor fan, this is caused by missing insulation in the loft. I redistributed it.

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. The secondary cause of high relative humidity is low temperature relative to the source of humidity, see surveyor.tips/humidity.

There is no externally ducted kitchen extractor fan.

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.
  2. Kitchen 30 l/s adjacent to hob; or 60 l/s elsewhere in kitchen.
Mould is growing in the front room.

This is caused by combination of insufficient ventilation, heat loss, and insufficient air circulation.

The thermal imaging camera highlights the risk of heat loss in this triangular bays.

Blue/green is about 5˚C colder than yellow/orange. The ceiling is also at risk of heat loss, and therefore mould and condensation, consider insulating it.

There is mould and condensation in the rear bedroom.

Again this will be caused by insufficient ventilation and heat loss.

The heat camera picks up areas of heat loss.

On the ground floor there is mould and there are calcium sulphate salts crystalising 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. 

Mould is inhibited by nitrates found in groundwater. 

The presence of mould at the bottom of a wall eliminates rising damp as the root cause and points toward condensation from unvented excess vapour.

Relative humidity is a measure of how much vapour is in the air compared to air’s capacity to hold vapour.


Relative humidity is a function of vapour pressure (quality of vapour) and temperature. As temperature rises, air can hold more vapour. Conversely as temperature drops, air holds less vapour until it meets the dew point at 100%RH, when dew or condensation forms. Mould forms when there is period of humidity above 85%RH for 6 or more hours.

I doubt the fish tank is having much impact, but it is an additional source of vapour.

Given the additional strain on the property, I’m sure the tenants will do what they can to help monitor and minimise humidity.

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