102M: top floor; mould condensation; suspected flat roof

The buyer requested a damp survey to determine if there were any major damp issues before instructing a RICS homebuyer survey – second time this year. here were have a modern second floor apartment over a shop. Water and mould can be seen everywhere. The buyer’s concern was is the flat roof causing penetrating damp?

Root causes

The root cause was excess vapour through poor ventilation and heat loss.

Surveyor Tips

  1. If there is ingress or penetrating damp from a flat roof there will be an obvious point of ingress and normally tea bag stain like discolouration.
  2. Ideally start monitoring before the tenant opens windows and changes the environment.
  3. In the words of Graham Cole (“PCA”); “mould is your friend”, it tells you that there is excess humidity, and that therefore even if there might be another root cause, which there wasn’t, then ventilation and possibly heat need to be addressed before determining any other root cause.


This is perhaps the mouldiest is property for over a year, which is saying something.

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.

I used a damp meter in radio-frequency mode to determine that there is moisture around the corner of the wall.

I tested walls with a Protimeter 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.

Despite looks, the ceiling was dry at the time of the survey.
This image was taken in a bedroom cupboard.

Condensation dribble marks have rolled down the wall.

The area around windows it’s full of mould.

Curtains trap humid air against the cold surface.

Mould can be seen growing on the curtains themselves.

Note the condensation on the double glazed windows.

Condensation is forming on the lintel above the double glazing.

Note that there are no trickle vents in the double-glazed windows. The windows are open, but I suspect this is because I was surveying the property.

Mould is even growing next to the passive vent.

Unless there is wind outside, passive vents do you little more then reduce the temperature increasing the risk of mould.

Condensation – Cold water comes into a property at about 8°C in winter.

You can see the condensation forming along the pipes. Consider insulating cold water pipes.

There was some dampness on an internal wall next to the bathroom.

I can’t be sure what is causing this dampness, but the three most likely causes are 1) water finding its way through cracks in the bathroom tiles, 2) spillages from the bathroom floor or mop, and 3) condensation forming on the nearby cold water pipes.

The kitchen extractor fan does not duct externally.

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.
The bathroom extractor fan is slow and stops as soon as lights are turned off.
There is no ventilation in the kitchen.

The mould is growing across the lintel. Lintels are normally made of concrete reinforced by metal.

I tested and found that there was metal in the lintel.

Metal causes rapid heat lost when it’s cold outside, this illustrates the insulation challenge, but not the solution.

The tenants are drying the clothes in doors.

Clothes add about half a litre of water per person per day, to an already humid environment.

You will notice some leaking from under the shower panel, this is easy to fix.

Missing sealant, cracked grouting and leaking shower panels are all common problems and should form part of your regular maintenance checks.

I climbed onto the roof.

It looks like somebody has spent a considerable amount of money and time trying to stop water penetrating, when in fact the water is coming from inside the flat, not from above. If there was penetrating damp then that would be brown stains on the ceiling.

Some of the coping stones have been sealed, they look in reasonable order.

I think the roof will probably last for a few years. If you are thinking longer term you should consider installing insolation and a seamless roof covering, such as EPDM. It will cost in the order of £5,000 to £10,000 to install. It is probably a freeholder decision and responsibility.

The challenge with the roof is the blistering under their roof coating and ponding.

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. Despite the windows have been open for 10 – 20 minutes before I was allowed in, the CO2 meter reading of 1,370ppm, which is exceptionally. 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.

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