102G: church conversion, mould, condensation and leaks

This is a pretty two bedroom ground floor church conversion flat. The landlady had been told that the problem was a roof leak. In fact the primary issue was mould and condensation from a broken internal bathroom extractor fan and various leaks.

Surveyor Tips:

  1. If suspicious that there could be a wastewater leak from a washing machine, ask occupier to turn it on and look at the waste water pipe during the drain part of the cycle.
  2. Ask tenant for information and images/film of defects – they will normally oblige.
  3. Ask tenants to send back data to your email address from data-loggers, show them how to first, with the first few data points.
  4. Don’t assume anything you are told by well-meaning contractors, occupiers or owners, until you’ve checked the evidence.


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The primary concern is mould and damp patches on external walls and under the roof.

Mould grows where relative humidity exceeds 85%RH for 6+ hours. Excessive humidity results from insufficient ventilation, poor air circulation and cold surfaces. See surveyor.tips/mould.

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There are signs of condensation across the flat such as on the metal window frame in this image.

Condensation results from too much vapour and insufficient ventilation.

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Water sitting on a cold surface such as in the image above, results from condensation.

There was a massive amount of condensation taking place in this property.

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When water passes through brick, or timber it tends to leave discolouration like a teabag stain.

There is a stain on the ceiling below the upstairs flat. This looks like it comes from a leak. It is not located near a cold surface. Kept

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By contrasts, there are stains by the Velux window that look like they come from the roof, but may not.

This form of discolouration can also result from interstitial condensation, that is condensation that forms within a building, such as around the metal of the Velux window.

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I could not see the roof clearly, so your roofer may be right that tiles need renewing around Velux window.

Looking at the image above, where the water stretches across the whole window, this is not normally consistent with a damaged tile, which would tend to have a single point of entry. Besides, condensation needs to be solved. Therefore it would be wiser to fix ventilation first.

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I measured the wall where mould was growing with a damp meter.

This section of mould wall was dry.

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Other sections of the wall were damp.

I tested with a Protimeter damp meter in conductance mode. These meters measure electrical conductance of salts in water, a proxy for damp. So there is dampness under the roof. But this is more likely to be caused by interstitial condensation not a faulty roof.

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The temperature of the wall by the window frame, was 14.2°C.

The calculated dew point was 14.2°C. Therefore the wall was cold enough for condensation to form during the survey.

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The internal bathroom extractor fan it’s not working. The ensuite fan was slow, without an overrun.

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.

 For landlords I tend to recommend a continuous flow extractor fan such as the Elta Mori DMEV II HT (Amazon £110).

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The tenant supplied me with a video showing a leak in the corridor outside.

I saw images that also looked like leaks from the flat above. I understand most of the leaks have been repaired.

During the survey I found a leak to the wastewater pipe from the washing machine.

Water was running down into the subfloor. There is a risk of rot. However, there appear to be sufficient subfloor ventilation to dry the water away. The cost of raising the floor to find out if there is rot will be in the order of £300. The cost of strengthening rotten timber is about £500. So it probably is not worth investigating further, so long water leaks are stop.

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There is timber rot called “blue stain in service” by the bathroom door.

I believe that the rot results from the leaking washing machine wastewater pipe. But it could come from water being walked out of the bathroom.

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There is some other rot also probably “blue stain in service” by the back door.

I suspect this rot comes from condensation forming on the back door as can be seen in the image above.

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There was damp staining in the kitchen by the cooker.

Measuring with the damp meter conductance mode, I found the surface to be dry.

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Using the damp meter in radio frequency mode the wall was determined to be damp deep inside.

This suggest that there’s been a leak, but that the wall is drying out. It can take 10 months for a wall to fully dry out.

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I saw no obvious signs of a leak on the other side of the wall.

It is possible that water is coming from underneath the tap. I felt no leaks under the tap. 

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I left a datalogger to measure humidity and temperature.

Unfortunately the tenants could not get a connection. I suspect that was because condensation cause the electronics to fail.

Unfortunately, the tenant was unable to email data to me. Consider installing a Wi-Fi data logger so that you and the tenant can monitor humidity and temperature through tracking improvements over time.

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Damp wall

Carbon dioxide results from respiration, not from excess humidity. I measured the CO2 in the 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 meter reading of 1,037ppm is high. The normal level is around 500ppm. High levels of CO2 suggests that windows are not being opened enough.

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.

Other matters

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There is water splashing onto the wall by the downpipe.

There is no sign of it this rainwater causing internal dampness. But I guess that the gutter or downpipe is partially blocked.

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The gutter has a “hedgehog” system for stopping it from becoming blocked.

I’m not a huge fan of hedgehog gutter systems as they can impede the movement of rainwater, causing water to spill over the gutter during heavy rainstorms. In my opinion, it would be better if the gutter was inspected and cleared twice a year.

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The roof appears to be in reasonable condition. 

Although moss is not ideal, it’s an inevitability and therefore better to check and clear gutters twice a year, than attempt to remove the moss every year.

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An overflow pipe is pouring water onto the Side of the building.

There were no signs that this water was causing penetrating damp. Ask a heating engineer to repair the cause of the overflow. This is not an urgent problem and may be a freeholder responsibility.

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