Leak above solid floor

Example from damp survey May 2022


Chelsea Victorian garden flat purchased about 5 years ago, after damp proofing treatment against rising damp. The treatment did not stop damp. Walls were treated again last year following a quote for £19,056. Again damp returned.

Although in the end the source of moisture was clear (and would have been to any surveyor) and easy to find, it was only found after obtaining permission for disruptive investigation. Prior to that the following all diagnosed rising damp as the cause:

  • 3 damp proofing contractors,
  • 1 independent damp surveyor and
  • 1 plumber specialised in leak detection and repair.

The leak detection company I recommended concluded: “Leak detection procedure undertaken at the address above didn’t locate a leak in the client’s property caused by service pipes Hot/Cold, wastewater and CH.… The source of these wet patches and lack of mould at the bottom of the wall along the corridor, can be linked with external moisture ingress, as rising damp or neighbouring property that had recent or ongoing leak.”

Their conclusions are disappointing as:

  1. Clearly there is and was a leak, see below. This calls into question the equipment used by the leak detection company and any reported negative results, including referrals to other clients.
  2. The leak detection company suggest rising damp despite the flat being in London where groundwater is pumped out.

Take away

The fear of being sued for damage and the extra time taken to hunt down the root cause of damp, can reduce the accuracy of damp surveys. Permission to disrupt a property should be obtained in advance, in case it is required.

Be careful to which contractors to referred clients to.

Costs, quotes & disruption that could have been saved

An estimated total £60,000, plus lost rent of £10,000.
Actual cost of repairs approximately £200.

Table of Contents

Evidence and explanations

Damp wall of primary concern

The concern was dampness along a corridor above a solid floor, despite 2 chemical damp proofing treatments.
Dampness is visible along a corridor above a solid floor, despite 2 chemical damp proofing treatments.

An independent damp surveyor reported in June 2021; “internal damp issues are caused by moisture making its way up from the foundations because of an aging damp proof course… Over time the movement of buildings caused by traffic vibration, changes in temperature and general wear and tear causes the DPC to become less effective.” 

The pathway being described is rising damp, which results from groundwater. No evidence of groundwater is provided. Groundwater, the water under the water table, is highly unlikely to be the root cause as the property is in London where groundwater is pumped out to more than 30M below ground. The British Geological Map shows “Rocks with essentially no groundwater” under this flat.

Walls have had the standard PCA chemical damp proofing and the Dutch method, neither addressed the source.

These two methods of treatments for rising damp also involve replacing absorbent, internal plaster with impermeable plaster to block or hide, rather than deal with the root cause. Both reduced permeability, and in the case of the Dutch method of aerating and therefore cooling the wall (red arrows) increase condensation risk, while doing little to hide moisture.

Primary cause of damp

Around the edges of the damp proofed wall, there are signs of brown staining.

The brown discolouration like a teabag stain is a sign of water moving through building material, typical causes are penetrating damp from rainwater, leaks and interstitial condensation. The wall is internal and above a solid floor. Therefore, penetrating damp and interstitial condensation are both very unlikely here.

The profile of dampness across a solid floor points to a leak. Loss of pressure suggests a mains water leak.

I tested for a leak with a mains gauge three times. Each time there was rapid pressure loss. There can be benign causes, such as a drip or cistern leak, I could not find another cause for the pressure loss. I could not see behind the dishwasher and washing machine, but if they were leaking the damp would be centred in the kitchen, which it is not. You should confirm the leak with a specialist leak detector before opening up the timber floor.

The floor is solid with a plastic membrane underlay, protecting the timber floor from moisture.

Cement-based screed floors are impermeable. Water and vapour can travel a long way between two impermeable surfaces. Screed can cause copper pipes to corrode and leak.

From the profile of damp, the most likely centre of the leak is somewhere along the front section of the corridor. This later proved to be incorrect.

Damp proofed walls can mask the centre of the leak. Consider instructing a leak detection company to use both a high-pitched noise detector and gas detector system to pin point the centre of the leak.

Only one section of timber appears to have been affected by moisture, that is in the bedroom, inside corner. This later proved to be incorrect.

There are early signs of rot, probably Coniophora puteana, or cellar rot, a brown wet fungus, see surveyor.tips/rot. Rot requires water to grow. Remove the water and rot will stop destroying timber. There are no structural implications.

There was also a high damp meter reading in line with pipes, with calcium sulphate salts, up to ceiling level.

I tested walls with a Protimeter damp meter in radio frequency mode. Water reflects radio waves at a set frequency. 999 REL indicates water absorbed at depth. High readings illustrate the extent and profile of dampness. Damp meters should never be used to determine the source of moisture without corroborating evidence.

Calcium sulphate is a key ingredient in cement and other building materials. Salts result from constant evaporation and condensation, commonly associated with excess humidity. Cold water pipes cause heat loss on nearby surfaces, increasing the risk of condensation.

There are condensation dribble marks on the bedroom external wall.
There are more calcium sulphate salts by the rear door to the kitchen.

The reveals around windows and doors are often poorly insulated increasing the risk of condensation. It is common for walls by kitchen doorways to suffer from condensation. It later transpired that the leak was near here, so the leak was more likely to have caused the salts, probably made worse by condensation.

Damp continues up the wall, in the cold window reveal.

The kitchen extractor fan appears to be sufficiently powerful and externally ducted, but is it used when cooking?
Also, I understand that tenants have been drying clothes in the kitchen during winter without ventilation. The extra weight of damp clothes is water. The drying process moves moisture from clothes to a cold wall unless a vent is provided. The bathroom extractor fan runs at 13.2 l/s which is only marginally below the Building Regulations required 15 l/s and better than most properties, but it does not stay on after lights are switched off. About 90% of vapour stays in a bathroom after a shower is turned off.

Given the signs and ricks of condensation, consider replacing the bathroom extractor fan with a continuous flow fan, such as the Elta Mori dMEV see recommendations. Consider installing a retractable drying rack above the bath and making tenants agree under the tenancy contract not to dry clothes internally without ventilation.

Heating systems can leak, but pressure is normal, and I understand it has not been topped-up for a while.

I considered the heating system as a source of moisture, but given the mains water pressure drop and normal pressure on the boiler’s dial, doubt it is a contributory factor.

Return after specialist leak detector failed to find leak

I re-tested the mainswater pressure and again found pressure loss when the flat’s stopcock was turned off.

This pressure test method for leaks is very sensitive and accurate. There can be benign reasons for pressure loss that are not related to a leak, such as a dripping showerhead. However, until the pressure stops dropping, it has to be assumed that there is still a leak.

There was a small leak on the stopcock. I don’t recall this leak.
Water was found dripping onto a magazine.

I repaired and stopped this leak by tightening the fitting.

Excess vapour causing: Mould

I checked all nains connections for leaks: taps, shower head, cistern and behind the washing machine.

During my original survey, I checked the assessable mains connections and offered to check the washing machine for leaks. However, your farther was reluctant for me to potentially damage the kitchen when it did not appear to be the central point. My PI insurance is as a surveyor, not repairman. This limits the amount of damage I can risk. On occasions, the only way to positively identify a source of water is by pulling things apart.

I removed the kicker plate, which was swollen and damaged from moisture, and unscrewed a piece of kitchen unit to the right of the washing machine to release it (blue arrow).A vapour source (local from penetrating damp and general from the poorly ventilated bathroom and kitchen). A cold external wall, relative humidity is a function of both vapour pressure and temperature. Reduced airflow such as behind books. Vapour moves through diffusion, like smoke, from high to low concentrations, furniture and other objects reduce the flow of heat from warm air currents.

White rot was growing immediately under the kicker plate, a sign that there is an ongoing source of water.Rot

White rot was growing immediately under the kicker plate, a sign that there is an ongoing source of water.

Cause of the leak

The leak was coming from the hot water pipe, upstream from the dishwasher inlet.

I tried tightening the red T shaped fitting and think I reduced the leak, but couldn’t eliminate it, so suspect that there is a defect with the fitting or pipe, and that it should be tested and replaced by a competent plumber. The mains water should be rechecked for pressure loss over 20 minutes, after replacement to ensure there is no ongoing or other leak.

Looking behind the unit, under the sink, we can see that the wall is exceptionally damp.

You asked:

  1. Why did water spread across to all four corners of the flat and appear to be centred in the corridor.
  2. How do we know if this leak is the cause of the all the dampness.

Answer in order:

  1. Past damp proofing treatment has obscured the effects of the leak, as has the kitchen. Looking behind units we see considerably more water than elsewhere. The damp is being transfer between an impermeable concrete screed and impermeable plastic underlay.
  2. I can’t promise that this leak and condensation are the only sources of moisture, but the profile fits with a leak. It is bad luck to a have leak, by exceptional to have two or more.

Data loggers

This graph comes from your data loggers, the property was vacant part of the time. Numbers detailed below.

Understanding the data logger diagram above

  1. During the period under investigation the surface relative humidity on the damp wall (green dotted line at the top of the graph) is at normal levels suggesting atmospheric is not the primary cause and not a concern, at least at this time of year. The property was vacant.

Conclusion from the data loggers

Vapour does not appear to come from the bathroom (albeit the property was vacant for part of the time). However, the data backs up my opinion that dampness is mainly a leak.

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