102Z: damp proofed lower-ground-floor

The owner left this lower ground floor garden flat vacant for months with no heating and no essentially ventilation.

Our buyers have asked for a price reduction on the property of circa 5K in reflection of Tapco’s and Kenwood’s quotation for remedial works, here are some of the sentences from the letter describing the work they believe is required given Tapco’s finding. We have not yet been given the full report.

The main finding in the report was serious damp issues throughout the whole flat. 
Both of these surveys reinforced the finding of rising damp caused by existing damp proof course having partially broken down or being bridged. Both surveyors [contractor quotes] informed us that work is essential before we are able to move into the flat. 
Also, we would need to redecorate the areas where work is carried out at additional cost to us. 

“First and foremost we do not want our buyers to be effectively ripped off, or allow a company to undertake invasive work that’s not needed or is inappropriate causing them moving in delays or un-needed stress. Most worryingly the claim that they are advised that the work is essential before they can move in, seems to be scaremongering as we strongly  believe the property is 100% habitable at the moment.”

Root causes

The root cause vapour from condensation and hygroscopic salts.

Surveyor Tips

  • Look for damp areas at the base of a wall. If sections of a wall is dry, there there can’t be rising damp, as groundwater rises evenly up a wall.
  • There is some degree of condensation in virtually every property. If there is no decorative spoiling, i.e. surface salts or colour change, why replace the plaster, causing potential problems going forward – see example of damage caused by replacing plaster.
  • Scan the chimney breast with a Protimeter in radio frequency mode, as you can normally pick up dampness from hygroscopic salts.
  • Hygroscopic salts may not show until it is humid, or a wall becomes damp from replaster, wall paper or other means, after which damp will show, and you won’t be thanked by the new owner.
  • You can save many thousands from needless chimney removal, roof replacement or replastering when in fact you you need to do is create an impermeable barrier, such as with an oil based primer.

Illustrations

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There were high damp meter readings at the base of the wall.

I tested the surface on the inside at the base of all external walls every metre, chimney breasts and a sample of internal walls with a Protimeter damp meter in conductance mode. These meters measure electrical conductance of salts in water, a proxy for damp. Readings below 20WME are dry. The range is 8WME to 99WME, see surveyor.tips/dampmeter. Walls measured were largely dry on the surface, most high readings were only marginally high. There is no sub-floor timber, so no further investigation into the root cause is necessary.

There were no stains, surfaces salts or damp patches, therefore no treatment is necessary.

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Many of the damp meter readings are low at the base of the wall. This is inconsistent with rising damp.

Groundwater is the source of water in rising damp. It rises horizontally upwards like floodwater or paper dipped in water. Later in this report I will detail why there is no risk of groundwater, and therefore rising damp. Groundwater can be easily evidenced by drilling about 1M below ground to find out if the base of the wall could be in contact with the water-table. Water rarely rises up more than two courses of bricks, about skirtboard height.

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There was a marginally high damp meter reading in the rear room.

There was no evidence of decorative spoiling in the rear room. The meter reading is well far more consistent with condensation, the primary cause of dampness in ground floor flats, and not especially consistent with rising damp, penetrating damp, or a leak.

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Again in the rear room the meter reading is low, i.e. dry, again inconsistent with rising damp.

The radiator in the rear room is away from the wall, this increases the risk of condensation.

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High meter reading by the front door, despite it being higher than the base of the dry reception wall.

Condensation will tend to form at the base of the walls and openings because this is the coldest part of a room. Gravity draws dampness down. A damp brick has half the insulating capacity of a dry brick, therefore condensation can become self-perpetuating if occupiers don’t ventilate sufficiently.

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There is a slightly higher damp meter reading in the bathroom.

Other matters

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Looking at the expose bricks, we can see a dark stain on the wall.

This dark stain on the external wall is consistent with hygroscopic salts.

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You can see the dark hygroscopic stain above the kitchen.

Hygroscopicity is not a source of moisture, but describes a force of attraction between certain salts and water. Dark bands are a symptom, normally of manure or historic burning of timber or coal. However, the PCA and some PCA contractors often state, without any scientific evidence, that these bands are a sign of rising damp. See later, and in video letter I sent to the CEO of the PCA on this matter.

It is true, groundwater contains calcium nitrate, a hygroscopic salt. But there is no scientific evidence of groundwater rising up to about 2M, as seen on this wall. Besides the slope is inconsistent with rising damp. It is more likely to result from rainwater splashing manure, from horses in Victorian times, onto the wall and up through repeated rain and evaporation.

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There is an off the scale damp meter reading on the chimney breast about 1.8M high.

There is no sign of dampness on the chimney breast, even when I tested the wall with vapour from a boiling kettle. However there is a chance that in summer-time damp patches will show on this wall. The simplest solution is to use Zinsser solvent-based stain block, it is more reliable and cheaper than damp-proofer’s of slurry, see you later in this report.

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Looking inside the chimney you can see dark patches of hygroscopic salts resulting from the burning of fuel.
This damp brick is above the opening and therefore could not result from rising damp.
I took a sample of salts and tested them for nitrates, using the Protimeter nitrate tester.

The right-hand sample, showed deep red, a positive for nitrates, and compares to the left-hand example where no nitrates were found at another property, on another survey.

The most common hygroscopic salt found near chimneys is calcium nitrate. It is a natural fertiliser, found in soil. Condensation starts above 50% RH, i.e. normal levels of humidity.

Rising damp risk assessment

Elevation is: 31M above sea level.                 The flood risk is: no risk.

Sub-soil rocks are: Rocks with essentially no groundwater.
Signs of groundwater: there are no signs of groundwater.

Therefore the risk of rising damp is a remote possibility, see a good explanation by Dr Robyn Pender of Historic England  https://youtu.be/Jo8oF9ubvtI

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The building has been damp proofed against rising damp.

This Dutch system of drilling out vent holes is more destructive than chemical damp proofing, hear what Dr Robyn Pender of Historic England says about them.

Despite the low risk of rising damp, there has been an attempt at a treatment against rising damp. Rising damp treatment is normally a sign of insufficient ventilation. The wall have been treated using the “Dutch” method, whereby holes about 1 brick deep and 50mm wide are drilled into the wall. The theory being that dampness in the brick will evaporate out. The side effect of this approach is reduced insulation increasing the risk of internal condensation.


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