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104Y: Leak & hygroscopic salts

This ground floor Victorian conversion had a leak from a rear extension. The wall was damp.

Surveyor Tips:

  • Look at neighbour’s side.
  • Look for chimney breasts hat have been removed.

Root cause

Leak below rear extension and hygroscopic salts on removed chimney breast.


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By pouring water on the roof above your extension, we determined last summer, that there was a leak.

The roof above the extension has been replaced and you wish to ascertain whether the wall is dry enough to plaster or not.

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There is slight residual dampness in the mortar between bricks in the column supporting the roof.

Damp meter readings below 20WME are considered dry, above this level could be damp.

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I used to damp meter in radio-frequency mode to determine that the most of the brick wall has dried out.

Radio frequency meters detect density, such as from water, up to 70mm below the surface. They help trace damp in a normal, homogeneous wall. Below 300REL is consider dry.

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There is a high damp meter reading below the tiles, suggesting residual damp, which will take longer to dry.

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There are high damp meter readings along the wall behind the plaster.

A section of chimney has been removed. It is common for chimneys to contain hygroscopic salts causing condensation at normal levels of humidity, see surveyor.tips/hygroscopic. Calcium nitrate is the most common hygroscopic salt in a house. They are often released after a leak, flood and replastering such as following chimney removal.

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The surface is damp and looks dark like that of hygroscopic salts. I tested and found nitrates in a salt sample.

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There had been rot. Hygroscopic salts don’t cause rot. The rot would have resulted from the rainwater leak.

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I tested for a mains leak by turning off mainswater while the tap was open – checking if water was sucked back.

The water was not sucked back up the kitchen tap over about 20 minutes. I conclude that there is no mainswater leak.

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There is dampness to the neighbour’s wall.

I did not perform a full survey to the neighbours property, but the most likely cause in their property is the leak from your roof extension.

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Their wall is still damp. It will take longer given the plaster, typically about 10 months for a wall to dry out.

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Some of the wall is dry.

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Looking through a thermal image, there appears to be heat loss by the party wall. Evaporation causes heat loss.

The infra-red lens highlights heat differences, it can sometimes help understand thermal dynamics. Blue is about 5°C colder than yellow.

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There is dampness at skirting board level. It is possible that the cold pipe in the kitchen is reducing evaporation.

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I checked your neighbour’s under-floor heating floats used to indicate a leak. There didn’t appear to be a leak.

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The roof above your extension did not have any unexpected thermal bridges (blue – heat loss).

It would be wise to have ventilation and insulation in the flat roof to reduce the risk of interstitial condensation.


Hygroscopic salts act like grease stains, they are benign, cause no rot or mould, but are unsightly, especially in summer, and cause problems selling or renting a property. The best solution is to cover the salts, with a significant overlap of at least 300mm.

There are three alternative methods, the last is most applicable to you:

1) the simplest, cheapest, best used with small areas, is Zinsser Cover Stain, a solvent-based, primer-sealer. For each patch, it takes about 1 hour and costs around £10 plus any labour if required, the great thing with Zinsser is it is easy strip off and reapply.

2) Sempatap see https://www.youtube.com/watch?v=_R1B44fGttE, or Wallrock lining paper. This creates a slightly nonporous “cavity” separating the lining paper from a wall. It takes about 1/2 day and costs around £400 for Sempatap and less for Wallrock.

3) Remove the plaster and install thermal insulating plasterboard, with a vapour barrier from floor to ceiling. It takes about 1 day and costs around £250 per linear metre.

Given that you have already removed the plaster, in your case I would dry-line the wall, either with thermal installing plasterboard or, if you want to reduce the width, dry-lined plasterboard and an organic solvent based adhesive. If you look at a case of misdiagnosed rising damp in a barn in https://youtu.be/EeT05HxvAs0 at 1:28mins, you will see the result of dry-lining with a standard water based adhesive.


This report is provided to help the owner of the property as a courtesy, without payment or benefit from recommended actions and therefore without accepting any responsibly.

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