103G: Render crack and hygroscopic salts

This Victorian semi-detached London house has dampness up the flank wall. The wall had been render. There were cracks.

Surveyor Tips:

  1. Penetrating damp normally has a brown stain.
  2. You would not expect to see penetrating damp on the ground-floor chimney breast, which is why hygroscopic salts are likely.
  3. Visible surface salts are normally calcium sulphate a key ingredient in cement and mortar.
  4. Visible surface salts are a signs of water in flow and evaporating.
  5. By contrast hygroscopic salts are generally colourless.
  6. It can be hard to determine the root cause when two causes are mixed together.

Root cause

Penetrating damp from render cracks mixed with hygroscopic salts from the chimney breast.


The most visible signs of dampness are on the left of the chimney breast in the rear reception.

I understand that a large section of the wall was replaced by damp proofers, presumably with damp-proofing slurry which is impermeable reducing the absorption of vapour. 

There are brown stains and surface crystals on the ceiling.

Staining results from water passing through bricks and over timber. These stains are a sign of a leak or penetrating damp, that is rainwater coming from the outside.

The pattern of them continues onto the chimney breast.

Light brown staining it’s evident through the paint

There crystallise salts on the surface.

There are calcium sulphate salts on the surface. Calcium sulphate is a key ingredient in cement and other building materials. If diluted in water salts tend to move to the surface. These can be removed with sandpaper and decorated.

Well I think the primary cause of dampness is rain penetrating through external render cracks. Given that it is a Victorian chimney breast which is been damp proofed, I can’t help thinking that hygroscopic salts are a factor. See you later in the report.

In the second bedroom, on the wall above, there is a high damp meter reading in radio-frequency mode.

Water reflects radio waves at a set frequency similar to mobile phone shields. Radio frequency 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. 

The render is cracked external. These cracks result from expansion and contraction.

Water is absorbed through cracks. Small cracks act like capillaries drawing water in. Water will flow down the render until it meets a brick, from there it will pass it to the building. 

Generally speaking, horizontal cracks draw in more water than vertical cracks. Likewise a crack low down a wall will cause more ingress than one higher up.

There is a risk of rot. I tested the subfloor and found no signs of rot but I was unable to test the first floor ceiling void. The risk is low. Consider drilling through the floor testing for high humidity, the sign of rot, with a hygrometer probe.

It looks like there has been a past attempt at filling the cracks.

Rendered walls, especially on the sunny side, are always at risk of cracking. From a damp perspective, it would be better to just have exposed bricks as the absorption and evaporation is more evenly distributed, like an overcoat. Conversely, render works like a raincoat. As a minimum rake out and fill with an external acrylic caulk.

Consider both filling and covering with external insulation.

Looking through a thermal imaging camera we can see the chimney breast appears warm during the day.

The issue with chimney breast is there is a flue running down inside with a single brick between what is effectively the cold outside air and the inside. So when it is very cold outside a chimney breast can lose a large amount of heat, increasing condensation risk.

Most properties have some condensation risks.

The kitchen extractor fan it’s not externally ducted.

The ground floor bathroom extractor fan is running at around 20% of building regs requirements.

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.
Clothes are being dried internally which is quite normal, however it will have an impact on condensation risk.

Other matters

There was a leak stain which I tested and found to be dry.

Leave a Comment

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.