Rising Damp in 3 Year Old Extension

This week I attended a property in London where the client was suffering from quite significant decorative spoiling, caused by rising damp above the skirting boards in the rear extension. She had only recently purchased the property and given that the rear extension was only around three years old, and works were signed off by her local building control department, understandably, she was a little worried by this.


The extension was of cavity walled construction, with a lightweight concrete block inner and outer leaf of masonry, and an 80mm cavity, fully filled with Rockwool batts. One initial observation showed that the sidewall of the extension was unfinished concrete blockwork, where even the mortar joints had not been pointed up. External ground levels were also significantly too high.

Exposed concrete blockwork
Exposed and highly porous concrete blocks left exposed to the elements. The wall had not even been pointed up.

Moisture Readings

Moisture readings to the hard plaster system were only slightly elevated, but high enough to warrant further testing for moisture at depth using calcium carbide.

High damp readings
Protimeter MMS2 shows slightly elevated readings above the 20% trigger point, where testing for moisture at depth is required.

We carried out testing for moisture at depth using calcium carbide and found a very high reading of over 20% total moisture content at wall base, and 12% at the next highest level. A rising damp moisture profile, showing that moisture is being sourced from the ground. Of course, you have to contextualise these readings in light of the material being tested, and these highly porous concrete blocks will soak up moisture very quickly. Had these been brick walls, then moisture readings would have been significantly reduced.

Calcium carbide testing
Saturated concrete block walls with rising damp moisture profile

How could rising damp affect relatively new construction?

Since we know that ‘Part C’ (Resistance to Moisture) of the building regulations requires the damp proof membrane (DPM) in the floor to form a continuous barrier with the damp proof course (DPC) in the wall, then clearly, if those guideline had been followed, then the property would not be suffering from rising damp. Invasive work was now required to locate the damp proof course in the wall, to firstly ascertain if a DPC was present, and if so, was it linked to the DPM in the floor, and was it possibly bridged?

We started to remove plaster from the wall base and thankfully, located the DPC in the wall, which proved to be bridged by the internal plasterwork. This also showed that the floor DPM was not linked to the DPC, a clear breach of building regulation requirements.

Bridged damp proof course
Bridged damp proof course not linked to DPM in floor.

Bridged Wall Cavity?

External observations showed high ground levels and with the render extended down to ground floor level, then clearly any DPC present in the outer leaf of masonry had to be bridged.

Bridged DPC
External render will bridge the physical DPC

This issue should not present a significant issue with regards to moisture transferring to the inner leaf of masonry, so long as the wall cavity is clear of debris, since the cavity will form a ‘moisture break.’

However, with the high moisture content recorded to the inner leaf of masonry, we suspected there was a problem with the cavity and opened the cavity up for inspection. As suspected, we found significant amounts of debris, bridging the cavity at wall base. The debris serves to transfer moisture across from the outer leaf of masonry, to the inner leaf of masonry.

Debris in wall cavity
Debris in wall cavity

Incidentally, the wall cavity was inspected with a borescope, but full fill insulation prevented a proper view of the wall cavity, which is why I decided to open up the cavity at the door reveal base.

Should we call in the damp proofers?

Had the client called in the damp proofers, it is almost certain that they would have diagnosed rising damp, using a hand held electronic moisture meter, recommended that the walls be injected with a retrofit damp proof course, and re-plastered the internal walls with a waterproof tanking plaster or render. The water proof tanking, may have provided a dry wall surface for a while, but would no doubt have failed in the not too distant future, since the underlying problem had not been addressed.

It will be almost impossible to remedy the lack of bond or connection between the DPM in the floor and the DPC in the wall but addressing the issues that can be easily dealt with should remedy this problem. Key actions to address this problem will include:

  1. Remove the skirting boards and remove all plaster from the internal wall base to fully expose, and un-bridge the physical damp proof course.
  2. Opening up the wall cavity to remove all debris, which is transferring moisture from the outer leaf of masonry to the inner leaf of masonry.
  3. Ideally, reduce external finished floor levels, so that they are a minimum of 150mm below internal finished floor levels.
  4. Remove external render from the wall base, ensuring that the render is a minimum of 150mm clear of external finished floor levels.

None of this is specialist work, and can be carried out by any reasonably competent builder., as is often the case when it comes to remediation work for damp.

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Magnesite Floor Failure

Property In London W7, with Magnesite Floor Failure
Property In London W7, with Magnesite Floor Failure

I’ve recently just completed a long Roadshow for the RICS, ironically, I was giving a talk on solid floor defects, and included a section on Magnesite floor failure, as a refresher. In the last talk I gave, I asked an audience of about 70 Chartered surveyors, how many had seen a magnesite floor recently; the answer was none. I myself have not seen for for quite a few years but only a week later, I attended a property in London to investigate an alleged problem with sulphate attack, and guess what I found?…

What is Magnesite Flooring?

Magnesite, or Magnesium Oxychloride, was a product widely used by the flooring industry between circa 1920 and 1960. It was especially used in industrial premises, as it was resistant to oil spillages; however, it was frequently used in residential premises.

It is a water based product, commonly reddish pink in colour, though can be pale yellow, or any other colour specified.

Magnesite Flooring
Magnesite Flooring

Magnesite flooring was made from a mixture of calcined magnesite and magnesium chloride solution with various fillers (e.g. wood flour, sawdust, asbestos).• It was typically laid between 10 and 25mm thick, but two coat applications could be up to 50mm thick. 

What’s wrong with Magnesite?

Firstly, Magnesite contains chlorides, so if there is any embedded steel reinforcement within the floor slab, then the concrete can be affected by Chloride attack, which will corrode the embedded steel. Corrosion is an expansive reaction, and cracking of the concrete is likely to occur, as the steel corrodes. You should also consider, that steel water pipes may be buried in the concrete, and these are equally at risk.

Many old concrete floors, do not have a damp proof membrane installed, DPM’s came into common use in the mid 60’s, but prior to this, many concrete floors had a waterproof oversite, a layer of bitumen was commonly used. It would be wrong to assume that Magnesite provides that same protection against damp, and in fact they are very vulnerable to dampness. Magnesite is water soluble, and will return to its previous state if exposed to enough water.

The Asbestos Risk

As discussed earlier, Magnesite can contain asbestos fibres, as a filler. Commonly, the way to deal with asbestos, once identified, is to remove it, using a licensed contractor, or to encapsulate it. However, you can’t encapsulate a Magnesite floor, as they are so vulnerable to deterioration when exposed to water. Obviously, if you tried to encapsulate by pouring a screed over the top, then you’d be introducing large amounts of construction moisture into the Magnesite. The underlying Magnesite, would then most likely turn to a Weetabix type consistency, and start to break up, leaving you with no suitable substrate support below the screed.

Magnesite Floor Case Study

In this particular property, a Chartered surveyor had recently attended, for a pre-purchase survey and noted heave, or an uneven concrete floor below the carpets; he then of course raised the alarm for a potential risk of sulphate attack.

I attended to sample the floor, but on pulling up the carpet, the cause of this uneven floor, was clearly Magnesite floor failure; the Magnesite having got saturated, subsequently expanding and causing large blisters in the floor.

It was still important to investigate the situation with the underlying concrete and I excavated a hole through the slab to sample both the concrete, and the underlying hardcore. However, on breaking through the 8″ thick concrete slab, we found that there was no hardcore, and the slab sat directly on wet clay, with no DPM installed. This of course means that the concrete is in direct contact with ground sulphates.

Concrete slab sitting directly on clay
Concrete slab sitting directly on clay

The concrete was also notably wet, and this moisture had transferred to the Magnesite, causing it to heave up, blister, and crumble. From the image below, you can see how the magnesite had delaminated from the concrete substrate, forming large blisters, which crumbled when you stepped on them.

Failed Magnesite Flooring
Failed Magnesite Flooring

We did take samples of the concrete for sulphate tests, but with a saturated slab and widespread failure of the Magnesite, my advice was to renew all the solid floors, with the only test required being for asbestos. Testing the magnesite for asbestos, was critical before any works to remove the flooring could proceed.

One final note worth mentioning for any surveyors looking to check for dampness in Magnesite. Magnesite is electrically conductive, so if using a hand held electronic moisture meter, it will always give a high reading for damp.

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