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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When to Specify Woodworm Treatment

Was woodworm treatment necessary?

timber decay

A screwdriver is useful for assessing timber decay but appears to be the diagnostic tool of choice for this particular damp proofing company.

I was recently asked to review a report, supplied by a large National damp proofing company, on behalf of a client, who spent thousands of pounds on unnecessary treatment, including timber preservatives. I say ‘unnecessary’ but perhaps it would be more correct to say that the woodworm treatment wasn’t proven as being necessary.

Perhaps it might be first useful to explain what woodworm are…

Woodworm

Woodworm is a collective term, which is used to describe all classifications of wood-boring insects.

Wood-boring insects use wood as a source of food, and in the UK at least, mainly consist of beetles.

Furniture Beetle, Woodworm

Furniture Beetle (Anobium Punctatum)
(Source: https://www.naturespot.org.uk/species/common-furniture-beetle)

The most common type of wood boring insect in the UK is the ‘Furniture Beetle’, or ‘Anobium Punctatum’ to give its latin name. It is brown, usually about 3-4 mm in length and has a prothorax that resembles a monk’s cowl.  Whilst less common in the UK, timber can also be infested by the much larger Death Watch beetle (Xestobium Rufovillosum) (which is considerably larger at 7- 8mm long), the House Longhorn and the Powder Post beetles. In all species there is a grub like larvae stage, lasting several years, whereby the larvae bores around under the surface of the wood. This is where the most damage occurs.

Note insect flight exit holes in timber

Note insect flight exit holes in timber

A furniture beetle’s Life cycle

The infestation process starts when the female beetle lays its eggs directly into the timber through cracks, crevices or even existing flight holes. The eggs are oval in shape and laid in batches of between 20-60. After about 5 weeks, the eggs hatch and the larvae (woodworm) tunnel down into the wood and spend the next 1- 5 years digging their way around munching through the timber until the next stage in its life cycle. It is in this stage that the greatest structural damage to the wood takes place, and this damage goes unseen due to its location within the timber. The larvae then carve out a ‘pupal’ chamber near the surface of the wood in which the larvae ‘pupates’ into a beetle over a 6-week period. The fully formed beetle then burrows its way to the surface and out, leaving a circular ‘flight hole’ about 1-2mm (roughly the size a dart would make in a dart board). This is the final stage of the beetle’s life cycle, with the females living for around 14 days, and the males only 4 days.

Wood boring beetles thrive in damp conditions; indeed many living trees can have beetle infestation. Species such as the furniture beetle prefer the softer ‘sap’ wood from which they feed off the cellulose, whereas the larger Death Watch beetle is especially fond of hard woods such as oak and beech. It is worth pointing out that the ‘heart wood’ of say, a piece of oak, is usually left untouched by beetles and just the outer sap wood is subject to beetle attack.

Signs of beetle attack

Again, the screwdriver is used in support of woodworm treatment with insecticide.

Again, the screwdriver is used in support of woodworm treatment with insecticide.

Apart from the obvious flight holes left by the beetles, a powdery dust called ‘frass’ can often be seen on floorboards and other wooden surfaces. Frass is the excreted wood that the larvae produce.

In old historic timber framed buildings, especially thatched cottages, there may be signs of beetle attack, evidenced by frass on the face of the timbers, in a honeycomb pattern. If the wood has clearly suffered beetle attack, there is a need to determine whether the timber still has structural integrity. The frass must be scraped off and the heart wood of the timber examined. It is very rare for a beetle attack to penetrate deeply into the heartwood of a piece of oak, so its structural integrity usually remains.

Old bore holes or new?

To establish whether beetles are still present in timber, where there is evidence of frassing, is quite straightforward. Once the frass has been scraped off, a piece of tissue paper can be stuck over several suspected areas of wood and sealed round all edges with a water soluble glue, and left from April to August (this is the flight stage in the lifecycle). On inspection, there may be larvae and beetles trapped or flight holes puncturing the tissue paper. All of these point to an active infestation. Also, checking spider webs in old timber framed buildings to see what’s entrapped can help to establish the types of insects that are present. You should also recognise, that many timbers were installed with insect flight exit holes, because wood boring insects also attack trees as well as cut timber. The majority of flight holes observed on timber from historic buildings are often at least 100 years old. This is because the infestation took place within the living tree before it was even felled, due to the moist conditions present in nature. Needless to say, whether the bore holes are old or new, forms a critical deciding factor in whether or not woodworm treatment is required.

Case Study

In august 2017, the property in question was subjected to a damp and timber survey, by a large national damp proofing company. The client knew they’d be expensive but wanted the assurance of dealing with a ‘brand’ name.

Damp & Timber treatments specified costing over £22,000

Damp & Timber treatments specified costing over £22,000

In total, this company specified over £22,000.00 worth of work, which included over £3000.00 for treatment of wood boring insects and timber decay. Post completion of the work, another surveyor attended the property and informed my clients that he was of the opinion that this work wasn’t necessary, and that my clients had been mis-sold these treatments. Obviously alarmed, they asked me to carry out a review of this companies survey report and survey methodology.

The Survey

There were many issues, both with their investigation for rising damp, and their assessment of timber decay, but for the purposes of this blog, we’ll remain focussed on their assessment of wood boring insects and the treatment specified.

The surveyor repeatedly made claims in his  report that there were, “current attacks by wood boring insects,” and of course used the word ‘current’ to justify his specification for timber preservative treatment.

In fact, no evidence whatsoever was presented proving a current attack, failing to even take or record timber moisture contents; a significant omission. BRE Good Repair guide 13, highlights the following as indicators of live infestation:

Indicators of insect activity are:

  • Freshly cut exit holes and recently ejected bore dust (frass), although dust may have been shaken from timbers by foot traffic.
  • Insect larvae extracted by probing the tunneled timber. In practice the larvae are difficult to find.Identification of the insect causing the damage is important, not only in deciding if any wood preservative treatment is necessary, but also in deciding if any other action is required.

From the flowchart below, it could be seen that the company concerned failed to demonstrate or prove that insect infestation was active, and therefore failed to reasonably comply with this guidance. If no live infestation was present, then the next step is to assess whether timbers contain more than 20% sapwood; if the answer is no, then no treatment is required.

BRE Good repair guide 13

Flowchart taken from BRE Good Repair Guide 13.

Further guidance is given in BRE Digest 307, which states that; ‘The presence of damage by wood boring insects does not always indicate a need for remedial treatment.’  The common types of damage found in building timbers have been divided into three categories:

BRE Digest 307

Taken from BRE Digest 307

Again, the table above highlights the importance of identifying the species before deciding on treatment measures, and throughout the report, wood boring insects were only discussed in general terms, with no identification being made. What we know unequivocally, that no proof. whatsoever was provided to demonstrate this timber treatment was necessary.

As discussed earlier, the client was charged over £3000.00 for timber preservative treatments, the insecticide, being a Permethrin based product. If you want to understand the mark up on these products, 5 litres of Permethrin based product, can be obtained from a brand name company, for around £9.00; enough to cover around 15-20 square meters of timber.

How we deal with woodworm organically

It is vitally important to get to the root cause of the woodworm issue so that a long-term solution can be implemented. Since the woodworm larvae thrive in damp timber, it makes sense to identify and remedy the cause of that damp, and by doing so the infestation will cease, and more importantly, not come back.

Generally speaking. woodworm like their meals with a little ‘gravy’ so you should focus on the root cause of the damp in timber and we would generally look to treat this issue in the following simple manner…

  1. Eliminate the source of moisture and introduce rapid drying
  2. Assess the severity and depth of timber infestation and damage
  3. Replace any structurally unsound timber
  4. If required improve ventilation to the affected area

It is a treatment plan almost identical to what we’d specify to deal with an outbreak of fungal decay such as dry rot.

It is a sad fact that the damp proofing industry has enjoyed great success by unnecessarily recommending that damp timber infested by woodworm is covered with litres of toxic chemicals. The main issue with this method is that it does not address the primary reason as to why the timber is infested in the first place, Since the timber is still damp after being treated with chemicals, there is a likelihood of re-infestation occurring.

Furthermore, these toxic chemicals can cause environmental damage and might even require a licence if being used in close proximity to any protected species. Thankfully though, the inadequacy of this practice is now becoming better understood and is well documented in BS7913: 2013 “Guide to the conservation of historic buildings”. It also identifies that chemical treatment does not actually penetrate deep into the timber, but only just under the surface and therefore any larvae, which are active inside, won’t necessarily be affected.

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