Cementitious Render Failure (Part One)

Cementitious Render Failure: Causes of Failure in External Render & What to do About it.

This will be a two part blog. In part one I shall discuss the causes of render failure and in part two I’ll cover remedial action.

Midlands property with substantial failure of external render system

Midlands property with substantial failure of external render system

Sometimes you get a run of particular problems to investigate, if two in a week can be considered a run. This week I was asked to investigate the cause of render failure on two separate buildings, one in southern England and one in the Midlands, though technically the Midlands building had already been investigated by a Chartered Surveyor. What I was actually asked to do on this building was to quote for writing the works specification and project managing the works. However, I was less than convinced at the previous surveyors conclusion that the render had failed due to ‘wetting.’  It was a vague conclusion that was of little use in understanding what specification was required so I went to see the building for myself.

Before I move on to the issues affecting each building it may be worthwhile outlining some general principles relating to the application of cementitious render.

Why do we render buildings?

Cementitious render  is applied to buildings for a number of reasons:

  1. It provides a weatherproof barrier to prevent penetrating damp into the building.
  2. It may be applied for purely aesthetic reasons to enhance a building with little architectural merit.
  3. It may be applied where buildings have been extended or altered and therefore have mismatched brickwork.
  4. It may be applied to hide underlying cracks in the masonry.
  5. It may be applied to hide heavy spalling of underlying brickwork.

Please bear in mind that we are not discussing EWI systems so improvement in the buildings thermal value is not a significant consideration.

General Principles for Installing Render

Render is mixed to a particular mix designation according to what is suitable for the background to which it is being applied, so you will only understand what specification is required after assessment of the building. It generally contains OPC mortar, a fine aggregate and possibly lime. Mix designations range from class i (Strong, relatively impervious with high drying shrinkage) to class v (weak mix to be used on weak backgrounds in sheltered locations).

I’ve previously discussed how mortar should always be weaker than the masonry units it holds together and similarly, OPC based render should be slightly weaker than the underlying substrate. This means that the  render mix proportions must be compatible with the underlying substrate. For instance you would not apply a class i mix designation to an old building constructed  with lime mortar, yet this is precisely what we see on many many occasions.

Rendering is generally applied in two or more coats, with the first coat being 9-13mm and the second or subsequent coats being thinner. However, the batch mixing must be consistent for each coat. There are some specially mixed renders that can be applied in single coats but where we see single coats, there is generally nothing at all special about the mix and this is generally a factor in its early failure.

In specifying render  you need to consider a range of factors:

  1. The type of aesthetic finish that you want.
  2. The type of underlying substrate, with particular consideration to strength and absorptivity.
  3. Local exposure to weather
  4. Primary purpose (Functional or aesthetic)
  5. Impact on buildings breathability (Is OPC render a suitable choice)

You will need to consider a number of factors relating to the underlying substrate and should give thought to:

  1. The strength of the masonry or panels to which the render will be applied. Strong masonry will require a strong render mix.
  2. The mechanical key. Some substrates such as concrete blocks offer a good mechanical key and even heavily eroded mortar joints can aid in providing a better mechanical key. Some backgrounds require you to provide a key for the new render.
  3. Resistance to damp penetration. Will the substrate provide this resistance if the render does not?
  4. The durability of the substrate. Some backgrounds such as wood will rapidly degrade if exposed to moisture.
  5. Suction of the underlying substrate. This is related to the absorptivity of the substrate. Some substrates have a high rate of absorption and will suck the moisture from the render before its had a chance to properly cure and bond to the substrate.

When applied the render should be fit for purpose, meaning it should both look good and provide adequate weatherproofing. It should also be highly durable and have good adhesion with the underlying substrate.

What Causes Render to Fail Prematurely? 

Sometimes the fundamental cause of render system failure is not immediately obvious and in such cases we may even send off samples of the render to our laboratory for SEM (scanning electron microscope) testing or indeed a range of other tests. However, in most cases  the cause of failure can be established during the site investigation. During the site investigation we assess the following range of factors:

  1. Was the general aesthetic finish acceptable? A poor finish is a prime marker for the installer having a poor understanding of the wider technical issues.
  2. Have bellcast moulds been properly specified and installed.
  3. Have window and door edge beads been properly specified and installed?
  4. Are we confident that this is an OPC, rather than lime based system?
  5. Is there any visual evidence of ‘shelling’ or debonding of the render system? This will often involve tapping the render in numerous locations to listen for that telltale hollow sound.
  6. Is the system intact or is underlying substrate exposed in places?
  7. What is the age of the system and might sulphate attack or generalised ageing be a factor? In constant wet conditions sulphates can be leached from the underlying masonry and can cause sulphate attack in the finished render.
  8. Are there underlying structural issues that may affect the integrity of the system such as subsidence.
  9. Are there underlying movement joints not replicated in the render system?
  10. Have building junction details been properly attended to? This means properly sealing window/door frame junction details with appropriate sealant and ensuring other potential points for water ingress, such as pipe or cable penetrations, are properly sealed.  This detailing is critical and yet is constantly overlooked.
  11. What surface finish is applied and has this been affective?
  12. Are there other failed critical building details, such as parapet walls, that will allow rainwater access behind the render system.
  13. Is the render system cracked and if so, what is the severity of cracking and are there any regular patterns to the cracking?
  14. If cracked is this because it is an overly strong mix. The stronger render are more susceptible to generalised cracking and shrinkage cracks during the drying/curing period.
  15. Is there any impact damage or local factors for building disturbance. I once assessed a failed render system close to a train track, which I felt sure was a factor in its failure.
  16. Is the underlying substrate sound?
  17. Is the underlying masonry saturated?
  18. Is the building highly exposed?
  19. Are rainwater goods sound and fully functional
  20. Is there adequate overhang at the roof eaves and verge details to prevent saturation of the render from rainwater runoff.

Case Study 1

Widespread cracking in 7 year old render system

Widespread cracking in 7 year old render system

Our first building relates to a substantially extended property in the south. The originally building had 225mm solid brick walls laid in Flemish bond. The building then had cavity walled extensions built on at either side of the property approximately 8 years ago. The brickwork was substantially mismatched so an external cementitious render was applied, primarily for aesthetic reasons.

What was immediately obvious was that the render had been installed to a very poor standard. The surface finish was poor and the render had been installed without the benefit of bellcast moulds to its base. The render showed widespread cracking on all elevations though the southwest and southeast elevations were more substantially affected by generalised hairline cracking.

Interestingly works had commenced to repair the render system and the contractor was midway through works when the alarm was raised with regard to potential wall tie failure and an alleged concern that this might be linked to failure of the render system. In fact there was no evidence of structural issues or wall tie failure but these concerns had stopped works until the alleged issue was addressed. The contractor has cut through all cracks in the render on the southeast and southwest elevations to open up the crack and provide a better key for the repair mortar and it was at this point that we inspected the building.

Widespread corrosion of incorrectly specified galvanised edge bead.

Widespread corrosion of incorrectly specified galvanised edge bead.

We believe that an overly strong render mix was used that suffered shrinkage cracks during the drying/curing period, hence the prevalence of cracking to the southeast and southwest elevations. However, the bigger issue was that edge beads had been incorrectly specified to the window and door reveals. British galvanised edge beads do not meet European standards and are not suitable for external environmental conditions found in the UK. Hot dipped galvanised beads, usually imported from Belgium, may be suitable but these can be difficult to locate so we would always specify stainless steel or UPVc edge and bell cast beads to prevent corrosion occurring within the render system.  In this particular case there was widespread corrosion within the edge bead system and they all needed hacking out and replacing with suitable edge beads. Rusting is an expansive reaction and will continue to cause the render to blow at its junction with the edge beading. Moreover from a purely aesthetic consideration, the edge bead corrosion was causing substantial and unsightly iron oxide staining to the render system.

Given the additional scope of works specified and the poor quality of the original installation we believe that there is a strong argument for complete system replacement but a cost benefit analysis will help the client decide on which course of action to take.  I’ll deal with case study two in part two of this blog where I’ll discuss in more detail what can be done to repair or replace failed render systems.

Widespread cracking to southeast elevation mid way through being repaired

Widespread cracking to southeast elevation mid way through being repaired. Will repairs be cost effective?

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The Hidden Dangers in Buying a New Home

The Hidden Dangers in Buying a New Home

New homes are a risky proposition

New homes are a risky proposition

Why would you consider buying a new home, is it simply because you want that feeling of owning something new? Perhaps you want cutting edge design and modern standards of thermal comfort with low energy bills, or perhaps you simply feel that buying a new home gives you peace of mind that nothing could possibly go wrong? You may even have reasonable expectations that nothing is perfect and perhaps you may find one or two snags but so what, your constructor will willingly correct defects in the first two years and after that you have NHBC insurance to cover further eventualities.  Moreover, even your solicitor has advised you not to bother getting a full survey because its a new property and you’re covered by the construction guarantee and NHBC warranty. Well a qualified solicitor must know what he’s talking about and given the expense in moving home you’re predisposed to taking this advice because you’re trying to manage a budget. Of course your solicitor is absolutely right in principle but in practice he is very very wrong because there are many hidden dangers in buying a new home


An old traditional property is surely a greater risk to the home buyer than a new build isn't it?

An old traditional property is surely a riskier proposition tor the home buyer than a new build…  Isn’t it?

I write this blog very conscious of the fact that I’m in a ‘well you would say that wouldn’t you’ position since I’m advising a course of action that I may potentially profit from, but to be frank this information is only going to come from a surveyor dealing with these issues at the sharp end.  I’ve carried out many detailed snagging surveys on new build properties and I can not remember ever signing off on a property that only had reasonably minor defects. So what is going wrong?

The Decline in Quality Standards

Firstly there has been a gradual and systematic decline in quality control linked to a number of fundamental issues:

  1. Developers self manage their  quality control pprocess. The Site managers primary focus is on managing productivity and site efficiencies and you will rarely see them employ a Clerk of Works anymore to oversee quality control.
  2. A mistaken belief that either building control or the NHBC are responsible for managing the quality control process. They aren’t.
  3. Big developers deliver the larger portion of their works through a wide range of sub-contractors and often very little of the work is completed by in-house staff. Developers then expect sub-contractors to self manage their own quality control process but they’re under the same commercial pressures as the developer and time is money.
  4. The lack of time served tradesmen and their subsequent replacement with multi-skilled, jack of all trade, operatives.


Delamination of solid floor in 6 year old new build.

Delamination of solid floor in 6 year old new build.

5.  British workers are increasingly refusing to carry out works on a price and this has resulted in an influx of Eastern European workers on the large development sites and even fundamental  communication problems on site. I often find myself on development sites where I genuinely struggle to encounter staff that speak English.

6.  The advent of MMC (Modern Methods of Construction) and ‘Lean’ construction principles has resulted in the development of some truly terrible composite products. For example, there are some really shocking composite I-beams on the market that are both cheap and nasty. This has resulted in a marked increase in the number of floors that are flexing under load, something that was rarely a problem when standard timber floor joists were used. MMC is actually the modern version of non-traditional construction and will have a significantly reduced life span when compared to traditional construction erected during the Georgian or Victorian era. It is being optimistic to expect a 100 year life span from MMC construction and I believe that most will have a significantly shorter lifespan due to poor  quality materials and low architectural merit.

7. The move away from traditional skilled building and plumbing techniques. Most central heating is plumbed in on cheap plastic push fit pipework and soldered copper pipes are in decline, you’d be amazed at the number of flooded properties we see due to poor plumbing. Roof leadwork is also being severely de-skilled on new build sites with leadwork sealed in place with mastic rather than being properly installed. Push fit pipework relies on a rubber o-ring to make a seal but how long do you think this o-ring lasts when compared to soldered copper pipework. I’ve always held a view that plastic push fit pipework is OK for Portacabins or other temporary installations but it is now in common use.

8. The lack of a site benchmark test wall. There was a time when a standard test wall was constructed on every site and used to benchmark the site build standard. You rarely see this anymore, though we’d advise that you should use the site show home as your benchmark build standard. They are generally significantly better than anything else constructed on site.

9. New home buyers rarely have a detailed snagging survey carried because why would you need to check build quality on a new home? Possibly even your solicitor informed you that a full survey was not required.  This is yet another missed opportunity  to challenge the developer on quality control and another factor for increased  levels of complacency on site. Incredibly we have even been refused access to site to carry out a quality inspection until the buyer had signed the contract.

10. Many defects are ‘patent’ or immediately obvious, but many are ‘latent’, which means they are hidden. If not picked up during key stages of construction inspection then often they’ll remain hidden until failure occurs. Key construction stages are simply not being checked and signed off as they once were. It has been said that this ‘removal of red tape’ facilitates the construction process but in reality it has meant further erosion of the quality control process, which has facilitated poor quality development.

So what can be done? 

No provision made for surface drainage and rainwater breaching door threshold and entering lounge. The property was 7 years old.

No provision made for surface drainage and rainwater breaching door threshold and entering lounge. The property was 7 years old.

Firstly accept that there is nothing that you can do as a potential purchaser of a new home to deal directly with site related issues. There is a National housing shortage and developers generally sell new homes as quickly as they are being built. New housing is what economists call a highly elastic product with demand often exceeding supply. Despite this, it was  reported in December 2015 that Britain’s nine biggest housebuilders have land banked 615,152 housing plots that have not yet been developed – four times the total number of homes built in the past year. The top four firms – Berkeley, Barratt, Persimmon and Taylor Wimpey – also hold £947million of cash, according to an investigation by The Guardian. Developers generally like to see a 25% gross profit on development, which compares very favourably to the wider construction industry. Land banking brings to mind a very interesting fact about the diamond trade… Did you know that diamonds are not actually that rare. In fact the market is carefully controlled by DeBeers who hold a massive stock of diamonds and carefully control their limited release into the market place. If Debeers chose to flood the market with their vast stockpile then diamond prices would collapse.  It is to my mind a thought provoking comparison.

We  speak from experience when we say that we have formed a firm opinion that buying a new home is a far riskier proposition than buying an old traditionally constructed property.  Of course many buyers of older properties would not consider signing off on what could potentially be their biggest ever purchase without investing in the surety that comes with a detailed home buyers survey. Very few new home buyers make that same investment because they have been lulled into a very false sense of security. Despite this, very few want to go through a process of suing their developer for breach of contract when things go badly wrong and why should they? They expect developers to remedy defects in the first two years and the NHBC Guarantee to cover the remaining 8 years. In fact very little is covered by these guarantees and claimants often realise that they are dealing with building guarantee companies with a general predisposition towards claim refusal. I recently enquired about buying a new home from a large National developer and I rang their sales office to discuss my requirements. Hello I said, I’m interested in buying a new home but I have no interest in the NHBC guarantee and  in fact I would insist that my contractual arrangement remains with you, the developer. I explained why I had no interest in the NHBC warranty but the sales person was stumped by my enquiry and promised to call me back once they’d looked into the matter. As expected, I never heard from them again. Far too risky for them to assume full responsibility for their own products.

Shocking standard of lead flashings and lead soakers on Durham new build

Shocking standard of lead flashings and lead soakers on Durham new build


In terms of dealing with what developers call ‘legacy’ issues, some are far better than others and again we speak from pragmatic experience. Some developers do have legacy people who will genuinely try to deal with customer complaints and correct your defects. Then there are some who simply ignore customer complaints and hope the customer goes away and sadly many customers do just that out of sheer frustration.

Be Objective Enough to Walk Away

As new home buyers we have to learn to be far more careful and far more discerning because development standards have been in decline for quite some time. I recently had a potential client who emailed me a number of pictures of his potential new home, which was under construction. He had numerous concerns about the build quality and asked me for my thoughts on correcting the issues. I agreed that it was shocking and my advice was “since you’ve not signed the contract then walk away now.” I never heard from the chap again because I suspect he did not get the answer he wanted; he was committed to buying it no matter what! Sadly the heart often rules the head when buying a new home.  If new home buyers consistently appoint a Chartered professional to carry out a detail snagging survey ahead of signing the contract then build quality would increase dramatically but as potential purchasers we also have to be prepared to put head before heart. Once you have signed the contract, all is not necessarily lost because you have up to 12 years to pursue a claim against the developer for breach of contract. Developers may like to think that risk transfers to insurers like NHBC after two years but this is not necessarily the case. If you have notified your developer of a defect within the first two years then you have between 6 or 12 years to pursue a claim from the date of cause of action. The NHBC would simply to refuse to pay out on any claim which had been notified to the developer within the first two years anyway and would likely refer it back to the developer for resolution. Insurers will not insure against pre-existing defects and why should they.

Whatever assumed level of protection you think you have in buying a new home please be forewarned that everything is geared towards providing very little protection. I recently read that you have more consumer rights when buying a tin of baked beans and I have far too many clients for whom we have discovered major defects on their new homes to believe otherwise. If ever there was a case of Caveat Emptor then purchasing a new home is it!











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