What Causes Render Damage – Cementitious Render Failure (Part Two)

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

When I started writing this blog I originally assumed that would be a two part blog but as it transpires it will need to be in three parts since the scope of issues under discussion is so broad.  In part three I will discuss specification and application of renders in more detail. In part one of this article I discussed the items that should be checked on site when investigation the failure of external cementitious render and before I examine potential remedial works I’d first like to discuss modes of failure.

Render can fail in a number of ways and failure need not be isolated to one particular mode of failure. It is not unusual to see more than one failure mode, particularly if you are dealing with more than one elevation of a building.

Failure modes may fall into the following categories:

Shelling or Debonding

Cement render debonded from whole front gable

Cement render debonded from whole front gable

In this particular image the render was thinly applied in one coat and the render had debonded from the whole front gable. A few very light hammer blows were all that was required for the render to fall away from the building. In this particular case the render was serving a weatherproofing function as the underlying brickwork was roughly  constructed and poorly pointed. The building was suffering from rainwater ingress due to complete failure of the external render system.

When shelling or debonding occurs, the render becomes structurally unsound due to not being adequately bonded to the underlying substrate, this happens  for a number of reasons.

  1. High suction in substrate not recognized and addressed during installation.
  2. Sulphate attack: Ettringite formation between render and masonry blows render bond with wall.
  3. Freeze/thaw action: water gets in behind render and freezes at the render/wall interface. Since water expands as it freezes the resultant hydraulic action blows the bond between the render and the substrate.
  4. Differential movement/expansion: Render characteristics not matched to the underlying substrate and substrate moves or expands and contracts at vastly different rates to the render. It is impossible to maintain a durable bond under these circumstances.
  5. Substrate poorly keyed to in preparation for the render and fully reliant on an adhesive bond, whereas a mechanical keyed bond is also required.


The render exhibits signs of cracking that will allow rainwater ingress through the cracks and consequently a risk of penetrating damp. The render may crack for a number of reasons.

  1. Shrinkage cracks due to render being applied in poor weather conditions.
  2. Chemical action: Sulphate or chloride attack.
  3. Differential movement/expansion: Render characteristics not matched to the underlying substrate and substrate moves or expands and contracts at vastly different rates to the render or junction detailing with incompatible materials fails to account fore differing rates of expansion.
Movement in steel lintels not accounted for and subsequent cracking to render

Movement in steel lintels not accounted for and subsequent cracking to render.


4. Hardness and subsequent inflexibility. Cement render is essentially a large inflexible sheet that cannot accommodate movement over large areas.

5. Junction detail failure. There is a frequent failure to adequately seal junction details with a suitable mastic or sealant. Rainwater then enters at these junctions where it can freeze and cause debonding and cracking.

Protective film not removed from windows and unsealed junction with render. A direct pathway for rainwater ingress.

Protective film not removed from PVCu windows and unsealed junction with render. A direct pathway for rainwater ingress.

6. Structural cracking. If the building is affected by subsidence or other structural or impact damage then it is inevitable that this damage will be mirrored to a greater or lesser degree in the render. Render can be repaired or replaced so long as you are satisfied that the underlying cause of failure has been addressed.

Cracked render caused by underlying structural movement. Note spreading roof tiles near roof verge.

Cracked render caused by underlying structural movement. Note spreading roof tiles near roof verge.

Consequences of Cracking

We have something of a chicken and egg situation when it comes to cracking. Is the cracking the primary cause of render failure or has the cracking resulted from from the primary cause of failure? Either way, cracking is not just an aesthetic consideration and cracks will form a direct moisture pathway for rainwater ingress behind the render system. Once there moisture can cause direct penetrating damp and  further cracking via hydraulic freeze thaw action and subsequent debonding. This process is self perpetuating and the trick is to establish whether cracked render is recoverable or whether it should be written off.  This can be a very subjective assessment but it needn’t be if a cost benefit analysis is carried out to establish repair versus renewal costs. Of course, you should really only consider repair if you are satisfied that the render is compatible and correctly specified in the first place.


We were commissioned to investigate why this render had failed prematurely

We were commissioned to investigate why this render had failed prematurely

Surface erosion is a fairly uncommon form of failure when dealing with cementitious renders. The accompanying image shows severe erosion in an external render system that was incorrectly specified to a London Docklands commercial property that was converted for residential use. The building was circa 200 years old and constructed with lime mortar, so to replicate the breathability and the required degree of softness the installer blended an extremely weak render mix comprised almost entirely of sand with very little Portland cement added. The surface was highly friable as a result and was literally being washed away by rainfall. It was also generally saturated at depth, which was causing secondary erosion through hydraulic freeze/thaw action. The system was also causing a number of problems with internal penetrating damp. Of course what should have happened here is that the installer should have specified a lime render system but wrongly assumed that limes characteristics could be replicated in a weak OPC render mix. The entire  system had to be removed and replaced with lime render.


Material Incompatibility

Hard OPC based render on historic property.

Hard OPC based render on historic property.

Old and historic buildings may be constructed of softer gauged brickwork and lime mortar, they are meant to breathe and will go through seasonal wet/dry cycles as they manage moisture; this is precisely what they are meant to do and applying hard cement renders will completely interfere with this process and cause a number of unintended consequences.  If buildings predate the Victorian era and originally had render applied this is likely to be a weak natural hydraulic lime or a non=hydraulic lime system; both of which allow the building to breathe and are soft enough to accommodate some small movement in the underlying substrate. Lime renders of this sort even have the ability to self heal where small cracks occur.  If original render fails, which of course it will over time, then it should be replaced on a ‘like for like’ basis. If you are using a non-conservation specialist then watch them like a hawk wherever lime renders are specified because they sometimes like to sneak a bit or portland cement into the mix in the mistaken belief that this will improve the mix. In fact it will virtually nullify any benefits that would have been gained from using the correct lime mix.

In this image we were dealing with a very old property in Leicestershire that was suffering from a number of issues with internal penetrating damp. The render was applied in an Ashlar finish but was severely cracked on all elevations and testing a small area at the corner of the building confirmed our worst suspicions that a very hard OPC render mix had been used that was completely incompatible with this building. It is one of those occasions were you hope that a very poor bond has been achieved with the underlying substrate but of course it was firmly bonded and incredibly difficult to remove.

Failure to Replicate Underlying Movement Joints

Cracking caused by failure to replicate underlying movement joints.

Cracking caused by failure to replicate underlying movement joints.

It stands to reason that if the underlying substrate has inbuilt movement joints then these need to be replicated directly above in the render coat. If the render coat cannot accommodate and mirror the underlying substrate movement then it will crack. The form of cracking seen is regular horizontal or vertical cracks often seen at regular centres.

The attached image shows one of our previous  investigations, a commercial high rise block in London that was renovated and converted for residential use. The render was applied for purely aesthetic reasons but failed within 12 months of being applied. The situation can be rescued by retrofitting movement joints but given access costs this will prove to be a costly oversight.

Inadequate or Absence of   Sealant to Critical Junction Details

This is without doubt of the most common causes of external render failure we see. Once newly applied render is complete then it is critical that junction details are sealed with a high quality mastic and that will mean attending to window and door frames, boiler flues, soffit to wall junctions, pipe and cable penetrations and basically anything else that forms a junction with the finished render. If this is not done then the render system will take in rainwater from pretty much day one and premature failure is guaranteed.

We are even seeing million pound external wall insulation schemes to high rise blocks that have had no sealant applied whatsoever to any of the critical building junctions and why would you risk having a £1m scheme fail all for the want of some sealant? I was shocked recently to find that a bricklayer working on one of my projects had never used a sealant gun and when I did ask him to seal around windows etc, he made a complete hash of it and it all had to be done again. Plasterers & renderers often see sealant application as work to be completed by others though rarely will they make a point of asking the client to ensure that everything is sealed once the render is dry.

Adhesive failure of recently applied sealant to window reveal

Adhesive failure of recently applied sealant to window reveal

Even choosing the correct sealant is fraught with pitfalls and I recently noticed that a client of mine specified low modulus silicone for everything and whilst low modulus silicone is the best choice for sealing UPVc windows and doors, there are better products applicable to the wide range of situations that you will encounter. I will no doubt write a blog on sealants in the not too distant future because the scope is too broad to include here. Even where sealant is applied it is not uncommon to see adhesive failure of the sealant due to wrong material selection or simply that it’s been applied to a poorly prepared or dirty surface.

Improper Flashing Installation at Critical Junction Details


Those who apply cementitious renders are often confused as to how flashing details should be dealt with. Where flashing details pre-exist for low level roofing then it is best to stop the render short of those flashing details and provide a bell cast drip detail running parallel and just above those flashings. In this image we can see where original stepped flashings were removed so that the render could be extended down to meet the roofline. Once the render was dry a channel was cut into the render and an apron, as opposed to the correct stepped flashing, was then installed. Lead has a high coefficient of expansion and therefore can crack render. Moreover when you consider that lead needs to be pegged to firmly secure it in place then how do you secure the lead when any attempts at pegging it would undoubtedly lead to cracking of the render?

Chemical Attack

Crystalline ettringite structure. Crystal formation is expansive.

Crystalline ettringite structure. Crystal formation is expansive.

External cementitious render can come under chemical attack, the most common of which is sulphate attack. This occurs when the tricalcium aluminate present in ordinary portland cement reacts with any sulphates present to form ettringite.  Ettringite formation is an expansive reaction so it can cause cracking, bulging or debonding in the render. Sulphates may be present for a number of reasons,  from traffic pollution, contamination of the render mix or most likely the sulphates are present in the underlying masonry. The reaction is expedited where permanent or intermittent saturation of the render occurs so failure to deal quickly with water ingress can lead to sulphate attack writing off the render system.

In part 3, I’ll deal with correct specification of cementitious renders.

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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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