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?

17 responses to “Cementitious Render Failure (Part One)”

  1. Mark Duckworth avatar
    Mark Duckworth

    Really interesting article.

    I undertook a similar exercise several years ago and found that a weaker mix had been applied with a harder mix on top, all of which was applied to expanded metal lathe on a victorian building!
    Hence it failed and was replaced with a hybrid lime base solution.
    Thank you for the authoritative comments.

  2. Joe Malone avatar
    Joe Malone

    Thanks Mark. I’ll be discussing EML and reinforcement in part two so your comments are a useful lead in.

  3. John Kellett avatar
    John Kellett

    I await part 2 but the render on my own house is failing because water laden moisture from inside saturates the wall because it cannot pass through the cementitious render easily. The problem made worse in recent years by filling the cavity which increases the temperature difference between inner and outer leaves.
    My solution will be to remove the cavity insulation, repair any structural damage and externally re-render and insulate with a more breathable system. Cementitious render is, and always was, bad design. What can’t get in, can’t get out! External renders need to be ‘waterproof’ not ‘vapour proof’.

    Any thoughts/comments?

    1. Joe Malone avatar
      Joe Malone

      Hi John,
      What you have proposed is a viable theory, however, is there any proof that this is the case? Essentially you are proposing that internal relative humidity is causing a very high HMC (Hygroscopic moisture content) in the wall. Have you had the wall tested so that you understand CMC (Capillary Moisture content), HMC and TMC (Total moisture content)? Moreover what proof do you haver that the cavity wall fill is transferring moisture to the outer leaf? External renders need to be appropriate to the building they’re applied to too if you have an old building with soft lime mortar then Ordinary Portland cement based render is not the right material choice. As I said, your theory is a viable proposition but I very much doubt that testing will prove it to be true. Technically, no render is vapour proof, it’s just that some are more vapour proof than others.

  4. Murray Bell avatar
    Murray Bell

    Thanks for the analysis, very useful.
    I am dealing with failure of a Sto render on a house – would you be interested in seeing some photos?

  5. keith walker avatar
    keith walker

    hi there
    I have been rendering my house, i am a plasterer of 20 years but im rendering my house and im getting mad crazy paving crazing after 2 weeks of laying on, im in the process of trying different patches now with different mixes, but their all crazing, after laying on and finishing top coat i am keeping moist by finely spraying and for 4-5 days afterwards am spraying down wall every half hour to an hour to aid curing, still cracking ,
    the mix is = scratch – 4:1 , sand:cement with 1 or 2 5 litre caps of freeflow waterproofer. sand is 2;2, building:sharp sand and cement is OPC
    Left at least 1-2 weeks
    Top coat – 5:1 , sand:cement with half a measure of hydrated lime to a 15;3 mix. and 100ml freeflow waterproofer added to a 12litre bucket of water, using wickes plastering sand and OPC

    Like ive said ive been trying loads of different mixes and their all crazing, please can you help me with this??
    Keith Walker

    1. Joe Malone avatar
      Joe Malone

      Hi Keith,

      I’d need more information first please.

      1. What is the substrate you are rendering to?
      2. What preparation have you done on the substrate? (If any)
      3. How old is the building?
      4. How many coats of render are you applying and what is the thickness of each coat of render?
      5. How long are you leaving the render between successive coats?


      Joe Malone

      1. Patrick O'Connor avatar
        Patrick O’Connor

        Hi Joe

        Have similar issue to Keith above is there any definitive reason for this crazy paving/ crazing effect?? My site is very open to the elements but was a new house plastered in dry cool conditions. Cracks only visible in wet cool conditions, once weather is warm and dry cracks not visible.

        Just wondering if you have any info on this type of failure.

        1. Joe Malone avatar
          Joe Malone

          HI Patrick,

          There’s really too little information for me to be sure as to why this happened in your case. It would be useful to know whether this was a site mixed render or a pre-bagged system, such as a Monocouche? Generally crazing is caused by the system drying out out too quickly before the render has had time to properly cure. In essence we want the system to achieve the chemical set, whilst managing how quickly the render dries out. Its why we sometimes see new render systems sheeted over to slow down the drying time. I’d say they’re most probably shrinkage cracks, possibly due to an overly strong render mix and/or that failure to manage the drying times when it was applied.



  6. Mike avatar

    Having rendered and dashed a small extension 5 years ago, during this severer cold spell with drifting snow and wind chill below -5 the dashing have become hollow.
    Please could you advise on the cause of this.
    Extension is built from old imperial solid 9 inch brickwork. It had one base coat of 4:1 opc cement/ sand and a top dashing coat of same mix with water proofer.
    There are no soffits on the building or overhang protection.
    Based in the heads of the valleys area in South Wales
    Extension is in an exposed area.

    1. Joe Malone avatar
      Joe Malone


      I’ve sent you a render mix designation table, sounds like you need a mix designation 2.

      The next issue is that it hasn’t been applied in line with the guidance in BS8000, essentially you needed three coats.

      It’s a strong mix so sealing around all junction details is critical to prevent water ingress and freeze thaw action, which will crack the render and cause it to debond.

  7. Damian avatar

    Hello Joe,
    I’ve come to you blog to see if (1) I can understand why the render on my house has failed and (2) what type of render specification I need. Thanks for your writings, been very informative.

    I’m an architectural technologist of 30+ years practicing, New build I’m OK with but identifying and rectifying existing is different.
    The render on my house is smooth to the east and west, has cracks and taps hollow in many places. The south & north is pebble dash / roughcast and seems to be sound. The cracking on the east & west elevations is random and ‘crazy’ pattern, ie: not seemingly following any horizontal, vertical or coursing lines.

    Last year when placing a ladder against the (smooth) west wall a small chuck of render fell off, Looks like a 3 coat application of 4mm 1st, 5mm intermediate and 5mm final coat. I have noticed the paint blistering so I’m assuming that moisture has been trapped behind. About 2-3 mm of the brick surface has also come off, Could this be the freeze/thaw spalling the brick, or a strong render pulling off the brick face?

    Over the weekend I spotted a lot of red brick dust on the ground below a large section of hollow render on the east elevation at the corner of the house, so I’ll need to get this sorted very soon.

    House is a conc block inner leaf and brick outer leaf cavity wall construction, built c.1965. Originally no CWI but we installed InstaFibre CWI about 7 years ago, which has a BBA certificate.

    What I don’t know is if this has done any damage to the outer leaf of brick that would prevent the application of new render, and if so, what options might be available. Mechanically fix a mesh and render onto this, but could this also create a void behind the render?

    Appreciate your thoughts and thanks again for your blog.

    Thanks you Damian

  8. Monika avatar

    Can someone comment on oozing render? With each rainfall transparent “jelly” pushes through the render causing holes. We are looking for adequate mending solutions.

  9. claire selby avatar
    claire selby

    I have a newbuild property (2011) large well known housebuilder; they blocked a gutter whilst doing the edge mortar on the gables. It led to damp ingress internally, which they resolved (4 months after moving in) but it appears that this has also led to damp behind the render. We had a big missy patch appear and part of the render has now blown and fallen off…how do we fix this?! Thanks

  10. Emily avatar

    Hi Joe, I wondered what your thoughts are on cork based render – promising or another industry scam!?

  11. Dorota avatar

    Very informative, but ‘my’ failure doesn’t seem to be on the list. The render is sandy and crumbles away when touched, from the bottom to about 0.5-1m high. Exactly the same on all four sides of the house. Less than 10 years old. Some told me it’s because it’s been rendered right to the ground, but most neighbours on the street had it done the same way and theirs are fine. Others said it was too hot on the day, or that the washipg up liquid was added to the mix. I am going to get someone to hack off and replace, but how do I avoid this in future? Any idea what might be causing the problem? Regards

    1. Joe Malone avatar
      Joe Malone


      The render should not be taken down to ground level, it should be at least 150mm clear of the floor to prevent rainsplash damage and/or the render wicking moisture up from the floor.
      Historically, some site builders added washing up liquid to cement to make it more workable and easier to apply. The downside to this is that it contains a lot of chlorides, which can cause issues with efflorescence or chloride attack. I very much doubt this is anything to do with your issue.
      The British Standard is clear, that if you are extending render to ground level then you must use a waterproof render system in this area. Ideally, you should stop the render just before the physical damp proof course so you also avoid a DPC bridging issue.


      Joe Malone

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