Concrete Boot Lintel Failure

Boot Lintel Failure
Boot Lintel failure

Understanding why Concrete Boot Lintels fail

Whilst carrying out a recent building condition survey in Nottingham, I encountered a case of concrete boot lintel failure, and thought it would be useful to outline the visual defects seen and the mode of failure.

What are Concrete Boot Lintels?

It is first necessary to understand that when looking at cavity wall construction, the inner leaf of masonry forms the structural load bearing element of the building, whilst the outer leaf of masonry forms the weatherproof outer envelope of the building shell. Concrete boot lintels were designed so that the lintel bearing surface only rests on the inner structural leaf of masonry. The outer face of the lintel is shorter and has no bottom bearing surface on the outer toe, with the head of the lintel supporting the masonry above window and door openings.

Boot Lintels were primarily installed in the 1970’s, and gave the opportunity to provide a cleaner design to the external facade, as the face of the lintel could be hidden behind bricks slips, though often. the concrete face of the lintel can clearly be seen seen above window and door openings. Where this is the case, then they are easily identifiable as the lintel does not extend into the masonry as either side of the opening. Thermal imaging often alerts us to the presence of concrete boot lintels, as we see direct cold bridging above the internal window openings, and of course, these areas can be a focus for cold surface condensation.

Typical appearance of concrete boot lintel on external face of masonry
Typical appearance of concrete boot lintel on external face of masonry

The image above shows how the lintel sits flush with the opening below, and does not extend into the masonry at either side of the opening. Typically, we’d expect to see a minimum of 150mm of bearing surface at either end of the lintel.

Visual Signs of Boot Lintel failure

Typically, we may see an open horizontal bed joint above the lintel, caused by the lintel dropping slightly, and the subsequent lack of support to the head of masonry above.

Internally we may see cracking to internal plasterwork to the head of window and door openings, as seen in the following image.

Internal cracking of plasterwork caused by rotation of the concrete boot lintel
Internal cracking of plasterwork caused by rotation of the concrete boot lintel

In these cases, we can’t fully understand the scope of cracking to the underlying masonry, without first removing the cracked plasterwork, though often this isn’t necessary due to the obvious cause of failure.

More commonly, we see the classic sign of boot lintel rotation, whereby stepped cracking appears at either side of the lintel to the head of masonry above.

Classic stepped cracking often seen with rotation of the concrete boot lintels
Classic stepped cracking often seen with rotation of the concrete boot lintels

Why do Concrete Boot Lintels Rotate?

Once you see the ‘boot’ shape of the lintel when viewed in profile, and further understand that they are only supported by bearings on the inner structural leaf of masonry, with the outer toe being unsupported, then it is easy to understand the forces at play, which cause the lintel to rotate, but this is best illustrated in the two technical detail drawings below.

Boot Lintel Technical Detail
Boot Lintel Technical Detail
Rotation of concrete boot lintel

Recommendations for Repair

There are specialist contractors who have developed remedial systems for repairing concrete boot lintels in place, which includes anti-rotation of the boot lintel, back into place, prior to installing a wall tie system that permanently secures the lintel. This wall tie system is often installed in conjunction with a helical steel bars, which form a composite masonry beam below the front toe of the boot lintel, thereby providing further support to prevent future rotation.

In. this particular property, the home buyers were looking to renew all external windows and doors, therefore, it would most likely have been more cost effective to simply swap the boot lintels for concrete lintels, which have a bearing surface to both the inner and outer leaf of masonry, or for a more discreet steel cavity lintel. For both cases, temporary support, such as ‘Strongboy’ props, would need installing to support the head of masonry above the openings, whilst remedial works are underway.

If there are no plans to replace the windows and doors then you should carry out a cost benefit analysis, on lintel renewal versus lintel repair, before making a decision on the best course of action to take.

Please follow and recommend our blog page:

The worst brickwork?: A new contender.

The worst brickwork we’ve ever seen.

Norwich New Build

Norwich New Build

Back in July we were called to carry out a snagging inspection to a David Wilson Homes site in Norwich, which had the worst brickwork we’ve ever seen. Our clients had signed up to buy a new build ‘off plan’ but started to have grave concerns relating to the quality of their potential new home as they watched the build progress. Our initial discussion related to the fact that the brickwork colour was mismatched and that the developer had employed someone to tint the bricks to match; when I arrived on site to carry out the snagging inspection the specialist was at work  painting individual bricks with a pot of red tint solution and a paint brush, a quite laborious task as you can imagine.

We’d agreed to inspect before the build was complete because our discussions with the client, and indeed pictures sent to us, gave enough cause for concern that this was necessary.  The following image slider will give you a feel for the quality of the work and the sheer volume of defects we encountered. Please view the slider on full screen to fully appreciate the illustrated defects.

Would you have bought this house?

Significant Defects

As you can see the defects were significant and we commented in our report that we found it difficult to believe that this brickwork was completed by a fully qualified brick layer. Of particular concern was the incredibly poor setting out, failed mortar bed joints, inconsistency in width and depth of mortar joints and last but not least, walls that were significantly out of plumb, well beyond the 8mm maximum allowable NHBC tolerance. As often happens in these cases it was relayed back to us from our client that the developer didn’t agree with our report and that a site manager of 20 years experience knows more than us and his view is that the brickwork was perfectly acceptable. I thought it may be useful to balance the Chartered professionals view with a second opinion  from a master bricklayer and obtained the following commentary from an acquaintance who is also a master bricklayer…

The master bricklayers view

“My name is Bill XXXXXX and I have been a brick layer for the last 30 years.  I have City and Guilds  NVQ level 3 in brick laying  and NVQ level  6  in site management. I’ve been asked by Joe Malone  for my opinion  about the workmanship of Plot XXX in Norwich.

There seems to be wide and inconsistent perpendiculars and significant variation in bed joints.

The pointing is of a very low and poor standard i.e. holes and not perps not ’top and tailed correctly’

Weep holes are protruding out of the brick work and should be flush.

Also bricks have been laid upside down allowing moisture to catch on the face leading to premature failure through ‘spalling’ aka frost damage.

The walls are significantly out of plumb. Variation in plumb should on good brick work be a maximum of 4l mm out of plumb one way or another. Brick courses seem to wander. Chipped bricks have been used rather than discarded. The Brick work is over sailing be 10mm in places below the DPC

There are large gaps around some windows which implies poor setting out.

Some bricks are cracked and should have been discarded. Two failed bed joints are apparent.

The brick work has not been washed or cleaned down.

The mortar colour varies implying it has not been ‘gauged’ and makes the building look patchy.

The damp course is protruding through the mortar.

Back straps for the garage have been missed. Roof ridge work is poorly finished and there appears to be no mechanical fixings

The brick work does not appear to be ‘fair faced’

In all a very poor standard of work has been delivered with a significant amount of snagging already required.

On a site managed by our company this work would be condemned and the brick layers replaced or forced to do the work again to our own companies’ standard.”

The only point on which we don’t agree with on this second opinion is with regard to the DPC being pointed over. DPC’s should not be pointed over, they should be exposed and clearly visible and if they are not then they are bridged.

The worst Brickwork

Our clients reached something of stalemate with their developer, because they were insisting that sections of the building were taken down and rebuilt, whilst the developer was offering minor remedial works that fell well short of dealing with the significant defects in this build. Their complaint was ultimately elevated to the managing director of David Wilson Homes and our client eventually informed us of the following outcome, “We have decided not to proceed with the purchase of the house. I think we always knew this was the outcome deep down. I have received a reply letter from the MD of David Wilson East division offering to rescind the contract and contribute towards ‘reasonable’ conveyance costs.”

When we last spoke our clients were looking to purchase an old traditional property and we completely understand why, moreover, we believe that they made absolutely the right decision to withdraw from this contract. A brave and sensible decision, especially when you consider that many clients purchase with their heart rather than their head.



Please follow and recommend our blog page:

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.

Please follow and recommend our blog page:

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?

Please follow and recommend our blog page:

Repointing Heritage Brickwork

Keeping Thin Joints Thin

Mortar joint width almost doubled due to poor repointing

Mortar joint width almost doubled due to poor repointing

We rarely see repointing done well in standard 10mm  mortar joints so imagine what we find when encountering thin joint construction or gauged brickwork with joints of around 1mm thick? We commonly find thin joint construction in old buildings and generally speaking, the thinner the mortar joint, the stronger the building. Buildings are either left with deep open joints in the belief that these buildings can’t be repointed or mortar specification is incorrect and looks like its been applied with a catapult. Poor application, incorrect tools and lack of specialist training are all problems but mortar specification is a primary consideration that is often overlooked.  Imagine trying to repoint a a 1mm mortar joint when aggregate size  is 1.5mm in diameter, the effect would be similar to trying to fit a square peg into a round hole.

Repointing Techniques

There are a number of techniques for repointing thin joint lime construction in old buildings but the primary consideration has to be the mortar mix. Aggregate size is critical and if you are to stand any chance of successfully repointing then you’d be wise to have the mortar analysed. There are a number of UK based lime companies who offer this service now.  If you are repointing a 1mm thick joint then the  sand diameter needs to be 0.5mm. This would produce an extremely fine lime mortar. Generally speaking, this would still be an NHL 3.5 lime mortar for most external masonry applications but consideration is given to BS8104, the British Standard for assessing building exposure; the mortar analysis service we use also supplies our mortar after blending it to match existing and provides a certificate of conformity. It costs nothing to send the mortar sample in, and as was said to us recently, “It’s as easy to get it right as get it wrong.”

Poor repointing

Thin joints smeared with poorly applied mortar.



We recently encountered a fully repointed chimneystack, all correctly done in hydraulic lime and to a reasonable standard. The only problem was that the mortar was not colour matched and was stark white when compared to the pale yellow colour seen in the original mortar, simple colour matching would have avoided this problem. Chimneystacks are one area where repointing needs doing particularly well due to high exposure and poor access for ongoing maintenance but repointing here is often no better than seen in other areas of external masonry. Here we have a beautiful ornate chimneystack constructed in 1877 but aesthetically, and no doubt structurally, damaged by poor quality, ill informed repointing work. Note the thicker joints where mortar has been smeared across rather than pressed into the thin joints.

Finding a true craftsman who can deal with high quality maintenance work to gauged brickwork or thin joint construction is incredibly difficult and when you do find one they are generally booked up for weeks or even months ahead. These skills are all but lost to the mainstream construction industry and I would prompt any builder with an interest  in heritage brickwork to sign up for one of Dr Gerard Lynch’s Courses, which can be found here… Heritage courses . Dr Lynch is one of the few remaining master craftsmen left in the UK and is thankfully passing on his skills.

Source: Practical Building Conservation. Vol.2

Specialised tools for dealing with heritage brickwork. Source: Practical Building Conservation. Vol.2

Gauged Brickwork

Gauged bricks or ‘rubbers’ are very soft bricks, hand cut and rubbed to size. Buildings constructed with gauged brickwork and built to incredibly high tolerances and joints are unlikely to exceed 3mm in width. Where mortar has eroded in gauged brickwork then it is often best not to repoint unless absolutely necessary since raking out mortar joints can damage the soft brick arris, thereby doing more harm than good. If repointing is absolutely necessary, for instance lets say that a brick has slipped in a brick arch, then this will need a specialised repair. You would use a fine hacksaw blade to to help ease the brick back into position prior to pegging it into position with slivers of lead. The joints are then temporarily sealed with 2 coats of liquid latex, which on drying is then injected with a fine lime mortar from a 50cc syringe. The recommended mortar will be lime putty normally mixed with refractory brick (Fire brick)  powder but in practice this can be extremely difficult to get hold of. Refractory brick powder or HTI powder, is a natural pozzolan that gives the lime putty its chemical set and is mainly comprised of silica and alumina. With regard to particle size, studies have shown that particles of 75 microns or less are pozzolanic, whilst particles of 300 microns or more act as porous particulates. You could have your own refractory brick powder made by having fire bricks crushed in a roller mill but we would generally use Argical M-1000, which is made from burnt clay and contains primarily silica and alumina; particle size is around 80 microns or less and you can find a data sheet for this product Here. The latex is peeled away from the joint once the mortar has set. Carpet tape can also be used as an aid to repointing thin joints; it is laid across the joint then split down the centre of the joint with a sharp craft knife. The folds are then pressed into the joint and this will aid you pressing fine lime mortar into the joint with damaging or spoiling the brick arrises. You would use extremely thin pointing irons for this work and sometimes they need to be purpose made. Assessing whether your contractor is capable of carrying out repairs to heritage brickwork may be a simple case of asking what’s in his tool bag because even many qualified brick layers will not carry these tools or understand the specialised requirements.


Thin joint construction that will present a real challenge if professional results are to be obtained when repointed.


As with most things good preparation is critical and only the most patient and exacting tradesmen are prepared to expend the time and commitment to ensure that joints are carefully hand raked to the required depth and cleaned, so be prepared to pay a premium. Joints should be washed out with clean water prior to repointing and should still be damp when repointing commences. This will reduce suction on the mortar and promote better adhesion in the joint. Generally speaking, joints should be raked out to a depth of at least two times the joint width. However, you will often find that this principle is fairly meaningless when dealing with thin joint construction since erosion often far exceeds this depth by the time repointing is required. Maintenance and repair of heritage brickwork is a large and extremely complex subject area that I can’t possibly cover within the limitations of this blog but if it is a subject area you’re interested in then I’d highly recommend reading Gauged Brickwork by Dr Gerard Lynch but volume 2 of Practical Building Conservation is a good starting point.

Please follow and recommend our blog page: