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.

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Bulging or Leaning Masonry Walls

16th Century Cottage with distortion to masonry walls

Typical Causes of bulging or leaning masonry in old solid walled buildings

It’s a fairly common occurrence to survey old solid walled buildings and find some distortion in the masonry; the walls often found to be bulging or leaning to some degree; and indeed we’ve encountered two such properties within the last four weeks, the latter case was particularly interesting and forms the subject of this blog. 

16th Century Cottage with distortion to masonry walls
16th Century Cottage with distortion to masonry walls

The building in question is a 16thcentury building, originally built with an Oak timber frame and brick infill panels. In fact the building was originally separated into three small cottages but over time was converted to a single large cottage. The building is grade 2 listed but has been derelict for some years but about to undergo substantial renovation and improvement works. With that in mind we were commissioned to carry out a full condition survey, focussing on structural condition and causes of dampness within the property. Little remains of the timber frame and the external envelope of the building is now of solid walled construction; only two timber end frames exist internally, along with the timber roof structure. which is now almost wholly supported on the external masonry walls.

Checking for Distortion in the Masonry

A 2 meter spirit levels shows a bulge of 80mm to the front wall
A 2 meter spirit levels shows a bulge of 80mm to the front wall

Commonly we’ll use a simple plumb bob and line or a large spirit level to. check for distortion in masonry. A large spirit level can be used quickly and effectively, and in this case we noted bulging to both the front and rear walls of the property. The front walls were at a much reduced height due to the cat slide roof, and despite 80mm of distortion, they were less worrying than the 50mm distortion measured to the rear wall, due to the greatly increased height of the wall.

50mm bulge measured to rear wall
50mm bulge measured to rear wall
Checking masonry distortion using plumb bob and line
Checking masonry distortion using plumb bob and line

Acceptable Limits for Leaning or Bulging Walls

Generally speaking you should be concerned with anything more than 25mm of distortion as it lowers the stability of the wall. There is a general rule known as the V3 rule, which asks that you consider the walls centre of gravity. When viewing a 225mm solid wall in profile, a plumb line dropped from the head to the foot of the wall, which passes through the walls centre of gravity, will not fall outside that centre of gravity at the wall base, (if the wall is perfectly vertical.) Where walls are leaning or bulging then the plumb line will fall outside that centre of gravity, and should be considered unsafe, where the plumb line falls beyond the outer edge of the wall base. In these cases you should seek advice from a qualified structural engineer.

V3 Rule
V3 Rule

Common Causes of Bulging or Leaning Walls

Bulging of the walls is caused by a number of factors: 

  1. Vibration from road traffic. 
  2. Increasing the floor loads or building on additional floors 
  3. The original walls being insufficiently thick in relation to the height. 
  4. Lack of lateral restraint between the external walls and floor joists, beams and partitions. 
  5. Thermal or moisture expansion of the walls outer surface

In our experience, the type of bulging seen in older buildings built with lime mortar, such as this, is often a lack of lateral restraint. It is a well-known principle that lateral restraint should be provided to arrest any potential movement in the masonry walls, and this is usually achieved by building floor joist ends into the masonry, or by bonding in internal partition walls at right angles to the outer wall. In this particular case, lateral restraint was meant to be provided by the first floor joists, running from front to rear in the building. However, when viewed internally, the joist ends were set in sockets cut in the central spine bresummer

Joists. supported by sockets cut into bresummer
Joists. supported by sockets cut into bresummer

Visual Inspection of Joist Ends

In this case visual inspection was fairly straightforward, since all the floor joists were fully exposed. However, it is more common for the floor joists to be hidden as they are sandwiched between the ground floor ceiling, and upper floor. Often, we may need to take up floorboards at first floor level to inspect joist ends, and in particular, to assess how they are tied to the masonry walls, and whether lateral restraint is being provided.

The joist end lap joints, seen above, are seen to have pulled clear of the sockets cut into the bresummer, which accommodate them. Ideally, these lap joints would have been secured with oak pegs, but they are unfixed, and have simply pulled clear of their sockets as the external walls have bowed. This technical detail was simply unable to provide the lateral restraint required by the front and rear walls of the property.

When checking some joist ends, we found as little as 20mm of bearing surface at the joist ends, which essentially tells us, that if another 20mm of deflection occurs to the outer walls, then the floor joists could collapse.

Only 20mm of bearing surface to joist end
Insufficient bearing surface to joist end

The floor joists are now too short and cannot be re-used, unless the outer walls are taken down and rebuilt so as to be perfectly vertical again. In this case, the joists should then be able to be fully inserted back into their sockets formed in the spine bresummer. However, with the amount of deflection measured, the walls should be perfectly stable, so long as adequate lateral restraint in re-instated to prevent further ongoing distortion in the masonry. Joists will be replaced with longer joists which will be adequately tied into both the outer masonry walls, and the central spine bresummer.

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Timber Frame Defects

Common Timber Frame Problems

Neo-Georgian with Timber Frame

Neo-Georgian with Timber Frame

We were recently called to investigate some damp and structural issues to a 12 year old timber frame Neo-Georgian 3-storey apartment block across the water from mainland UK. The building had been affected by both water ingress and a number of structural issues for quite some time and two previous technical reports had reached broad agreement of the fact that the timber frame was showing signs of distortion due to shrinkage, shrinkage commonly occurs in timber frames after construction and one engineer estimated the height of the timber frame may reduce by up to 30mm, a degree of shrinkage that wouldn’t be replicated in the outer non-structural leaf of masonry. In fact the outer masonry leaf tends to expand as it takes up moisture during the first couple of years after construction, so it is in fact moving in the opposite direction to the timber frame.

Wall Ties

It is for these reasons that timber frame movement ties are specified for developments over four storeys, as these are required to accommodate the additional vertical movement in the timber fame and differential movement between the inner and outer leaf. However, this is a 3 storey development and so long as vertical movement stays within expected limits then a standard fixed wall tie should suffice.

Standard Timber Frame Wall Ties

Standard Timber Frame Wall Ties

Timber frame movement tie

Timber frame movement tie

 Structural Cracking & Movement

Significant structural cracking

Significant structural cracking

Structural cracking to the outer masonry leaf of timber framed buildings can often occur where this differential movement between the inner and outer leaf falls outside of acceptable limits due to inherent design flaws or poor build quality.

When inspecting the building externally we noted that door and window frames were often slightly deformed and out of square, which resulted in extreme difficulty in opening the softwood timber french doors leading out onto the apartment balconies. We also noted significant stepped cracking in a number of areas to the outer leaf of masonry.

Starting from the Top

It was initially thought that defective balcony detailing and waterproofing arrangements were the cause of water ingress into the building and in fact the initial instruction was very much about investigating potential balcony defects, but of course you must approach these investigations with a blank canvas and an open mind. Whilst there were a number of relatively minor issues with balcony upstand detailing and parapet wall box gutter outlets, it was clear that these were not responsible for the water ingress or the structural defects seen.

Logically, I like to start from the top and work my way down once I start the internal inspection  and starting from the top meant inspecting the balcony that fully surrounded the building at top floor level.

Keeping the Timber Frame Dry

Open bed joints to parapet copings and no throating detail to underside

Open bed joints to parapet copings and no throating detail to underside

I found a number of serious and critical defects relating to the high level parapet walls that in my opinion have been allowing rainwater ingress into the wall cavity for a number of years, possibly since the building was constructed. Of course, if this was the case and water ingress was as bad as I believed it to be then the the greater probability is that the timber frame has swelled and expanded, rather than shrunk. The net result of course is the same, which is the potential for excessive differential movement between the inner and outer leaf. Moreover, there is a further potential for timber decay in the structural timber frame and perhaps even structural failure as timbers are affected by fungal decay.

Open perp joints between coping stones

Open perp joints between coping stones

Defects Causing Consequential Damage

Adhesive and cohesive failure of sealant to coping bed joint

Adhesive and cohesive failure of sealant to coping bed joint

We noted that the parapet wall copings were not fit for purpose and had been poorly installed off centre so the outer wall face had a 70mm overhang, whilst the inner parapet wall face only had a 30mm overhang. To meet the requirements of BS5642 then a minimum 45mm overhang was required to either side. However, more critically there was no throating detail to the underside of the parapets meaning that rainwater would flow along the underside of the coping overhang and straight into  cracks or open joints that may exist to the coping mortar bed.

Lead apron proved not to extend across the width of the cavity

Coping stone removed. Lead apron proved not to extend across the width of the cavity


Of course, this shouldn’t be a problem, because there’s bound to be a physical damp proof course installed under the copings as a secondary line of defence… or at least there should be!  We removed a parapet coping and as we suspected there was no physical DPC installed. So water was entering the wall cavity from the underside of the failed bed joint to the copings and the open joints and cracks in the coping mortar perp joints.

On finding these defects we of course had serious concerns as to what effect this long term water ingress was having on the timber frame. On checking the timber moisture content to the head of the timber frame we recorded a moisture content of 21.2%, proving the real current and ongoing risk of timber decay to the structural timber frame.

High moisture content to timber frame

High moisture content to timber frame









We recommended and specified urgent works to correctly waterproof the balcony parapets and further recommended opening up sections of the wall cavity where cracking had occurred to inspect the integrity and condition of the structural timber frame.

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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.



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An Introduction to Calcium Silicate Bricks

Calcium Silicate Bricks: A Case Study

Residential scheme with calcium silicate bricks

Residential scheme built with calcium silicate bricks

Some time ago I was asked to investigate structural cracking in a large residential scheme in the West Midlands.

On visiting the scheme and noting cracks on the building I had a strong suspicion that the building had been constructed in Calcium silicate bricks  but it should be noted that there is no definitive site test for identifying Calcium Silicate Bricks; positive identification can only be gained after laboratory analysis, specifically XRD (X-ray diffraction) where peaks in both quartzite and calcite will positively confirm calcium silicate construction. However, a basic understanding of these bricks and their properties can go some way to helping with correct site identification. Since we know there are a number of known problems associated with Calcium Silicate brick construction, it was of primary importance to identify the form of masonry construction.

Calcium silicate (sand lime and flint lime) bricks are manufactured by mixing lime, sand and/or crushed silicaceous or flint stone together, with enough water to allow the mixture to be moulded under high pressure. The bricks are then steam autoclaved so that the lime reacts with the silica to form hydrated calcium silicates. Pigments can be added during the mixing stage. In their natural state, calcium silicate bricks are white to a creamy off-white colour, but the addition of ochres (buff or cream colours), iron oxides (pink, red, brown or black) or chrome oxide (green) can enable a very wide variety of colours to be produced.

Close inspection of the bricks showed what appeared to be small particles of flint embedded up to 3mm in size.

Calcium sulphate bricks

Embedded flint visible and bricks scraped very easily on their surface.

This would be consistent with a calcium silicate brick as would the fact that a surface scrape of a brick proved them to be extremely soft. They also have no ‘fireskin’ as would be seen with a clay brick. They are often confused with concrete bricks but these are much harder and would not scrape quite so easily. Finally the factor which swung the balance of probability in favour of calcium silicate bricks was the colour differential below and just above DPC level. Calcium silicate bricks have a propensity for all colour variants to darken quite noticeably when wet. The wetter bricks below DPC level and just above dpc level (where the dpc has been bridged) is noticeably darker.




Calcium silicate brick identification

Unusual colour change phenomena often seen in calcium silicate bricks













Defects Noted at During Visual Inspection 

  1. Mortar considerably harder than the masonry units.
  2. DPC bridged by mortar
  3. Regular and consistent stepped shrinkage cracks throughout scheme
  4. Pointing to movement joints in building corners
  5. Loss of protection to movement joints in corners.
  6. Discolouration of brickwork below and just above dpc level.
  7. Oversailing brickwork at DPC level.

Stepped cracking in calcium silicate bricks

Regular stepped cracking & poor repair work. DPC slip plane should also have been installed at first floor level.

Explanation of Defects noted at Scheme

    1. Mortar considerably harder than the masonry units: Not in itself a defect but Calcium silicate bricks are prone to shrinkage or expansion cracks therefore the mortar needs to ‘give’ with brickwork. This isn’t possible where an overly strong OPC mortar mix has been used. Ideally a lime mortar would be used that would have a similar co-effecient of expansion as the masonry units. The overly strong mix has undoubtedly contributed to the widespread shrinkage crack problem across this scheme.
    2. DPC bridged by mortar: This is of course an issue that might give rise to future damp problems but more importantly the dpc forms a very important part of calcium silicate brick construction. The DPC acts as a slip plane for the brickwork above and allows the brickwork above to move in a more controlled manner without cracking. Pointing around the dpc joint only serves to prohibit movement of the slip plane with a danger that uncontrolled shrinkage cracks will occur elsewhere in the building.
    3. Regular and consistent stepped shrinkage cracks throughout scheme: I do not believe that these cracks give any cause for concern over and above the fact that repointing is required to improve the aesthetic and to weatherproof open joints. There was nothing to indicate that these cracks are caused by anything other than shrinkage/expansion.
    4. Pointing to movement joints in building corners: Movement joints by their very nature are meant to move therefore you do not seal them with mortar against the elements as this is inflexible and will crack and fall out. This is precisely what had happened at this scheme and the mortar fillets should be removed and replaced with a flexible polysulphide mastic.
    5. Loss of protection to movement joints in corners: As for point 4 but the replacement of mortar with sealant will re-instate weather protection to the movement joint.
    6. Discolouration of brickwork below and just above dpc level: There are no concerns here beyond the different aesthetic appearance of the darker brickwork. There are no technical issues associated as Calcium Silicate bricks have a good level of frost protection
    7. Oversailing brickwork at DPC level: Worthy of mention but not in my opinion really a defect on this form of construction; it simply demonstrates that the slip plane at dpc level is acting as intended in certain areas.

calcium silicate movement joint.

Compressible fibreboard movement joint installed but prevented from functioning as intended by hard cement mortar. Joint should be sealed with flexible mastic.

The Range of Historical Problems associated with Calcium Silicate Bricks

  1. Thermal movement is likely to be about 1.5 times that of clay brickwork. Calcium silicate brickwork, unlike clay, usually undergoes an initial irreversible shrinkage on laying (clay brickwork tends to expand) but so long as the propensity for movement is understood and catered for in the design, there is no reason why the brickwork should not perform adequately. Often this factor is not catered for in the design and this results in widespread cracking.
  2. Calcium silicate bricks should not be used in solid work with clay facings or backings, this is because of the propensity of the bricks to shrink in contrast with the expansion of clay brickwork. If solid walling is to be contemplated, backings of concrete bricks or blocks should be used, as these have similar movement characteristics to calcium silicate bricks. We often see an inappropriate choice of walling material for the inner leaf and this sets up opposing forces due to differential expansion, again resulting in widespread cracking.
  3. General construction detailing is often not attended to, particularly with regard to providing sufficient flexibility in the wall ties to permit the differential movements and allowing for discontinuity around cavity closers to prevent cracking.

4. The requirement for inbuilt slip planes is often not attended to. Internally, walls of calcium silicate brickwork need to be bedded on a damp proof course to act as a slip plane and so facilitate longitudinal movements to occur – this would be equally necessary at upper floor levels, a detail that had been missed at this scheme.

5. Movement control in walling is not the only issue – also consider  building elements that could provide a restraining influence. For example, concrete columns or walls cast up against bricks should be avoided unless a slip membrane can be provided. – as should any form of construction that will prevent free movement. At this scheme pointing of the movement joints and dpc both provide this restraining influence.

6. It is not unusual to see some form of displacement with calcium silicate bricks due to thermal expansion, for example brickwork sliding off a damp proof course, cracking at corners or evident disruption. By contrast, shrinkage cracking does not generally produce these manifestations.

Calcium silicate DPC acting as slip plane

DPC pointed over but movement across DPC slip plane has caused mortar failure and so reinstated the natural slip plane function.


Calcium silicate bricks are often given a bad press due to the issues highlighted here; however it should be said that they are an excellent building material so long as the construction detailing required to deal with shrinkage or expansion is understood. Unfortunately more often than not this detailing is not understood and buildings are generally constructed in the same fashion as would clay bricks. They outperform clay bricks in some respects, particularly with regard to frost resistance.

The question for this particular scheme is whether the construction detailing was so bad as to cause significant concern for the long term future or viability of these blocks? In my view there were no significant concerns; the blocks are structurally sound and the cracking should be attended to as an aesthetic detail. The quality of previous pointing has been quite poor and this has to a degree scarred the blocks with unsightly or inappropriate work and there is little that can be done to reverse this damage. The pointing should be removed from the dpc to enable it to act as a slip plane and to stop the damp rising above dpc level. Additionally the vertical movement joints in the corners of the blocks should have the restraining mortar fillet removed and then be sealed appropriately with a high quality polysulphide mastic.

There is little that can be done with regard to the colour differential around dpc level but then this is purely aesthetic and it is a subjective view as to whether individuals like or dislike this colour change.

In general terms I saw no reason why these blocks should not continue to provide accomodation for another 40-50 years given nothing more than reasonable maintenance attendance and costs.


Weather exposure

Weather exposure

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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.

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Cutting Corners – Builder Short Cuts

A Number of Builder Short Cuts Adopted by Builders

Builder Short Cuts – We come across so many short cuts adopted by builders that I thought I’d start a regular post highlighting some of the strange decisions made by site trades people to save on time or money. As you’ll see, some of the decisions taken make no sense whatsoever. See for yourself.

Short Cuts 1


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Do physical damp proof courses fail?

Why DPC Injection Work is Rarely Required. 

The damp proofing industry in the UK commonly promote two statements that are fundamental to this industry. Firstly, they promote rising damp as a common occurrence and we can comfortably state that this is simply untrue. It is an academically proven fact that  rising damp is incredibly rare.

The second claim, which is also fundamental to an industry that sells retrofit chemical injection and re-plastering is that physical damp proof courses commonly fail.  We have reviewed many many reports from these ‘specialist’ companies and the absence or failure of an existing physical DPC is commonly cited as justification for installing a retrofit chemical injection system. Moreover, you have all commonly seen retrofit chemical injection work installed where physical DPC’s already exist.

Do Damp Proof Courses Fail?

There are of course legislative requirements for the insertion of a physical dpc in new buildings. Approved document C, Section 5.2, states that walls should: resist the passage of moisture from the ground to the inside of the building; and not be damaged by moisture from the ground to any part which would be damaged by it. This requirement is met if a damp proof course is provided of; bituminous material, polyethylene, engineering bricks or slates in cement mortar or any other material that will prevent the passage of moisture.  However, relatively speaking this is modern requirement and we have many thousands of properties in the UK that do not have have a physical damp proof course installed and yet they manage moisture perfectly well despite non-compliance with the modern requirement for a physical DPC.

I personally carried out a comprehensive review of this very question and  what became clear is that the majority of academic commentary cited bridging rather than failure as the key issue, in fact it is fair to say that there was general agreement on this point. We  found only two cases where commentators cited their view that DPC’s fail, in both cases these were unproven opinion rather than proven fact. Here is an opinion given by Trotman P, Sanders C, Harrison H (2004)…  Physical dpc’s can fail occasionally, particularly those formed by engineering bricks or overlapping slates, following breakdown of the mortar; bitumen felt dpc’s can become brittle with age.  The ‘breakdown of mortar’ is the most interesting point in this statement but the idea that an engineering brick can fail is simply wrong. The authors do not go on to explain their point but we can only assume that this idea is linked to occasional building movement that results in cracked engineering bricks at DPC level. A crack in a brick or a slate DPC will not result in capillary rise in those units and we are firmly of the opinion that engineering brick DPC’s do not fail. Moreover they are the simplest physical DPC to visually inspect. The key controversy must focus on hidden DPC’s installed to the mortar bed joint. These can be formed from a wide range of materials including poured bitumen, bitumen felt, lead, copper, overlapping slates and probably one or two more that currently escape my mind. They are  often not even visible at the bed joint and this may be due to being hidden by high external ground levels, or more commonly, they have been pointed over. Both issues are clearly bridging issues rather than DPC failure and if you have a bridge then the simple solution to that problem is to remove the bridge.

To my knowledge no one has carried out a piece of research into alleged DPC failures and published their findings. It can’t be done by the damp proofing industry because they have a vested interest in promoting the idea of DPC failure. It would need to be an independent piece of work  that to my mind would be a valuable piece of research. I have considered co-ordinating this with a demolition company so that every time a building is taken down we can thoroughly inspect the DPC in the process. We have removed bricks from walls on many many occasions to inspect cavities and where we do this we have consistently found the old physical DPC to be intact and fully functional.

We have previously written that Portland cement degrades over time, initially it is resistant to rising damp until after many years of degradation it then becomes the major moisture pathway for rising damp. Where a continuous physical barrier is installed then clearly this is not a problem but this fact may well form at least a partially valid argument towards a claim that an engineering brick DPC has failed. Technically there would be nothing wrong with bricks but the mortar perps may allow rising damp via diffusion. Interestingly we have seen where perp joints have been left open on engineering brick DPCs and this would completely mitigate for this potential issue.  However, in all alleged cases of DPC failure,  what we commonly recommend is that so long as there is a provision for adequate wall base ventilation then this does not become an issue. It is all about maintaining moisture equilibrium, which is ensuring that moisture is evaporating off the wall as fast as it is rising.  Similarly, where we find that physical DPC’s are hidden we simply treat the building as though a physical DPC is not installed so that if external finished floor levels are a minimum of 200mm below internal finished floor level then this need not be a problem. There are thousands of properties in this country that perform perfectly well without a physical DPC and they generally do so because moisture equilibrium is maintained in their walls due to the fact that they are left bare, they are correctly  repointed with lime mortar, there is adequate subfloor ventilation, external finished floor levels are not too high and local ground moisture is managed.  You can of course apply all or most of these principles to a building that has a physical DPC installed, even one that has allegedly failed and you would mitigate for the alleged failure.

We are lucky enough to carry out a great deal of survey work on the Crown Estate. We deal with some very old historic buildings that were originally built to a very high standard. We are seeing properties over 150 years old where ordinarily we would not expect to see a physical DPC installed but on this Estate they do,  and this gives us a rare insight into some quite unique properties. Many of the images contained within this blog are from the Crown Estate and we are consistently finding perfectly functional DPC’s in some of the oldest properties to have physical DPC’s installed.  I may not have proven through this blog that physical DPC’s don’t fail but I can state with certainty that no one has proved that they do. We do not believe that physical DPC’s fail so if one is installed then you should give careful thought as to why you would even consider installing another unproven retrofit chemical injection system in the absence of any proof that the existing physical system has failed. We have always taken a balanced view on retrofit DPC injection because pragmatically there are times when lowering external ground levels may not be an option but the fact remains that we very rarely have a need to specify these management solutions because our focus is always on curing rather than managing or hiding the problem.



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What happens when you apply waterproof render to old buildings

Managing damp problems in old buildings

Damp building made worse by specialist contractors.

A while back we were commissioned to investigate a damp problem on an old public library. The library had previously been used as as a Town Hall and when the decision was taken to convert the building, damp proof contractors were called in to deal with a damp problem that we do not believe was particularly serious. Stripping external masonry paint from the wall base plinth, lowering perimeter ground levels, improving sub-floor ventilation and dealing with defective and blocked rainwater goods was all that was required to cure the problem.

However, the specifier got in a ‘specialist’ to deal with the damp and this resulted in the internal walls being rendered with cementitious tanking to dam in the damp. A treatment that is completely inappropriate for this very old and historic building. Interestingly the internal render was applied to a height of around 4 to 5 meters and work was obviously very poorly scheduled because electricians followed the damp ‘specialists’ into the building and chased out their cement render in several locations to install electrical cables. These areas were then patched with a standard plaster system. Can you guess what happened in these areas?

It took less than two years for the system to fail and you can see how and why it failed in the next two video clips.

Induced rising damp: Part 1.

Induced rising damp: Part 2

You absolutely must recognise the difference between a management solution and a cure. Management solutions, as offered by many specialist damp contractors/salesmen  are by their very nature, destined to fail. The vast majority of damp problems can be cured with nothing more than minor building works and specialist contractors are not required.  Your aim should always be to cure rather than manage the problem and on that basis, specialist treatments will rarely, if ever, be required.





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Wall Base Plinths – Are they a Good Idea?

Are Wall Base Plinths A Good Idea? 

Wall base plinths are generally poorly understood and I attribute this to the confusion caused by a damp proofing industry fad for installing cementitious wall base plinths in the 50’s, 60’s and 70’s. It was a perceived treatment for wall base damp long before retrofit chemical injection became heavily marketed. The basic idea for installing brick wall base plinths, as seen in many thousands of old properties, was a good one, but a whole damp remediation industry ran with a sound principle and because they didn’t understand it, they ruined it. Original brick plinths generally preceded the widespread installation of a physical damp proof course, though you will occasionally find both an original brick wall base plinth and a physical damp proof course, usually of slate. Generally speaking, our experience has been that if you see an original brick wall base plinth then there won’t be a physical DPC installed.


Original brick plinths work well if correctly maintained. This one was built in 1880.

The thicker wall base gave extra protection against rainsplash and penetrating damp and generally acted as a larger buffer for damp. If you read my last blog then you may remember that I talked about buildings being built on the ‘overcoat’ principle. Well this is precisely how original wall base plinths work, they are generally constructed with lime mortar and so long as breathability is maintained then this thicker ‘buffer’ zone at wall base does a great job of protecting against wall base damp.  Unfortunately a remedial damp proofing industry latched onto this idea and started to install retrofit cementitious plinths in the mistaken belief that these too would help protect against wall base damp, which couldn’t be further from the truth. Anecdotally, retrofit cementitious plinths are said to help ‘shield’ the wall base from rainsplash but that function is outweighed by virtue of the fact that they shield against moisture evaporation from wall base.

When it comes to protecting against wall base damp, wall base ventilation  and subsequent moisture evaporation at wall base are absolutely critical. If a retrofit cementitious wall base plinth is installed then this critical ability is lost, moreover we often see retrofit plinths that have actually bridged an existing physical damp proof course so now you have double trouble, a bridged DPC and zero wall base ventilation.

Screen Shot 2015-08-05 at 00.48.37

Retrofit cementitious plinth bridging original physical DPC









Occasionally we also see heavily spalled brickwork to original brick plinths, which results in a render coat being applied to form a new cementitious plinth over the top of the original brick plinth, an action that will most likely lead to severe wall base damp, as in the images below.

To round up, if you see a retrofit cementitious plinth installed then you should have it removed. If an original brick plinth is installed then you should respect the importance of this technical detail and maintain its breathability by keeping the brickwork bare and only repoint using breathable lime mortar.


Cement plinth formed over original spalled brick plinth





















Retrofit cementitious plinths are a bad idea.


Original brick plinth rendered ineffective by the application of external masonry paint.

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