When to Specify Woodworm Treatment

Was woodworm treatment necessary?

timber decay

A screwdriver is useful for assessing timber decay but appears to be the diagnostic tool of choice for this particular damp proofing company.

I was recently asked to review a report, supplied by a large National damp proofing company, on behalf of a client, who spent thousands of pounds on unnecessary treatment, including timber preservatives. I say ‘unnecessary’ but perhaps it would be more correct to say that the woodworm treatment wasn’t proven as being necessary.

Perhaps it might be first useful to explain what woodworm are…


Woodworm is a collective term, which is used to describe all classifications of wood-boring insects.

Wood-boring insects use wood as a source of food, and in the UK at least, mainly consist of beetles.

Furniture Beetle, Woodworm

Furniture Beetle (Anobium Punctatum)
(Source: https://www.naturespot.org.uk/species/common-furniture-beetle)

The most common type of wood boring insect in the UK is the ‘Furniture Beetle’, or ‘Anobium Punctatum’ to give its latin name. It is brown, usually about 3-4 mm in length and has a prothorax that resembles a monk’s cowl.  Whilst less common in the UK, timber can also be infested by the much larger Death Watch beetle (Xestobium Rufovillosum) (which is considerably larger at 7- 8mm long), the House Longhorn and the Powder Post beetles. In all species there is a grub like larvae stage, lasting several years, whereby the larvae bores around under the surface of the wood. This is where the most damage occurs.

Note insect flight exit holes in timber

Note insect flight exit holes in timber

A furniture beetle’s Life cycle

The infestation process starts when the female beetle lays its eggs directly into the timber through cracks, crevices or even existing flight holes. The eggs are oval in shape and laid in batches of between 20-60. After about 5 weeks, the eggs hatch and the larvae (woodworm) tunnel down into the wood and spend the next 1- 5 years digging their way around munching through the timber until the next stage in its life cycle. It is in this stage that the greatest structural damage to the wood takes place, and this damage goes unseen due to its location within the timber. The larvae then carve out a ‘pupal’ chamber near the surface of the wood in which the larvae ‘pupates’ into a beetle over a 6-week period. The fully formed beetle then burrows its way to the surface and out, leaving a circular ‘flight hole’ about 1-2mm (roughly the size a dart would make in a dart board). This is the final stage of the beetle’s life cycle, with the females living for around 14 days, and the males only 4 days.

Wood boring beetles thrive in damp conditions; indeed many living trees can have beetle infestation. Species such as the furniture beetle prefer the softer ‘sap’ wood from which they feed off the cellulose, whereas the larger Death Watch beetle is especially fond of hard woods such as oak and beech. It is worth pointing out that the ‘heart wood’ of say, a piece of oak, is usually left untouched by beetles and just the outer sap wood is subject to beetle attack.

Signs of beetle attack

Again, the screwdriver is used in support of woodworm treatment with insecticide.

Again, the screwdriver is used in support of woodworm treatment with insecticide.

Apart from the obvious flight holes left by the beetles, a powdery dust called ‘frass’ can often be seen on floorboards and other wooden surfaces. Frass is the excreted wood that the larvae produce.

In old historic timber framed buildings, especially thatched cottages, there may be signs of beetle attack, evidenced by frass on the face of the timbers, in a honeycomb pattern. If the wood has clearly suffered beetle attack, there is a need to determine whether the timber still has structural integrity. The frass must be scraped off and the heart wood of the timber examined. It is very rare for a beetle attack to penetrate deeply into the heartwood of a piece of oak, so its structural integrity usually remains.

Old bore holes or new?

To establish whether beetles are still present in timber, where there is evidence of frassing, is quite straightforward. Once the frass has been scraped off, a piece of tissue paper can be stuck over several suspected areas of wood and sealed round all edges with a water soluble glue, and left from April to August (this is the flight stage in the lifecycle). On inspection, there may be larvae and beetles trapped or flight holes puncturing the tissue paper. All of these point to an active infestation. Also, checking spider webs in old timber framed buildings to see what’s entrapped can help to establish the types of insects that are present. You should also recognise, that many timbers were installed with insect flight exit holes, because wood boring insects also attack trees as well as cut timber. The majority of flight holes observed on timber from historic buildings are often at least 100 years old. This is because the infestation took place within the living tree before it was even felled, due to the moist conditions present in nature. Needless to say, whether the bore holes are old or new, forms a critical deciding factor in whether or not woodworm treatment is required.

Case Study

In august 2017, the property in question was subjected to a damp and timber survey, by a large national damp proofing company. The client knew they’d be expensive but wanted the assurance of dealing with a ‘brand’ name.

Damp & Timber treatments specified costing over £22,000

Damp & Timber treatments specified costing over £22,000

In total, this company specified over £22,000.00 worth of work, which included over £3000.00 for treatment of wood boring insects and timber decay. Post completion of the work, another surveyor attended the property and informed my clients that he was of the opinion that this work wasn’t necessary, and that my clients had been mis-sold these treatments. Obviously alarmed, they asked me to carry out a review of this companies survey report and survey methodology.

The Survey

There were many issues, both with their investigation for rising damp, and their assessment of timber decay, but for the purposes of this blog, we’ll remain focussed on their assessment of wood boring insects and the treatment specified.

The surveyor repeatedly made claims in his  report that there were, “current attacks by wood boring insects,” and of course used the word ‘current’ to justify his specification for timber preservative treatment.

In fact, no evidence whatsoever was presented proving a current attack, failing to even take or record timber moisture contents; a significant omission. BRE Good Repair guide 13, highlights the following as indicators of live infestation:

Indicators of insect activity are:

  • Freshly cut exit holes and recently ejected bore dust (frass), although dust may have been shaken from timbers by foot traffic.
  • Insect larvae extracted by probing the tunneled timber. In practice the larvae are difficult to find.Identification of the insect causing the damage is important, not only in deciding if any wood preservative treatment is necessary, but also in deciding if any other action is required.

From the flowchart below, it could be seen that the company concerned failed to demonstrate or prove that insect infestation was active, and therefore failed to reasonably comply with this guidance. If no live infestation was present, then the next step is to assess whether timbers contain more than 20% sapwood; if the answer is no, then no treatment is required.

BRE Good repair guide 13

Flowchart taken from BRE Good Repair Guide 13.

Further guidance is given in BRE Digest 307, which states that; ‘The presence of damage by wood boring insects does not always indicate a need for remedial treatment.’  The common types of damage found in building timbers have been divided into three categories:

BRE Digest 307

Taken from BRE Digest 307

Again, the table above highlights the importance of identifying the species before deciding on treatment measures, and throughout the report, wood boring insects were only discussed in general terms, with no identification being made. What we know unequivocally, that no proof. whatsoever was provided to demonstrate this timber treatment was necessary.

As discussed earlier, the client was charged over £3000.00 for timber preservative treatments, the insecticide, being a Permethrin based product. If you want to understand the mark up on these products, 5 litres of Permethrin based product, can be obtained from a brand name company, for around £9.00; enough to cover around 15-20 square meters of timber.

How we deal with woodworm organically

It is vitally important to get to the root cause of the woodworm issue so that a long-term solution can be implemented. Since the woodworm larvae thrive in damp timber, it makes sense to identify and remedy the cause of that damp, and by doing so the infestation will cease, and more importantly, not come back.

Generally speaking. woodworm like their meals with a little ‘gravy’ so you should focus on the root cause of the damp in timber and we would generally look to treat this issue in the following simple manner…

  1. Eliminate the source of moisture and introduce rapid drying
  2. Assess the severity and depth of timber infestation and damage
  3. Replace any structurally unsound timber
  4. If required improve ventilation to the affected area

It is a treatment plan almost identical to what we’d specify to deal with an outbreak of fungal decay such as dry rot.

It is a sad fact that the damp proofing industry has enjoyed great success by unnecessarily recommending that damp timber infested by woodworm is covered with litres of toxic chemicals. The main issue with this method is that it does not address the primary reason as to why the timber is infested in the first place, Since the timber is still damp after being treated with chemicals, there is a likelihood of re-infestation occurring.

Furthermore, these toxic chemicals can cause environmental damage and might even require a licence if being used in close proximity to any protected species. Thankfully though, the inadequacy of this practice is now becoming better understood and is well documented in BS7913: 2013 “Guide to the conservation of historic buildings”. It also identifies that chemical treatment does not actually penetrate deep into the timber, but only just under the surface and therefore any larvae, which are active inside, won’t necessarily be affected.

Please follow and recommend our blog page:

Thatched Roofing

An Introduction to Thatched Roofing

1950's Partially Thatched Roof

1950’s image shows a mix of clay pantiles & Thatch

Grade II Listed Thatched Cottage in Nottinghamshire

Grade II Listed Thatched Cottage in Nottinghamshire

We carried out a full condition survey last week on a beautiful 600 year old cottage in Nottinghamshire. From my perspective the key point of interest was the thatched roofing, and understandably we had a client who was particularly interested in its condition and the potential cost of any repairs or maintenance. When dealing with buildings of this age, then obviously there is a strong focus on conservation so that often involves doing a great deal of background research into the history of the building and in this particular case there was an anomaly to clear up… Doing the background research, we came across an image of the building taken in the 1950’s that clearly showed the roof to be only partially thatched. We were confident that the thatch predated the clay pantiles seen in the image but it was

important to prove it.

This is a grade II listed building and the provenance of the roof may well influence any listed building consents that may be required. More important, since it was immediately obvious that the building needed a re-thatch.

What we managed to find was two comparable street scenes taken over a century apart that proved that the thatch predated the partial clay pantiling.

Thatched Ridges

We also noted that in the early streetscene that roof was seen to have a flush ridge, as opposed to the  current decorative patterned ridge. The flush ridge, gives rise to the possibility that the thatching material has been changed over the decades.  Long straw is generally installed with a flush ridge. However, the patterned ridge, as currently installed, was a post world war 2 fashion and it is more likely that the change in ridgeline simply followed the fashion for patterned ridges. It should be noted that patterned ridges are more problematic and have a reduced lifespan. This is because the hazel liggers, used to hold the ridge in place, impede rainwater runoff from the ridge, creating depressions and moisture pathways into the straw, which promotes rotting of the straw. There is a strong argument for returning the roof to a flush ridge should the cottage be re-thatched. If the current patterned ridge is retained then the client should be prepared for more frequent ridge changes.

Streetscene showing existing Thatch Roof

Streetscene showing existing Thatch Roof

Streetscene proving thatch predated the partial clay tiling

Streetscene proving thatch predated the partial clay tiling

Types of Thatch

Exposed horizontal sways & spars due to loss of thatch material

Exposed horizontal sways & spars due to loss of thatch material

Traditionally there are three types of thatching material, Water Reed (Traditionally from Norfolk though most reed is now imported due to high demand), combed wheat reed (straw), or long straw. Water reed requires a complete roof strip back to timber when it is renewed, whereas combed wheat straw (reed) and long straw can be partially stripped and re-thatched. The material on the roof of this particular cottage is combed wheat reed.

Thatch Defects

we noted a number of issues, which included:

• Heavy build up of moss to some areas of the roof, particularly near the chimneystack to the northeast corner of the building.

• Substantial stripping of straw material and exposure of both the hazel fixing sways and plastic fixing spars, particularly to the south elevation of the roof.

• Complete failure of the mortar fillet flashings around the base of the chimneystacks.

Exposed plastic spars (Staples)

Exposed plastic spars (Staples)

It was particularly interesting to note that the ‘U’ shaped fixing spars are manufactured plastic fixings, as opposed to the more traditional hazel fixing spars but this is unsurprising, since handmaking hazel fixing spars is incredibly labour intensive and something of a lost art.  It’s also  worth noting that even the wire netting, installed to prevent birds from stripping material from the roof, can help aid identification since only the ridges are generally netted for water reed, whereas straw is completely netted.

Cost of New Thatch

It is clear the roof needs re-thatching in the short term, and this would also include installation of a new ridge. You should generally budget for circa £120.00 per square meter, though thatcher’s often quote by the ‘square’, which is equal to 100 ft2 or 9.3 m2. The total cost for a square would be in the region of £1116.00 net.  In this particular case we calculated circa 130 square meters of thatch so replacement may cost in the region of £18,720.00 inclusive of vat at 2016 prices. For context and an historical perspective this is circa two thirds of the National average wage.

New ridge to thatched roof

Decorative ridges need more frequent attention than the main roof

Moss build up causes underlying thatch to rot. You have the roof 'demossed' as required.

Moss build up causes underlying thatch to rot. You should have the roof ‘demossed’ as required.

When inspecting the roof structure we would always check the timber moisture content of the roof to ensure that the roof  timbers are not at risk of timber decay. The ideal moisture content for roof timbers is between 8-12%. This is slightly elevated but well below the 20% danger threshold and what roof timbers could be visually inspected,  appeared to be sound.

Fire Risk

Chimneys and flues are a particular concern when dealing with thatched roofing since a high percentage of fires started in thatched roofing are caused by chimney and/or flue defects. However, that should be contextualised against the view that fires in thatched roofing occur no more frequently than they do in more traditional roof coverings and the frequency of fires has been greatly exaggerated. Statistics prove this point and insurers are well aware of this fact but the comparable incidence of roof fires is probably down to the fact that owners of thatched roofing understand the risks and take the necessary sensible precautions to mitigate the risk. Some of the general fire safety considerations required for ownership of thatched roofing include:

• Have the chimneys swept twice a year

• If solid fuel fires are being used then burn only seasoned wood or coal

• Flues should be lined with a suitable liner if using solid fuels or a stove

• Chimneystacks should be kept in good repair

• Always have hard wired smoke detectors installed to the property

• Ideally, rely entirely on gas central heating and decommission chimney flues.

There has been a trend towards fire proofing the underside of thatched roofing by installing Rockwool slabs or spraying fire retardant onto the thatch but there are some concerns that the Rockwool could impede airflow through the thatch, thereby promoting rotting of the thatch, and similarly there is a risk that fire retardant chemicals may also promote rotting of the thatch.

Chimney Defects

Substantial failure of cement mortar fillets

Substantial failure of cement mortar fillets. Was ‘P L’ the last thatcher?

We noted a number of obvious visual defects to the chimneystacks

• Repointing was required to the two high level stacks on the main roof ridgeline. Previous attempts at repointing have been poor and the mortar has already failed.

• There is substantial failure of the cement flashing fillets to the base of both chimneys to the main roof ridgeline.

• We also noted that the cement flaunchings, which secure the clay chimneypot, appear to be failing on the central chimneystack.

We do not have a problem with the use of cement mortar fillets though lead can also be used; however, they have probably failed prematurely because hard Portland cement rather than more flexible NHL 5.0 lime mortar was used.

Thatched roof new ridge in progress

Thatched roof new ridge in progress (Summer 2016) but chimney flashings not yet complete.

There is no doubt that thatched roof ownership comes at a high comparative cost when you consider that straw thatch may require a re-thatch every 20-30 years and ridges need replacing every 10-15 years but these are often, as in this case, very special buildings, and they are worth the high cost of ownership.

Please follow and recommend our blog page:

Collyweston Stone Slate

Possibly the best material you could have on your roof. 

Leicestershire property with Collyweston stone slate roof.

Leicestershire property with Collyweston stone slate roof.

We do a lot of conservation and heritage work and we surveyed another old grade 2 listed historic building this week, which was particularly fascinating for its roof covering of Collyweston stone slate. Indeed, the roofing material is probably at least partially responsible for the buildings grade 2 listing. Walls  to this building were  circa 500mm thick, with the original part of the building being around 400 years old and constructed of random rubble limestone. Collyweston stone slate gets its  name from the village in Northamptonshire, which is where these slates are made and it is a  material whose use is generally restricted to areas running along the limestone belt so can be found in Northamptonshire, South Lincs, Rutland and Cambridgeshire; the property in question is in Rutland. Collyweston stone slating has never been a large industry but it is now extremely rare and we believe that there are only two roofing businesses operating that specialise in this material.

About Collyweston Stone Slate

Collyweston slate is not actually slate, in fact it is limestone dating from the jurassic period that splits naturally along its bedding plane to form slates. Making these stone slates is incredibly labour intensive and skilful. In the 19th century the process was known as ‘foxing’ and involved a miner laying on his side and tapping away at the overhead seam with foxing picks. At some point the overhead seam would fall and miners would build up temporary supports for the seam using columns of waste stone. If the seam did not fall by the days end then a ‘lions tail’ would be used to lever the seam down; it would hopefully smash into manageable pieces when it hit the floor and hopefully not land on a miner. These pieces were known as ‘logs’ and it was important for the logs to remain damp because they were then left out in the open on a bed of shale so that freeze/thaw action could initiate splitting of the log into slates.

Diminishing courses on Collyweston stone slate roof

Diminishing courses on Collyweston stone slate roof

Even today, slaters rely on frost to split the log. Slates are dressed into various sizes and when you view a Colllyweston slate roof you’ll immediately notice that the slates are laid in diminishing courses towards the roof peak. To accomplish this, the  underlying timber laths, usually 0.75″ x 0.75″ sections are also laid in diminishing courses. They may be spaced at around 6″s near the eaves slate and decrease to a lath spacing of around 2.75″ near the roof peak. The slates are secured with oak pegs fixed through a hole in the head of the slate. These days the hole is drilled but traditionally they were made with a bill and elves.

Life Cycle Costs

Incredibly, Collyweston stone slates are capable of almost continuous reuse, which makes them possibly the cheapest roof covering you can buy if you calculate life cycle, rather than upfront costs. It is the oak pegs or underlying timber laths that are likely to be the weak link so for very old roofing it is not unusual to have to strip and relay the roof to renew oak pegs or laths. This roof was stripped and relaid around 1989 but all the slate was reused.

Erosion to limestone parapet copings

Erosion to limestone parapet copings

Substantial oak frame takes the load

Substantial oak frame takes the load

As you can imagine, there is substantial weight in a Collyweston stone slate roof so the underlying timber frame has to be substantial and you will generally find an impressive and substantial oak frame taking the load, as can be seen in this case.

OPC mortar fillets in remarkably sound condition.

OPC mortar fillets in remarkably sound condition.

There was very little wrong with this roof, bar the heavy erosion seen to the edges of some limestone parapet wall copings and the fact that the base of the chimneystacks and parapet walls flashings had been filleted with OPC mortar. We’d have preferred that NHL 5 lime mortar was used but since both the slates and the parapet walls are limestone then there is little cause to worry about differential expansion and subsequent cracking to the mortar fillets.  The fillets were in remarkably sound condition and generally where we encounter OPC mortar fillet roof flashings, they have generally cracked or failed altogether.  After some relatively minor repair work to the parapet copings and occasional ongoing maintenance work, we feel pretty sure that the roof will be good for another 400 years.

As a postscript to this piece, I noticed that it was found and retweeted by the last company still mining Colllyweston stone slate, Claude N Smith & Co. That in turn led me to one of their Youtube videos, which is absolutely fascinating if you can spare a few minutes to watch. Mining Collyweston Slate

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:

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:

Rare Timber Framed House

Timber Framed and Technically Obsolete.

Rare 1940’s Timber Framed Building

I was asked to carry out a survey on a fascinating building this week, a very rare 8 bedroomed timber framed building. I’m told that there are only two of these buildings remaining in the whole of the UK so its highly likely that I won’t see another one and most  of us won’t see one of these buildings in our whole career. The building remains largely unchanged from the day it was built.

The building currently has a wall U-Value of around 2.1 W/m2K which is as bad as you’re likely to find and well below the current UK building regulations requirement for 0.16 W/m2K. There is no functional central heating system installed and residents gain what little warmth they have from an AGA in the kitchen, which is where they spend most of their time. It’s fair to say that they are built of sterner stuff than most of us, myself included.

Failed Hip Shingles

You won’t find any reference to this particular building in BR282, ‘Timber frame housing 1920–1975: inspection and assessment.’ The building was constructed in 1941 when masonry construction was the norm but I think there was a need for rapid construction and anecdotally I’m told that the building was used as accommodation for land girls during WWII.

Most timber buldings constructed at this time were platform framed but we’d need a more invasive inspection to confirm the exact method of construction.  We think that this is post and beam construction, or a structural frame of widely spaced timber posts with infill studwork set between the sole plate and wall plate. Joisted  and planked flooring and  4″ x 2″ rafters fixed to the wall plates at 400mm centres. Roof battens are 3″ x 1″ and closely centred to allow for a large headlap to the cedar shingles, because the roof has no sarking membrane installed. The internal walls are clad with plasterboard, which came into common use during the 1930’s.


Closely centred roof battens allow for a large shingle headlap

 The building’s timber frame is built off traditional brick footings, to which a timber sole plate is attached, the timber frame is then built off the sole plate. There should be a physical damp proof course between the brick footings and the sole plate and indeed, the timber sole plate can be weak spot in these buildings as they are subject to timber decay. For a full assessment of these buildings the sole plate and the base of the timber posts should be opened up for inspection. However, for a timber framed building approaching 75 years old this in in remarkably good condition and serves as an interesting historical marker with regard to the need for rapid construction in the 1940’s.



Typical Directly Clad Stud Timber Frame


The walls of our building have  4″ timber studding at 400mm centres  that is externally sheathed with timber, however where the technical detail differs from the image is that this building has no insulation installed and rather than external timber boarding, this buildings is clad with cedar shingles. The roof is also clad with timber shingles. Whilst the technical details are interesting, the very obvious problem is that this building is technically obsolete and can not provide the level of thermal comfort required for modern living.  In fact, there is an oil fired central heating system installed but heat losses were so great in the building that once the system failed, the occupants had no interest in getting it repaired due to the high cost of heating the building. Unsurprisingly, the occupants tell us that the building is incredibly hot in the summer and incredibly cold in the winter.


Shingle clad chimneystack

We understand the need to retain the external appearance of this building and we see no reason why this building should not provide accommodation for another 75 years, however, to achieve that aim, a substantial upgrade in thermal insulation is required, either internal wall insulation (IWI) or external wall insulation (EWI). Installing IWI, using something like 93mm Gyproc Thermaline would future proof this property without affecting the external aesthetic appearance but there would be a loss of internal floorspace, not a particular concern in a property of this size, however, this would involve major upheaval for the occupants and would also not deal with another concern relating to poor security. You’d gain entry into this property within two minutes with a decent battery powered circular saw.


Cedar shingles can be retained.

Our preference would be to remove and store the cedar shingles and clad externally with a structural external wall insulation (SEWI) system, Structherm or similar,  the building lends itself very well to this approach since it has a wide soffit detail. The SEWI could then be over clad with the existing cedar shingles and the aesthetic appearance would be retained bar the fact that windows would be noticeably set back, a feature that would afford better weather protection. Whatever approach is adopted, this is a fascinating building that thoroughly deserves a new lease of life.


Please follow and recommend our blog page:

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


Please follow and recommend our blog page:

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.



Please follow and recommend our blog page: