June 7, 2019
In this post, Chartered Surveyor Stephen Boniface discusses structural movement and cracking, and covers the various reasons why cracks may appear in a building.
Perhaps the most common concern that surveyors are asked to address when inspecting properties is whether there is structural movement to the building. In recent years we have witnessed drier summers followed by drier winters and the incidents of movement seems to be on the up. However, there are many reasons why a crack might occur in a building. In this brief article I cannot hope to cover all causes of cracking, but I attempt to provide an overview that readers may find helpful.
The reality is that all buildings will undergo movement of some description. The issue is how that building copes with the movement. With most modern buildings they are designed to accommodate a degree of movement, particularly if large, unusual, or in an area where problems may occur regularly, e.g. earthquake zones.
As this article is aimed at the residential homeowner based in the UK, we do not need to concern ourselves about some forms of movement.
The materials we construct with will have a significant bearing on the likelihood of cracking occurring. The crack is usually the first visible sign of any form of movement. Not all cracks are significant or such that need cause concern.
The main factor that determines whether a building cracks is the flexibility of the materials and of the structure generally.
Some materials are quite brittle and will crack quite quickly under any stress or strain, whereas other materials can accommodate a greater degree of movement before cracking occurs. For example, cement-based mortars and renders are more brittle than those that are formed using lime mortar. Therefore, we tend to find that modern materials and buildings often exhibit cracking far earlier than traditionally built properties when subjected to the same stresses and strains. This sometimes results in quite significant distortion to an historic building with no cracks apparent whereas a modern building might have a number of cracks with no specific distortion apparent.
Therefore, an important issue to consider is the nature of the building and the materials from which it has been constructed.
Broadly, temperature has an important part to play, but it is not only air temperature or the temperature of materials; it is also the moisture content in the air i.e. humidity.
Some materials react to temperature changes with significant changes in dimension. This means that materials might shrink or expand according to temperature and/or humidity. When assessing cracking it is important to note when the cracking first appeared or was noted to have worsened because weather conditions generally might be a significant influence.
However, when thinking about temperature, the issue of heating within the building can also be a factor. I have seen a number of historic buildings that started to move and cracks occur after central heating had been introduced. The problem being that the materials in the building had reached an approximate equilibrium in terms of moisture etc, but the introduction of central heating dried out the interior of the building such that moisture was taken out of the materials causing them to dry out further. Whilst this may not lead to any major structural problems it can cause some alarm for the occupant. It highlights the importance of careful consideration being given to the introduction of heating into a building that had previously been unheated, or where there is to be a major change in the nature of the heating.
Drying out is a common problem, particularly for new build. When buildings are constructed a significant amount of moisture goes into that construction. My grandfather was a builder back in the mid-20th century and he used to tell me that they did not allow anyone to occupy a building for the first several months after it had been constructed because they wanted it to dry out and settle so that they could deal with any snagging, filling of cracks etc before anyone moved in. Nowadays we tend to occupy buildings before the paint has even dried. It is no wonder that the occupiers then experience problems including cracks caused by drying out.
However, when repairs are undertaken to a building and a filler or areas of new plaster are used for repairs then some drying out should be anticipated. Sometimes cracks can occur around a repair.
Different materials will have different rates at which they will dry out. Also, materials will respond differently to temperature and moisture content. If different materials are placed alongside each other differential movement and cracking can occur.
In recent years we have seen an increase in the number of people wanting to build extensions of oak and sometimes green oak is used. This is oak that has not been subjected to extensive seasoning and has not been kiln dried. It will therefore still have some drying out to go through. If the design of the building does not allow for such movement significant cracks can occur where the oak abuts plaster or other materials.
We also know that metal can have a significant response to temperature and where there are metal elements incorporated in a structure these might expand or contact at different rates to the surrounding masonry etc. This then can lead to cracking at or close to the interface of the different materials.
Sometimes one sees these two words used almost as if they mean the same thing.
Settlement is where the building moves slightly when first constructed because the ground is taking the new weight above it and/or the structure is settling down partly due to drying out but also as the loads are distributed throughout the building. Such initial settlement is not unusual. Of course, any distortions that may be related to this initial settlement will be with that building for the rest of its life and when looking at an older building one has to bear in mind that some of the distortions might have existed almost from the day it was constructed.
Settlement can also occur when alterations are carried out to a building because loads within the building may have new load paths and the load may be distributed in a slightly different way. Therefore, some settlement following significant alterations and extensions can occur. I quite often find that there has been slight differential settlement between parts of a building constructed at different times.
On the other hand, subsidence is where the ground under the building moves. This movement under the building leads to movement of the structure above.
Subsidence in many parts of the country is most frequently associated with shrinkable subsoils such as clay. During dry periods the clay shrinks and the building above moves. Quite often the shrinkage of the soil is caused or at least exacerbated by trees and vegetation taking the moisture out of the soil. Although you will regularly read of the common culprits such as willow trees, poplars and oak, I have seen examples of subsidence caused by privet hedges and even large rose bushes.
Subsidence can also occur if fine material in the soil is washed away or if the subsoil is easily eroded by underground water. In such situations I have often found that leaking drains, gulleys etc have caused a problem. This usually leads to a localised area of subsidence to the building. In some parts of the country subsidence may be caused by mining and other such activities under a building.
Where subsidence is suspected the cause of the ground movement needs to be established. When I first became involved in dealing with subsidence issues in the 1980s it was quite common to underpin buildings but by the late 1980s and certainly into the 1990s it was realised that underpinning was not the panacea and in some instances caused more problems than it resolved.
The approach now usually taken is to establish the most likely cause, deal with the cause and monitor the movement to see if the movement ceases. It is only if removing the cause does not stop the movement, for whatever reason, that underpinning might need to be considered.
If subsidence is suspected monitoring the movement over a period of time is usually the first thing to be undertaken. As part of the investigation it may also be necessary to look at the nature of the subsoils and perhaps take samples of the soils and anything found therein (such as roots) to see if this gives any indication as to the cause.
Subsidence can be quite worrying and is a lengthy process to resolve. It can also lead to quite dramatic cracking. However, in many instances the problems can be resolved.
Alongside subsidence one has to consider heave. This is where the soil swells up and therefore lifts the building above it. This is most commonly associated with clay where perhaps the clay has had trees growing in it or nearby that took out moisture each year. If the trees are then removed the moisture stays in the ground rather than being removed and the clay expands. This can then result in heave.
In the past, I was involved with a building where the structure itself had not undergone any movement, but the owners of this new house reported drainage smells. To cut a long story short we found that the drainage under the building, within a void, had been crushed and moved by heave of the soil under the property. The area had previously been a woodland and whilst the foundations of the building had taken adequate account of this the void under the floor was inadequate. Therefore, when the ground heaved it damaged all the drains. The solution was quite simple in that the soil had to be dug out to create a deeper void. As you may imagine the task was not the most pleasant for the contractors involved!
Landslip will not affect a property on a level site and rarely affects a property on a very gently sloping site. However, where buildings are constructed onto or into a slope there is a risk of landslip. Landslip is often triggered by weather. Perhaps a flash flood or excessive rain over a period may cause destabilisation of the ground and therefore slippage. Leaks to drains and pipes underground can wash away material and cause landslip. In some areas erosion along a coast can cause landslip.
Whenever cracking occurs to a building built into a sloping site landslip should be considered as a possible cause.
The construction of a building incorporates a large number of materials and therefore a number of various chemicals are introduced into the property. Many of these are not problematic, but sometimes chemicals can result in movement.
For example, some chemicals will dissolve in moisture, but if the moisture evaporates the chemicals crystallise. The act of crystallisation usually results in expansion because crystals take up a greater amount of space than the dissolved chemical (we often refer to them as salts). Therefore, salt expansion can cause cracking.
In some situations, exposed elements of a building will be subject to weathering. Weathering can react with chemicals within the mortar (usually by dissolving and then the salts crystallising) to cause expansion. This can occur with chimneys, for example.
I was once called in to advise on some cracking at first and second floor level within a Georgian terraced town house in London. At first floor level was the principal reception room and the owners, who had recently moved in, had installed a bookcase along one wall. I quickly realised that the load of the books had caused overloading to the floor and this had resulted in cracking above and around the area.
In that particular instance we had an engineer design a method whereby the load of the bookcase could be transferred to the wall rather than being directly onto the floor.
In another instance I dealt with the refurbishment of a first floor flat in west London where the clients specifically kept a significant number of books. Engineers had calculated that the load would be too great for the floor. Therefore, as part of the refurbishment the floor had to be strengthened and stiffened.
In the above examples books were the cause of overloading and in my experience, homeowners often underestimate the weight of books and paperwork. When we find movement occurring or cracks around upper areas one has to think about whether there had been some overloading. This may apply to floors but can also apply to structures generally. Another common cause of overloading is baths. Homeowners often underestimate the weight of a bath full of water and a body.
The load paths should also be considered. We recently inspected a fairly modern building where there were significant cracks at first floor level. It was noted that the ground floor room was one large open space whereas the first floor above had been subdivided into at least three areas. The walls forming these subdivisions were of blockwork and in one area there was a boiler with hot water cylinder installed. Directly above this was a cold-water storage tank in the roof. All of this load was directly onto the floor that was over a void with no partition or other support below. There had been significant overloading of the floor thus causing the movement.
Whenever there is distortion and cracking the paths of loads through the building should be followed and considered.
All buildings will eventually deteriorate with age. How we maintain and look after them will determine how quickly or slowly they may deteriorate.
Of course, if that deterioration results in rot or infestation to elements within the building these can result in movement.
Some years ago, I was asked to look at a problem of dampness within a Grade II* listed pub. Upon inspection I realised there was active death watch beetle and clearly problems far worse than just dampness. Some of the cracking at first floor level appeared recent.
I asked the contactors to carefully remove the cement render from the timber frame so that we could investigate. When I returned the next day, the builders had only removed a very small part. Upon enquiring why, they informed me that the building had moved. It turned out that the render was the structural element of the wall because the frame behind had deteriorated to such an extent that it was no longer able to support the loads above it.
This was an extreme example of where rot and infestation resulted in movement, but of course failure to maintain a building is likely to result in various problems and one result of this could be movement if rot and/or infestation cause deterioration to timbers.
Over the years I have come across many Georgian and Victorian buildings with internal timber partitioning where alterations had created problems. The belief of some homeowners was that because the internal partitions were timber, they could not be load bearing. This was in fact wrong because with many of these buildings the whole of the interior is timber construction and much of it does provide load bearing support for the floors and roof above. In some instances, principal areas of bracing within the framing had been removed and movement had occurred above. Such matters are simply where the alterations were ill considered and poorly executed with inadequate allowance for re-distributing load paths.
A few years back, I was asked to investigate a problem of bulging to a wall that had been identified by a mortgage valuer as a serious problem. The building dated back to the late mediaeval period and had started life as a mediaeval hall house with no chimney or upper floor. Within a few minutes of entering the property I had realised what had happened. When the chimney and upper floor had been inserted some years after the house had been built, some of the principal members of the original hall house frame had been cut away. This resulted in lateral movement and hence the bulging. Nonetheless, it is quite likely that the builders undertaking the work at the time realised their mistake very quickly because where the floor had been inserted it followed the bulge precisely and in fact the floor was now providing the required lateral restraint for the building.
In that instance I was able to determine that the movements had occurred several hundred years before and was not an ongoing problem.
It does however highlight the fact that alterations can have a significant impact on a building and any changes need to be very carefully considered. Where cracking and movement occurs to a building that has been altered one needs to consider what took place and whether that might be the cause of the present problems.
The key to identifying causes of cracking is to understand the nature of the building, its materials, construction, the site it sits on and what alterations might have been undertaken in the past. It is only when all this information is available and can be properly considered that the cause of cracking can be determined.
Cracking can be quite an emotive problem. Where clients notice a crack, they can become fixated on it; I have had some contact me on a regular basis to tell me the crack is worsening. Upon investigation it is not, it is simply that they have become so focused on the crack that they perceive a problem when there is not one. Of course, sometimes, there is a problem and that is where the understanding of the building, monitoring the situation and proper assessment comes into play.
If you have a problem with cracking or are concerned about movement to a building, we here at Whitworth are able to provide advice. Call us today on 01284 760421 or visit our contact page here.
About the author
Stephen Boniface is a Chartered Surveyor at whitworth. Stephen has extensive experience in historic buildings and conservation work, and on a daily basis carries out a variety of work from surveys to expert witness.