Bre Digest 361 Do Buildings Crack
- BRE is a world leading, multi-disciplinary, building science centre with a mission to improve buildings and infrastructure, through research and knowledge generation. We use our cutting edge research to develop a range of products, services, standards and qualifications that are used around the world to bring about positive change in the built.
- BRE (Building Research Establishment) is an independent, research-based consultancy, testing and training organisation, operating in the built environment and associated industries. Why do buildings crack? (DG 361) was first published by BRE in May 1991, and the original content largely remains valid. The current edition was published on 21.
Cracking up
29 January 2018
Tim Kenny looks at what building surveyors can do to identify structural movement
News & Information. In order to help building surveyors to identify when ground movement had occurred and when structural intervention was necessary, BRE carried out an assessment of 130 properties that had suffered from subsidence. The results of the study enabled a way of categorising cracks to be developed.
Nearly all buildings are designed to allow for some movement. If they are not then they very soon fail, because where you get different elements moving at different rates, cracks will occur.
Some cracks can look significant, leading to assumptions about the state of the foundations, but then ultimately prove to be nothing more than a decorative issue; others can appear quite harmless, but develop over time to become a major, costly problem for the owner.
This can make the job of a surveyor tricky, particularly when they are asked to assess a crack from a single visit – say, as part of a building survey. Combined with concerns over complaints and insurance claims, this might lead some to pass the assessment of the problem on to someone else, but they need not do so.
Any good surveyor should at least be able to determine whether what they are seeing is a structural issue or just a cosmetic problem. Most should also be able to offer their client some advice on the probable causes and remedies.
Severity of cracks
The first step is to consider the severity of the cracks, and BRE Digest 251: Assessment of damage in low-rise buildings provides a good starting point.
Following a study of 130 properties suffering from subsidence, 5 kinds of crack were identified, categorised primarily by width. In simple terms, the study found that categories 1 and 2, cracks of less than 5mm, were essentially aesthetic problems only and unlikely to indicate a significant structural issue. Cracks in categories 3 and 4, of between 5mm and 25mm, were found to cause serviceability issues, such as sticking doors or penetrating damp. Anything in category 5, being more than 25mm, was considered to require structural intervention.
Figure 1: Example of a category 3 crack
This is a simplified approach only and other factors should be considered, such as the number of cracks. For example, several 3mm cracks may only cause a serviceability issue whereas a lot of 1.5mm cracks could indicate a problem with the structure. Categorisation is, however, a good place to begin.
Once the surveyor is sure there is a problem, they should consider what the cause may be. There are many different forms of structural issue, but a few are more common than others.
Lateral restraint
One common cause of cracks in solid walls is where the force of the roof pushing out at the top is not offset by beams at floor and ceiling level applying force to pull the walls back together; this is known as lateral restraint. The balance of forces in such cases holds the wall in a vertical position, but it depends on those forces being equal.
Where the wall loses its mechanical connection to the floor joists, normally as a result of rot, it can start to pull away, leaving a noticeable bulge in the outside wall. This is easier to see when the masonry has been left exposed, but when a property is rendered it can sometimes be mistaken for bulges caused by blown render. Instances of lateral restraint, as opposed to blown and bulging render, will also normally be accompanied by excessive movement or spring to nearby floors.
A surveyor is more likely to come across this problem when a roof covering is changed from something lightweight, such as slate, to something much heavier. The common example is when heavy concrete tiles are used on a late Victorian terrace’s roof, which would originally been covered with slates: the increased load causes the top of the walls to be pushed out, and because masonry walls do not have much flexibility, horizontal cracks will appear just a few courses down from the eaves.
Cavity wall-tie failure
The outer skin of a cavity wall has almost no structural stability, being too slender to support its height. Instead, it relies on the support of the internal wall to hold it upright. To connect the 2 walls but maintain a suitable cavity, a wall tie is used. In modern buildings, these are normally stainless steel and generally pose no problems, provided they are installed correctly. However, when they first came in to use – around 1910, or a little earlier in some regions – it was more common to use iron.
In many ways iron was suitable for the job, but it does tend to rust and when it does it not only loses strength but also expands. As the wall ties are bedded in to the mortar between the bricks, the expansion forces the courses apart and creates regularly spaced cracks through the mortar matching the ties’ locations. This is a good example of several category 2 cracks indicating something more than a decorative issue. Bear in mind that wall ties can also fail for other reasons without causing cracks to the outside walls, and this can only be seen by inspection inside the cavity.
Falling lintels
Most windows and doors are not structural so will need a lintel to take the load of the structure above and distribute it to the surrounding walls – a simple idea that has been a part of construction since people decided that caves were not the most desirable form of property.
Although lintels have been around for a long time, mistakes are often made. One such mistake is where a lintel does not have enough bearing; that is, it may not be extended far enough into the surrounding wall. When this happens, one end of the lintel can often slip where the pressure of the wall above causes a diagonal crack through that wall.
Figure 2: Movement around a lintel
Usually the lintel will stop in a secure position and the cracking will be minor, in category 1 or 2. More severe movement and cracking – category 3 or 4 – can result in sticking doors or windows and repairs will be required.
Even where any movement of the lintel has stopped and is secure, it can still be a problem if the windows beneath are replaced. The older window may have been providing a degree of support, and as soon as it is removed the wall above could collapse. For this reason, it is important to make a client aware of even historic movement of lintels, particularly if the windows are older or in a degraded state.
Where there is any doubt about whether the movement may be ongoing due to insufficient bearing then remedial repairs are required.
Subsidence caused by trees
Subsidence is often treated as being separate from structural movement, and rightly so. Structural issues develop due to defects in the building, while subsidence is caused by problems with the ground under the building. This is an important distinction because standard building insurance policies will cover subsidence but not repairs related to building defects.
The most common cause of subsidence that a surveyor will see is that caused by trees, which disturb the ground by drawing up water. Even small trees can take a significant volume of water from the ground, while a large deciduous tree can need 50,000 litres per year and in drought conditions can have an effect down to a depth of 6m.
When dry ground shrinks, particularly where it is largely clay, any foundation sitting on it will also move and if the shrinkage only happens under part of a building then cracks will appear. These are normally tapered, visible both inside and outside the property and extending below the damp-proof course; but any cracking to a building that is likely to be over the root system of a large tree should give cause for concern.
As a rule of thumb, a root system is usually of the same diameter as a tree’s crown. As with most rules of thumb there are exceptions; for example, the safe distance for a willow tree is 40m, much greater than the usual crown width.
Just because a tree’s roots may be growing under a building, it does not necessarily mean that they should be removed as this can often cause its own problems. If a tree is not there to take up the water, then the ground expands and will start to push against a building, which is referred to as ground heave.
Inspection and investigation
Although all the above issues will need further investigation to diagnose properly, it does not mean that a surveyor cannot provide meaningful advice to a client from just a one-off, non-invasive inspection.
Tim Kenny is a residential building surveyor at Tim Kenny Surveying
Further information
- Images © Tim Kenny Surveying
- Related competencies include Building pathology
- This feature is taken from the RICS Property journal (December 2017/January 2018)
- Related categories: Modern methods of construction, Building elements, Common defects
Abstract
Bre Digest 361 Do Buildings Crack Download
Purpose – There have been several recent cases reported in Malaysian newspapers of complaints about the impact of construction works on the physical conditions of the surrounding buildings. The purpose of this paper is to present the findings of a study conducted on a complaint received by the Local Authority in Selangor, Malaysia, about the impact of adjacent new construction works on the conditions of existing buildings in the neighbouring vicinity. Design/methodology/approach – The sample case study consists of 28 units of two‐storey terrace houses, and the instrument used was the inspection form for building inspections. A building survey was carried out for all the buildings in the case study. Two teams conducted the survey. Each team was led by a professional building surveyor and assisted by two assistant building surveyors. Apart from basic surveying tools, a digital calliper was also used to measure cracks. Crack‐width data were collected and analysed using the statistical package for social sciences (SPSS) software. Findings – The findings show that the adjacent new construction works have impacted the existing buildings nearby. A numbers of cracks of various magnitudes were identified in all the existing houses. One significant finding is that there appears to be no significant relationship between the cracks on the interior and the exterior, nor between the original and the renovated parts, of the existing buildings. Originality/value – The significant number of cracks identified in the existing houses implies that the cracks were associated with the impacts from the new construction works. However, the primary cause of the building cracks was difficult to determine. It is therefore proposed that the Local Authority should establish guidelines at the local level for new construction works to prepare an inspection report on the condition of existing buildings in the surrounding area before the project commences. Building condition reports can then be used for reference in the event of claims received from the surrounding building owners alleging that the new construction works have adversely impacted the existing conditions of nearby buildings.
Data Collection
Journal
Structural Survey – Emerald Publishing
Bre Digest 361 Do Buildings Crack Cost
Published: Aug 30, 2011
Keywords: Building condition report; Building survey; Construction impact; Cracks; Local authorities; Visual inspection; Construction industry; Malaysia