When does a minor plaster crack become a serious structural issue

In our first article, we discussed in detail what is a crack and why we worry about them. In this second article Simon Pole BSc C Eng FIStructE MICE MRICS MAE of Pole Structural Engineers Reports looks further into when a minor crack becomes a structural issue.

Introduction to cracks

Many people often have concerns about cracks present in their homes or those they plan to purchase. Whether the cracks are small or large, there is often uncertainty regarding their severity. This second article in the series explains “cracking” in more detail and provides guidance on when it is necessary to be concerned. Additionally, it aims to reassure readers that not all cracks are indicative of serious issues.

Background to cracks

It is only natural that cracks in a property can cause concern. Whist many cracks turn out not to be serious, those which become a genuine concern, can start off as small, minor cracks. Cracks in buildings can also cause concerns among surveyors, lenders, and insurers, which can result in challenges when buying and selling property and occasionally impact property prices.

Firstly, it’s important to say that most building materials occasionally crack. At the smaller end of the scale they shrink and expand due to changes in the environment. Those changes might be due to stresses and strains, which can lead to more significant implications, or just changes in moisture or temperature levels from one season to the next. These are less problematic. The more brittle the material, the more likely it is to be prone to cracking when exposed to any changes in its environment. This is why modern plaster and some external renders crack so easily due to their brittleness.

The most common building materials seen with cracks are either internal plaster or external brickwork and cement renders.

Most people notice cracks inside their homes first, as they are constantly exposed to this environment. And cracks on the exterior may also be quite noticeable, especially those near the front door. However, cracks in less obvious areas outside, such as the sides of the house or hidden behind garden shrubs, often go unnoticed by many homeowners. We often point these out during our surveys revealing that the owner were unaware of them.

Some small cracks might be recent but are continuing to widen so could be a problem in the future, whereas a larger crack might be very old and dormant, therefore less of a concern.

We need to differentiate between cracks which are a personal concern of the homeowner, and those which present a structural concern. The two are quite different and so Engineers and Surveyors need to appreciate this point. Cracks which worry many homeowners, and indeed those Surveyors associated with mortgage Surveys, may only range from a hairline width to perhaps 2 or 3mm wide.

In contrast, a Structural Engineer and the building industry does not, in general, become concerned until a crack widths is typically 5mm wide AND is becoming progressively wider. That said, as Structural Engineers we understand the concerns of owners and buyers and the practical issues associated with obtaining mortgages and Building Insurance.

A good Structural Engineer will appraise the implications of any cracks and present the report in a way that is readily understood by the public and crucially reflect both their concerns and the practical implications for obtaining mortgages and building Subsidence Insurance.

When construction projects are in progress and in the event, neighbouring buildings are damaged, the Party Wall Etc Act 1996 usually deals with the resulting damage and there is time to investigate and propose alternative solutions. In contrast, at the point of sale, Structural Engineers do not have the luxury of time to investigate, dig holes and monitor cracks, so a more decisive conclusion must be reached to assist the buying process.

The much quoted Burland Scale which categorises cracks together with the British Research Establishment Document number 251 1995, will be the subject of a future article.

Minor cracking

The following might help you when looking at cracks between a hairline to 3mm range.

Cracks which are not usually a problem

  • A crack that is located in a non load-bearing structure, such as a lightweight partition or in a piece of wooden joinery. Be aware that some timber stud walls can be load-bearing.
  • An internal crack that occurs at a junction between abutting a wall or between a wall and the ceiling.
  • A crack which only occurs internally, within an external wall, but does not occur in about same location externally.
  • A very old, dark and dusty crack, which occurs in a structural wall, inside and outside in the same location but shows no signs of ongoing movement. Simple correct resin repairs may be the solution to keep the property strong and robust. A further article regarding crack repairs will appear soon. 

But what about more serious cracks?

Cracks which are either larger, more than 5mm, and smaller cracks of a particular type which progressively become larger, may be a cause for concern.

As we have said previously, there is a distinction between cracks which are of personal concern for the homeowner compared to cracks which are of a structural concern and where major repair might be required. A homeowner who isn’t planning on moving may well be able to take longterm pragmatic action towards the cracks which occur in their home, providing they receive the correct structural advice and are subsequently reassured. In contrast, even small but progressive cracks can cause disruption to the buying and selling process and so it’s important to consider the position of the reader.

Identifying more serious cracks

In general terms, cracks which are of some structural concern usually include the following:

  • Recent cracks in external walls, throughout the thickness of the wall, appearing both inside and out and seem to be getting wider year on year. In London and the Home Counties, London Clay and trees combined tend to cause most subsidence cracks. A future article will look at geology, trees and the general causes of subsidence.
  • Diagonal AND tapered cracks in external walls, inside and out. These types of cracks tend to propagate from the top of a building and move downwards, as a result of what we call rotational movement of a building. This is perhaps counterintuitive when considering foundation movement and subsidence as many people might reasonably look for cracks close to the ground first! Subsidence and crack pattern will be explained in a future article.
  • Cracks wider than 2 or 3mm may concern a pre-purchase surveyor and impact the sale transaction process even though the damage is slight in structural engineering terms.
  • Cracks wider than 5mm start to become more serious to the structural engineer and whilst safety is not an issue, it becomes more important to establish causation and properly engineered repair solutions.
  • Cracks larger than about 20 to 25mm can start to impact structural performance and possibly safety so must be taken very seriously.


Cracks can elicit strong emotions in property owners, buyers and lenders, and Insuring Institutions.

The reasons for worrying about cracks are understood by an experienced and expert Structural Engineer, who can normally provide reassurance and clear guidance for action and repairs.

In general, Structural Engineers and the building Industry generally tend not to worry about smaller scale cracks as they rarely have an impact on the structural performance of a building and can usually be repaired easily and inexpensively, much like a dent in the bodywork of a car. Preventing cracks from reoccurring, however, is often more challenging, similar to preventing dents on a car.

In summary, cracking in general tends to worry many people perhaps unnecessarily, particularly during the stressful period of buying and selling property.

This article gives advice for what to look for in cracks, either minor or more serious, but must be regarded as very general advice since each and every property is different.

A Chartered Structural Engineer with specialist knowledge on cracks in residential buildings is the most appropriate property consultant to advise you on such matters.