Interpreting whether a crack is of structural significance is usually seen as one of the more challenging aspects of a Surveyor’s role when establishing the condition of a building.
There can be a tendency to err on the side caution and conclude that cracks which have appeared in a wall are naturally symptomatic of ground subsidence. Subsidence is, however, comparatively rare and the number of cases where further action is required is minimal. The Surveyor should therefore first try discount less serious causes.
To do this requires only a relatively basic understanding of the principles of physics and building construction.
Bricks and stone are very strong in compression but conversely weak in tension. When materials that are weak in tension are pulled apart, they tend to crack at right angles to the direction of the force acting on them.
Therefore, if we were to draw an imaginary line at a right angle to the crack it would provide us with the direction of the force causing the crack.
With this information we can predict that a vertical crack is likely to have manifested due to horizontal forces acting on the wall. These horizontal forces tend to be from thermal expansion of the wall in hot weather. This is one reason why movement joints are now added to walls that are in excess of 10m long.
A common crack pattern above window openings is shown on the attached image. Here, there are two cracks running diagonally above the opening. Again, following the same principles, when we add our imaginary line of force, we see the two arrows merge to point downwards to the location where there is no lintel support and where the forces are occurring.
Following the above principles will help as a basic guide when diagnosing the causes of cracks and will provide a reasonable degree of accuracy; however, there are always anomalies so please do call us if you would like a professional opinion and we would be delighted to assist.
For those interested in further reading on the subject I can highly recommend Malcolm Holland’s ‘Practical Guide to Diagnosing Structural Movements in Building’.