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Understanding Cracks in Walls

The most important thing to learn about cracks is that they are the visible symptom of a possible problem, not the problem itself. Consequently, to truly fix a crack you must first determine what has actually created it. To help you with this please refer to the section ‘Why Cracks Occur’ for a handy list of possible causes.

Thankfully the majority of cracks tend to be superficial, will not cause any major problems and are very easily fixed. However, since this is not always the case it is important to be able to recognise the various forms of cracks and their severity. This will allow you to establish the level, extent and urgency of any necessary repair works.

Where are Cracks Typically Located?

Cracks are a mechanism for releasing stresses which have built up within a structure. Therefore, they will naturally exploit inherent weaknesses in a buildings’ design and will consequently often be found running from the corners of window and door openings. They can also be located in associated lintels and still stones.

Why Cracks Occur

Structural cracks can be caused by a variety of problems, which might include, but are not limited to:

  • Damage due to damp.
  • Crushed or decayed internal structural timber.
  • Mining subsistence.
  • Shrinkage of clay soils.
  • Tree root growth.
  • Unstable adjacent walls.
  • Lack of foundation.
  • Temperature influence.
  • Wind loads.
  • Fractured masonry.
  • Lowering of ground water.
  • Road traffic vibration.
  • Air traffic.
  • Damaged drains.

It should also be remembered that the crack may be caused not only by one of the above factors, but by a number working together to create subsidence, bowed walls, delaminated bricks and general deterioration of the masonry structure.

Size of Cracks and Determining Severity

You can use the width of a crack to find out its severity. Cracks are typically categorised into 5 groups:

Negligible – In general, hairline cracks, with a dimension of less than a millimetre in width, are of little concern; despite being unpleasant to look at. It is most likely that to tackle this problem you just simply need to redecorate. 

Slight -  Over time, once structural movement has ceased and stabilised, cracks that measure between 1 and 5mm in width can generally be fixed by filling the open voids and then redecorating the interior. You will also need to re-point the affected area on the exterior.

Moderate – If your crack is between 5 and 15mm then it may mean that building work is necessary. Potentially, it would be a good idea to involve a professional who can find the real cause of the cracks and help to determine the most appropriate remedial work which should be undertaken. With these types of cracks it is more than likely that associated problems will also emerge and need to be fixed, for instance some localised replacement of fractured elements such as window sills, door lintels etc.. Work may also need to be carried out to mend any associated concerns with respect to the weatherproofing of the building; since this may have been compromised through cracks disturbing roof coverings etc..

Severe – If the crack is up to 25mm wide then it is usually a good indication that extensive structural repair works is needed. This will probably mean the replacement of affected sections of the building or structure. These types of cracks may also be accompanied by a variety of smaller cracks. With these forms of cracks you should always seek professional advice regarding the possible causes and the type of remedial work needed. It may also be pertinent to install temporary support scaffolding until the remedial works have been undertaken.

Very Severe – If the crack is greater than 25mm in width this generally means very severe structural damage. This form of damage normally leads to major repair work; these repairs will probably involve the partial or complete rebuilding of the affected area. With these types of cracks structural underpinning may also be necessary, since there is a danger of associated collapse and instability. We highly recommend that you seek professional advice as soon as possible. It is also a good idea to install a temporary support until the remedial work has been completed.


The Shape of Cracks

Another useful way of identifying cracks is through their shape, which can provide a good indication as to their cause.

Fine hairline cracks which run across the face of a wall may simply indicate that a small degree of shrinkage has occurred. Since most walls constructed in pre-1919 buildings use lime mortar, the structure can accommodate such fine defects with little worry. In fact hairline cracks may be seasonally driven and consequently appear and disappear depending on the climatic conditions.

A diagonally running stepped crack, similar in appearance to a set of stairs when viewed from the side, can denote that structural settlement is happening, which may be because of upheaval at foundation level or some other type of slippage. These types of cracks generally start off as hairline crack which tend to follow the alignment of the horizontal beds and vertical joints in the structure. If foundational settlement continues the crack can grow in width and individual stones or bricks may even loosen and become dislodged. With these types of cracks we advise that you seek the advice of a professional as soon as possible.

If the crack is vertical, or near vertical, this can also be a sign of a serious problem. If the cracks appear to be wide at the top and tight at the bottom this may indicate that one or both ends of the buildings’ foundation are dropping, or in fact that the middle of the foundations are rising. It should also be noted that vertical cracks also usually mean that the stresses within the buildings’ structure have been sufficiently severe enough to crack individual stones or bricks in the wall to such an extent that could make the broken pieces unsafe. Alternatively, if the cracks are wider at the bottom and tighter at the top then the opposite effects may be occurring. Parallel sets of vertical or near vertical cracks can also display variations on these symptoms which indicate complex foundational movements are taking place. Again we highly recommend that you seek professional advice as soon as possible and that you also undertake temporary works.

Continuous horizontal cracks which follow the masonry beds between the stones or brickwork should also be considered as very problematic and due caution should be shown. These type of cracks may indicate that walls are in an early stage of failure, particularly if the wall is also acting in a retaining capacity. Such crack patterns can also be found on parapet walls. If this is the case these should be investigated at an early stage, due to the added risk and safety problems that would be created if the parapet became loose and dislodged as a result of the structural failure and the real risk that the crack could be allowing rainwater to enter the building interior.

Useful Products

We have a wide range of crack fixing products located in the CrackFix category on our website, so why not have a browse and see what will work best for you. To get you started, though, we have highlighted a few useful products below.

For negible and slight cracks:

Decorators Caulk (Product Code: N02130): This handy little product will help repair small interior cracks in just 60 minutes.

For Moderate & Larger Cracks:

SkilledBuild Spiracoil Crack Stitching Bar (Product Code: N10000a) are free of expansion stresses when they are installed, so no further stresses will be introduced into the structure. In addition, because they are easy and fast to install the integrity and stability of the structure is restored quickly and economically.

In most situations these helical wires are installed externally causing the minimum amount of disruption to both the structure and the occupants.

Your best option is to buy one of our Crack Stitching Kits which include 10 SpiraCoil bars and everything you will need to successfully install them.

For more detailed information on crack Stitching please refer to the relevant ‘how to guides’.


PLEASE NOTE: We always recommend that a full survey is carried out by a qualified Structural Engineer before any work commences.