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Cracks in Buildings

The sight of cracking can set off alarm bells in anyone considering purchasing a property

Cracks in Buildings. How Serious Are They?

The sight of cracking can set off alarm bells in anyone considering purchasing a property, although, most properties will exhibit cracking at some point in their lives. As materials age and settle to accommodate changes in the building’s environment, cracks can appear if the builder has not allowed for this environmental change by installing control or expansion joints.
While cracking is often not a great cause for alarm, affecting only the appearance, some cracking can also be a sign of a serious problem, such as the stability of the building being under threat.
Because of this, all cracking, insignificant or otherwise, should be considered seriously and checked by an expert.

What To Look Out For

Wall cracks fall into three main categories:

  • Interior wall cracks which occur in plasterboard or gyprock.
  • Exterior wall cracks such as cracks in the surface, brickwork, or in the rendering of external walls.
  • Cracks in foundation walls.

All three can be serious and signs of structural failure.
In order to assess how serious the problem really is, you need to look at the location, the type of crack, and whether there is a separation in the materials used to construct the wall or if a separation has appeared between the wall and the framework.
Cracks should also be monitored over time to see if they are smaller or bigger, or staying the same size. A crack that starts off small in size may change into a larger crack, being an indication of more significant movement to the building’s structure that will require remediation.

Shapes Of Cracks

The form and size of the crack can sometimes indicate the underlying problem. Cracking can be horizontal, vertical, stepped, cogged or a combination of all these. When assessing cracks, the width of the crack is often more important than the length of the crack.
Stepped cracks tend to follow the lines of horizontal and vertical joints in buildings, such as beds of mortar between bricks or blocks and may indicate structural movement.
Vertical cracks may indicate that structural components such as bricks or blocks have failed and so can be a sign of significant stresses within the building structure.
Cracks that are wider at the top or at the bottom may indicate that there has been foundation movement, with the direction of the widening giving an indication of the likely direction of the movement.
Horizontal cracks may indicate that an element such as a wall is failing and this may present a safety concern.
As a general rule, the following sizes and descriptions apply:

Damage to Walls caused by movement of Slabs and Footings, and other Causes*

Crack Width Limit Typical Description and Repair
< 0.1 mm Hairline cracks
< 1 mm Fine cracks that do not need repair
< 5 mm Cracks noticeable but easily filled. Doors and windows stick slightly.
5 mm to 15 mm (or a number of cracks 3 mm or more in one group) Cracks can be repaired and possibly a small amount of wall will need to be replaced. Doors and windows stick. Service pipes can fracture. Weather tightness often impaired.
15 mm to 25 mm but also depends on number of cracks Extensive repair work involving breaking-out and replacing sections but also depends on walls, especially over doors and windows. Window and door frames distort. Walls lean or bulge noticeably, some loss of bearing in beams. Service pipes disrupted.

* Guide to Standards & Tolerances 2015, Building Commission

Cause And Action

Cracks fall into 3 main categories.

Category 1: Small (Minor) Cracks – includes hairline cracks <.1 mm and fine cracks <1 mm including cornice cracks


The most common cracks you will find in buildings are hairline cracks, particularly in plaster, which is prone to shrink and is very sensitive to movement.

The main cause is often settlement of the walls onto the foundations or movement in the building associated with changes in moisture and/or temperature.


Most of the time small cracks don’t require any specific repair, as the issue is likely to only be cosmetic.

If required, you can engage a contractor to do the job but most hairline cracks can be resolved with a DIY job.

It might seem counter-productive, but first you should create a deeper hairline crack (using a screwdriver, scraper or utility knife) before starting the repair work. This “opens up” the crack so that you can clean out the loose paint, debris, concrete or old filler (using a dry brush, cloth or vacuum) before mending. This also helps the filling solution to properly fill the insides of the crack.

After cleaning the crack, fill it with filling solution/joint compound. Make sure the filler covers the entire crack.

Even out the surface with your scraper and let the filling dry out for a day. Don’t rush the process!
Once the surface has dried, sand it down so the surface is completely smooth and even.

Then apply paint. If you are applying two coats, always make sure the first coat has properly dried before applying the second coat.

Category 2: Medium Cracks – includes cracks from 1 mm to 5 mm


Whilst aesthetically concerning, cracks up to 5mm can generally be patched and painted. In newer homes the cracking almost inevitably relates to settlement and or vibratory action from within or adjacent properties. With older properties it can be the onset of one of the key contributing factors associated with Category 3 cracking (see below).

In addition, one of the major contributors to wall cracking is when downpipes are not connected to soak wells, the concentration of water around the footings at the downpipe positions can lead to subsidence of the footings and cracking of the house walls.


The key issue with category 2 cracks is to determine significant changes over a relatively short period of time, say 6 – 12 months (or even shorter). If the cracking gets worse, preventative action should be taken sooner rather than.

If there are any significant contributory causes to the cracking, like rusting lintels or significant cracking to concrete which is facilitating water ingress, preventative action should be taken immediately. While the cracking may only be minor now, unless the underlying issue is addressed it will only get worse. Taking care of the cause is more important than fixing the crack itself.

It is also very important to monitor any changes in the crack over time. Once you become aware of a crack, your eyes will constantly be drawn to it and it can often seem like the crack is getting bigger. Photography can greatly assist with monitoring cracks. By using your smart phone or digital camera (ensuring the date stamp is turned on), you can take regular pictures of the cracking and monitor changes. It is a good idea to place a coin or ruler next to the crack as a point of reference to properly determine whether the crack is growing.

Category 3: Large (Moderate to Severe) – includes cracks from 5 mm to 25 mm


Category 3 cracks are important to address immediately as there could be a significant safety issue. There can be a range of issues which can cause severe cracking, such as:

  • Issues with footings and or concrete pads (subsidence)
  • Water ingress, moisture/damp in the walls
  • Concrete cancer and rusting lintels
  • Impact damage
  • Issues associated with the roof frame or other load bearing problems
  • Concentration of water around the footings


As a general rule, cracks within this category shouldn’t be ignored and left untreated. At the smaller end of the scale, the crack needs to be filled to avoid any potential water ingress, particularly if it is an external wall. At the other end of the scale, the wall may need to be inspected by a structural engineer to determine what remediation work is required. There is a possibility that a section of a wall needs to be rebuilt.

At the very least, an inspection by a professional and a plan of action needs to be determined – and acted upon as soon as possible.

Do you want to better understand what causing cracking? Read our article on “The Cause Of Wall Cracks”

Concerned about cracks in your walls? Contact BeSafe Property Inspections to discuss your wall crack concerns on (02) 9410 3740 or via our website.

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