HOW CAN YOU TELL IF A CRACK IS STRUCTURAL?

When your family home starts to show signs of cracking, it can cause fear and concern. Your home should be a place where you and your family feel safe and secure. So when cracking does appear, it can be hard to know what you should do. So how can you tell if a crack is structural? Also, how can you tell if a crack is serious and requires immediate attention? This article provides some insights into different types of cracking you may find in different parts of your home.

This article is designed to provide homeowners with some guidance and direction when their house shows signs of cracking. If a crack has come to your attention and it is causing concern, the best first option is to consult a local Structural Engineer to assess the cracking and provide advice to you on possible strategies for repair and maintenance.

QUICK REFERENCE GUIDE IF A CRACK IS STRUCTURAL

Here is a quick table indicating a summary of cracking for various structural elements you may find at your home. Click on the structural element which interest you to jump straight to the specific location in this article for further explanation:

Home Element When it’s a Structural Crack and Potentially Serious
Brick Walls – Crack width greater than 1.0mm
– More serious for load-bearing masonry compared with brick veneer construction
Residential Slabs and Footings – Crack width greater than 1.0mm
Suspended Slabs – Crack width greater than 0.3mm for slabs within enclosed buildings
– Crack width greater than 0.3mm for external slabs more than 1km away from coastline
– Crack width greater than 0.25mm for external slabs within 1km from coastline
– Crack width greater than 0.2mm for external slabs in salt-rich arid zones
Plasterboard (or drywall) – Crack width greater than 1.5mm
Guidance table to help you tell if a crack is structural. This should be used as a guide only and you should always seek advice from a local Structural Engineer

The best course of action when you discover cracking at your house is to talk to your local Structural Engineer for a detailed assessment and further advice on what to do for your specific issue.

Sometimes, even the experts can get it wrong when assessing cracking. Prior to its collapse, the FIU Bridge in Miami Florida showed signs of over-stress which was evident through significant structural cracking. These were later found to be serious structural cracks after further inquiry into the collapse, however at the time during construction, they were deemed not to be serious by Structural Engineers. This is an example of how hard it can be to tell if a crack is structural. For a full overview of how and why the FIU Bridge Collapsed, take a look at THIS article.

Image of a crack on the FIU Bridge in Miami Florida, prior to its collapse, an example which shows sometimes the experts may not readily tell if a crack is structural and serious in nature.
Image of a crack on the FIU Bridge in Miami Florida, prior to its collapse, an example which shows sometimes the experts may not readily tell if a crack is structural and serious in nature.

STRUCTURAL AND NON-STRUCTURAL CRACKS

Before taking a deep dive into the specific cracks you may be seeing at you home, its best to first define what a structural and a non-structural crack may be.

Non-Structural Cracks: Parts of your home or structure which do not form part of the main structural load bearing and support system can generally be deemed as non-structural. This may include:

  • Cornices
  • Base boards (or kick boards)
  • Tiles
  • Other decorative finishes

While usually less serious, “non-structural cracking” within these building elements can indicate that something is going wrong with your homes structure. It can be an early warning sign into future movement or distress your home may undergo which can sometimes lead to more serious structural cracks.

Structural Crack: In strict technical terms, any crack you see within a structural member of your home can be deemed as a “structural crack”. In some cases, cracking is not only normal for some structural elements but accounted for by the Structural Engineer during design. The key is understanding what structural cracks indicate that your home is under stress and what cracks are considered normal and part of your homes regular service life.

For the purposes of this article, reference to “structural cracks” are the cracks within structural elements which cause serious concern, and “non-structural cracks” are those in structural and non-structural elements which are of lesser concern.

Structural cracks are generally found in elements which are constructed from brittle material such as concrete and masonry. Timber may also be susceptible to a form of cracking but is generally referred to as splitting or splintering. Eequally however, splitting and splintering in timber may be a sign that your structure is undergoing excessive movement and/or distress.

Structural elements of your home or building which may be susceptible to cracking may include…

  • Concrete slabs
  • Concrete footings
  • Support beams
  • Concrete retaining walls

This article is broken up into separate sections, each one representing a part of your house which is commonly suspectable to cracking. If you would like information on the specific crack at your house, use the links via the list below to jump straight to the relevant section of this article. If you are interested in understanding a holistic view of potential cracking to a wide range of elements within a house structure, read from top to bottom:

HOW CAN YOU TELL IF A CRACK IN YOUR BRICK WALL IS STRUCTURAL

Brick is a diverse building material used in quite a number of applications for domestic construction. You may find brick walls at your property at the following locations:

  • Home exterior walls
  • Retaining walls
  • Boundary fences

The Masonry Structures design code for Australia (AS3700) specifies the maximum crack width limit for brick structures which is 1.0mm. This is the design standard which Structural Engineers use for code compliance, therefore, if you see cracking in your brickwork that exceeds this limit, chances are its serious and you require the immediate assistance from a qualified Structural Engineer:

Extract of the Australian Masonry Structures Design Code (AS3700) indicating crack width limit for masonry structures.  This can be used as a guide to tell if a crack is structural or non-structural.
Extract of the Australian Masonry Structures Design Code (AS3700) indicating crack width limit for masonry structures. This can be used as a guide to tell if a crack is structural or non-structural.

The weak point in masonry construction is generally where the mortar joints bond each brick unit together. This is often the junction where cracking is evident. There are also occasions where the brick units themselves may crack, as we will explore further down below…

Home Exterior Walls

Brick is used on exterior house walls in two applications. In “brick veneer” type construction it is used as a cladding material. This means that it’s not strictly speaking a structural support element. Brick veneer construction is quite a common domestic construction technique in Australia. If your home has a brick exterior with a secondary stud wall lined inside it, it is most likely brick veneer type construction.

Your garage however may have brick walls as the primary support structure. If the bricks are visible from the exterior and interior of your garage, chances are that the bricks are indeed a primary support element. A common construction method for brick garages in Australia is a reinforced brick pier with a single or double skin of brick wall spanning between.

It can then be assumed that a crack in your brick wall can be of lesser concern for brick veneer type construction however a crack within a brick wall which is part of your garage or part of the primary structural support system is more cause for alarm.

Cracking in brick walls can take a number of forms:

  • Step/stair cracking: This is a diagonally orientated crack pattern in your wall and generally follows the mortar joints of the brick work. Due to the stitch pattern of how bricks are laid, this often results in a zigzag or stair shaped crack pattern.
Example of a diagonal step or stair crack in a masonry wall.  The crack takes the path of least resistance between brick units which is a zigzag form where the mortar joint bonds each brick together.
Example of a diagonal step or stair crack in a masonry wall. The crack takes the path of least resistance between brick units which is a zigzag form where the mortar joint bonds each brick together.

Step or stair cracking may be an indication that the support foundation has undergone differential settlement. Therefore, further investigation may be required to determine if this is the cause of the issue. A way to further investigate is to inspect the interior of the wall in question. If the plasterboard has undergone cracking as well, then it is likely that the support structure (the stud wall) is undergoing strain and also being affected by the foundation.

Evidence of foundation settlement may be that doors and/or windows within the affected wall may bind within their respective frames and become difficult to open and close.

The image below shows an example of a step crack on a multi-level masonry structure. The foundation movement has been so significant that a visible bend in the wall has occurred.

Excessive settlement to a foundation supporting this outer masonry wall has resulted in a bend and subsequent step crack within the wall itself.
  • Horizontal Cracking: Horizontal cracking usually occurs at a mortar joint where one course of bricks lies above another.
Example of a horizontal crack within a brick pier.  it was found that this crack was caused by the brick pier bending or bowing.  The top portion of the pier had been pushed outwards (into the direction of your computer, tablet or phone screen)
Example of a horizontal crack within a brick pier. it was found that this crack was caused by the brick pier bending or bowing. The top portion of the pier had been pushed outwards (into the direction of your computer, tablet or phone screen)

Horizontal cracking may be an indication that your wall is bowing and/or undergoing rotation at its base or top connection. To further investigate this, use a spirit level to check the out-of-plumb orientation of the wall. Checking at various locations and heights of the wall can give you an indication if the wall is bending inwards or outwards relative to your facade.

The cause for such cracking may be that the foundation has undergone settlement which has caused it to rotate. It also may be an indication that the brick ties which connect the brick veneer to the stud frame have started to come lose. It may also be an indication that your stud wall is also bowing. A spirit level should be used to also check the plumbness of the internal stud wall to determine if the brick wall is moving in isolation to the stud wall or the issue lies within both the brick wall and stud wall.

Horizontal cracking is usually an indication that the masonry wall is losing its stability. Generally, for both structural and non-structural masonry these cracks can mean that something serious is happening to the support structure and further investigation is required.

  • Vertical Cracking: Vertical cracking can also occur to brick walls. Unlike the other crack patterns outlined above, a vertical crack means that the mortar joints have not only cracked but the brick units themselves crack (usually at their centre point) up the height or partial height of the wall.
Example of a vertical orientated crack pattern in a masonry wall.  The width and shape of a crack can help tell if a crack is structural.
Example of a vertical orientated crack pattern in a masonry wall. The width and shape of a crack can help tell if a crack is structural.

Vertical cracking may be an indication that thermal expansion and contraction has occurred within the wall leading to the resultant crack. This may be due to insufficient expansion joints being provided within the wall at the time of construction.

It should be noted that the effects of seismic activity can cause a combination of all three crack pattern types at your property. If your house is located close enough to the epicentre of an earthquake, the masonry walls (whether structural or not) are quite susceptible to cracking. The ground accelerations which generate forces in your house through earthquake shaking can result in sliding, shear and bending within your masonry walls which can result in horizontal, vertical and/or stair cracking.

Regions remote from fault-lines can also be susceptible to earthquake activity. My hometown of Melbourne has experienced a number of earthquake events which has caused varying levels of damage to peoples houses. If you live close to an earthquake event, you may be interested in THIS article which explores the effects earthquakes can have on domestic construction and your family home.

Retaining Walls

Masonry retaining walls can also be susceptible to cracking. They may exhibit the same three types of cracking as those outlined previously for regular masonry walls.

Masonry retaining walls can either be reinforced or un-reinforced. A crack within an un-reinforced masonry retaining wall is much more serious than a crack within a reinforced masonry retaining wall. Although both may be cause for concern if the cracks are greater than 1.0mm in width.

The reason why a crack within an un-reinforced masonry walls is more serious is the main force resisting mechanism within the wall is the bond between the mortar and the brick units. Once this has failed, the capacity of the wall can reduce significantly. Signs of cracking can indicate that the bond has been compromised.

In reinforced masonry walls however, the presence of reinforcement (or rebar) within the wall provides additional load carrying capacity rather than the mortar and brick bond alone. In these instances, a minor crack within the wall may not reduce the walls capacity as significantly compared with an un-reinforced masonry wall.

Boundary Fences:

A popular application for masonry construction is within boundary fences or garden fences. Although these elements may not be supporting significant loading, they often tend to be the most cracked types of masonry walls in your neighbourhood!!

This is generally because they may receive less design input either through the expertise from a Geotechnical Engineer or a Structural Engineer. They also tend to be less looked after compared to the main house structure and often don’t sit at the top of the priority list of homeowner repairs.

An example of a significantly cracked boundary fence (or garden fence) to the front boundary of a house.
An example of a significantly cracked boundary fence (or garden fence) to the front boundary of a house.

Taller masonry boundary fences can be quite dangerous if left to age without maintenance. A tragedy occurred in Melbourne where a boundary fence was undergoing modifications through attachment of construction hoarding which later collapsed killing innocent bystanders (article). This a sobering reminder that even lesser structural elements can pose significant danger to the general public if not adequately maintained or checked by a qualified designer.

HOW CAN YOU TELL IF A CRACK IN YOUR HOUSE FOOTING OR SLAB IS STRUCTURAL

Your house may have one or multiple foundation systems depending on its construction type and design. For a full explanation of domestic footing types and which foundations are best for houses, take a look at THIS article.

Often your homes foundation and ground floor slab are tied together and are part of the same system. This is true for a stiffened raft slab or waffle raft, two very common foundation types used in Australia. Structural Engineers use the Australian deign code for Residential Slabs and Footings (AS2870) for the design and specification of your homes foundation system.

The Residential Slabs and Footings design code considers that cracks with width up to 1.0mm are acceptable and don’t necessarily require repair. This should be used as a guide when assessing your own slab foundation. Cracks wider than 1.0mm are considered to be not code compliant. While there may be no immediate danger for cracks slightly more than 1.0mm, the long-term durability and lifespan of your foundation may be affected.

Extract of Australian Design Code for Residential Slabs and Footings (AS1870) this can be used as a guide to tell if a crack is Structural or non-structural
Extract of Australian Design Code for Residential Slabs and Footings (AS1870) this can be used as a guide to tell if a crack is Structural or non-structural

Cracks in your slab or foundation can be caused by one or multiple reasons which may include:

  • Poor drainage to your property
  • Large trees and other such vegetation planted close to your property
  • Plastic settlement
  • Cracking caused by shrinkage which occurs during the construction and curing of the slab
  • Thermal cracking due to heat of hydration
  • Corrosion of the steel reinforcement within the slab or foundation
  • Alkali Aggregate Reaction
  • Cracks caused by overstressing to the slab or foundation (due to bending and/or shear forces)

For a detailed article on each of these issues which may cause your house slab or footing to crack, take a look at THIS link. This article also goes through some repair methods that your Structural Engineer may specify as part of your slab or foundation remediation to your home.

HOW CAN YOU TELL IF A CRACK IN YOUR SUSPEDNED SLAB IS STRUCTURAL

For suspended concrete slabs, the acceptable crack width is stricter compared with concrete foundations and masonry walls.

The design code in Australia which Structural Engineers use to specify suspended concrete structures is AS3600 (Concrete Structures). The acceptable crack width is generally dependant on the location of your house and how aggressive the surrounding environment is in terms of corrosion:

Location of Concrete Element Acceptable Maximum Crack Width
Within an enclosed building sheltered from weather 0.3mm
External to a building and more than 1km from coastline 0.3mm
External to a building and within 1km from coastline 0.25mm
External to a building and within a salt-rich arid zone 0.2mm
Guidance on acceptable maximum crack widths for suspended slabs as per the Australian Structures design code (AS3600), this can be used as a guide to tell if a crack is structural or non-structural.

HOW CAN YOU TELL IF A PLASTER CRACK IS STRUCTURAL?

Plaster can either be structural or non-structural. In most applications, plasterboard (or drywall) is simply a cladding used to line internal stud walls of your house making it non-structural.

However, in some cases your houses stability may rely in part to the contribution of the plasterboard to its overall bracing system. Your homes lateral bracing system is what gives it strength and stability under wind or earthquake loading. To learn about how your homes bracing system may work, take a look at THIS article.

The Residential Timber Framing design code of Australia refers to this type of bracing as “Nominal Wall Bracing”. This allows Structural Engineers to account for the contribution of your plasterboard in conjunction with the additional structural bracing provided in the design. Therefore, any cracking in your plasterboard or drywall should not be instantly dismissed as non-structural in nature…

An extract of the Australian Residential Timber Framing code which allows Structural Engineers to consider the contribution of plasterboard to the overall stability of your home.  This can sometimes make it difficult to tell if plaster cracking is structural and serious in nature or not.
An extract of the Australian Residential Timber Framing code which allows Structural Engineers to consider the contribution of plasterboard to the overall stability of your home. This can sometimes make it difficult to tell if plaster cracking is structural and serious in nature or not.

Design code AS2870 provides guidance on crack width limits for plasterboard walls. This indicates that cracks within plasterboard with width of less than 1.5mm can be considered minor and not requiring repair (apart from for cosmetic purposes) …

Extract of Australian design code AS2870 indicating acceptable maximum crack widths.  This can be used to tell if a crack is structural within your plasterboard or drywall partition wall.
Extract of Australian design code AS2870 indicating acceptable maximum crack widths. This can be used to tell if a crack is structural within your plasterboard or drywall partition wall.

FINAL THOUGHTS

For the best piece of mind, if you have seen a crack to an element of your house and it is causing concern, the best first option is to seek further advice from a Structural Engineer.

Your Structural Engineer may provide guidance on repair and maintenance strategies. For less severe cracking, the recommendation may be to simply keep monitoring the crack over time to determine if it is getting worse or it has stabilised.

Published by:

Quentin Suckling is our founding director.  Prior to starting Sheer Force Engineering, he spent almost 2 decades working as a practicing Structural Engineer at Tier 1 engineering consulting firms delivering multiple billions of dollars worth of projects and managing large multi-disciplinary engineering teams. View More Posts

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