Free-standing walls are usually designed to suit the purpose for which they are constructed. However, over the life of a wall, a lack of maintenance and a number of external factors can affect its structural integrity.
Another issue is that walls constructed many years ago may have footings that complied with the standards at the time of construction but are far less stiff than footings required by current design codes. Older walls can also have weak lime mortar, which has minimal tension capacity, making them more susceptible to failure as a result of high winds or impact loadings.
Practitioners and consumers who have a concern about the structural integrity of an existing wall should contact their local council. Cracking to brickwork of free-standing walls can weaken the walls considerably and can be a sign that the structural integrity has been compromised. This will depend on the type, severity and location of cracking. If unsure it is best to err on the side of caution. It is also important to note that unintentionally free-standing walls, such as during construction or after demolition, are unsafe even if they have not cracked.
In addition, practitioners should be cognisant of precautions and temporary measures they should take when constructing a free-standing wall. WorkSafe Victoria has published an alert which gives advice on this matter.
What is a free-standing wall?
A free-standing wall is one which is 'standalone' along its length. That is, it is not connected at its ends to a building or other structure and is not restrained along the top. Brick front fences are typical examples of free-standing walls. It should be noted that long walls that are connected at their ends to another building or structure can behave as free-standing walls for the sections that are away from the ends.
What causes a wall to deteriorate?
Some common causes of walls deteriorating are:
- Rotation' of the footing or part of the footing - causes the wall to lean or crack.
- Differential movement of the footing along the length of the wall - causes the wall to crack.
- Introduced 'pressures' being placed on the wall - causes the wall to lean or 'bow'.
- Impact of vegetation and roots - causes wall to crack, lean or bow, or a combination of both.
Rotation of footing
This occurs when the footing of the wall rotates away from its original horizontal position. This can be caused by roots of trees 'pushing' up the inner or outer side of the footing. As the footing gets lifted on one side, the wall commences to lean out towards the other side.
Another cause of rotation is changed conditions in the soil. Clay soils are particularly prone to changes in moisture content, as they shrink in dry conditions and swell in wet conditions. If a clay soil is dry on one side of the footing and wet on the other, the soil will tend to shrink on the dry side and swell on the wet side. This causes the footing to rotate higher on the wet side and lower on the dry side. As a result, the wall will lean out towards the dry side. Tree roots are a common cause of drying shrinkage. Footing rotation can also occur if the soil on one side of the wall becomes excessively wet, as a result of poor site drainage or plumbing leaks.
It can be caused by a number of factors including different moisture content of the soil along the length of the footing, or other factors such as tree roots that might cause a section or end of the footing to rise or fall in relation to the rest of the footing. This usually results in cracking in the wall.
Long walls that do not have articulation joints are also susceptible to cracking.
A wall may be adapted for a use that was never intended. For example, a front garden wall may be used by a later owner as a retaining wall if they decide to create a raised garden bed behind it. This can cause horizontal pressures on the wall for which it was not designed and as a result may cause it to 'lean' out or 'bow'.
Impact of vegetation and roots
The roots of trees can cause significant damage to a footing, the masonry or brickwork. Large underground roots can disturb a footing and cause rotation and/or lifting. However, even light, fibrous roots can impact and dislodge mortar to an extent that the stability of the wall is compromised.
Equally importantly, tree roots can dry out the soil in which footings are founded and cause subsidence and rotation - even if trees are a considerable distance from the wall.
Incorrectly constructed walls
Walls that have not been constructed correctly are also a danger. This includes single skin brick and pier walls (such as garage walls) where the piers have not been keyed or tied into the single skin wall. In this case the piers are not carrying out their intended purpose of providing stiffness to the wall, and can separate from the single skin wall making it unsafe.
Walls that are unintentionally free-standing
This can occur during the construction stage and during demolition works. An example is walls that have been constructed but not yet connected to the roof structure (for example, garage walls). Alternatively, it may be the case that the roof or other structure the walls were connected to has been demolished, leaving the wall as free-standing. An example is when a masonry façade is retained for heritage significance. It is essential that walls in these cases are adequately propped to provide lateral stability. They must either be propped on both sides or have suitable tension/compression props on one side. Props must be securely connected to the wall and to a suitably large footing at the other end to provide adequate capacity to resist wind loadings on the wall.
It is also vital during construction of brick veneer walls that cavity ties in the brick skin are connected to the stud wall framing immediately after brickwork construction, and not left unconnected for a period.
The Victorian WorkCover Authority has resources available that provide information about ensuring the stability of buildings and structures:
- Preventing structural collapse – Safety alert
- Stability of buildings during construction - Guidance note
- Masonry structures – Instability leads to collapse
- Construction industry program incident report – Masonry walls collapse in strong winds, 2002
- Preventing masonry structures from collapsing – A health and safety solution