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December 2015

Question

I own a three-bedroom bungalow in the country with a lovely garden. However, I have recently noticed that the rear garden wall is in need of some repair. There is a vertical crack which currently extends across the whole height of the wall. It has penetrated the bricks in some places and the mortar in others. I am concerned that the crack is a symptom of subsidence. There are no such cracks within the house and there is nothing to suggest that there is any subsidence within the house. Am I exaggerating the potential dangers? Any help would be greatly appreciated. 

Answer

When subsidence occurs to a structure it is normally abrupt and sudden as a result of a change in ground conditions. If the cracking that you describe has occurred over a period of time it is unlikely to be a subsidence issue. There may be a number of possibilities that require examination in order to confirm the cause of the cracking, however, initially it should be determined if the cracking is affecting the structural stability of the wall which would give safety concerns. Light cracks (0-2mm wide) are common in boundary walls and may not indicate a serious problem. Cracks up to 5mm are described as “slight” and can be easily repaired. For wider cracks it is advisable to seek professional advice.

Vertical cracking in a number of locations to the wall could be as a result of an inadequate foundation or a foundation taken to an inadequate depth. A small trial hole will confirm this. If there is a tree near the wall there is a risk of damage by the roots and/or wind-blown branches.

Alternatively, if there were any large trees removed from within the boundaries of the property or within the curtilage of the adjoining lands adjacent to the wall, this could change the ground water table locally, causing heave to occur, resulting in cracking. This will be dependent on the species of the tree as some have a higher rate of water absorption than others.

A wall that is longer than nine meters in length should be provided with a vertical movement/expansion joints. Omission of these joints can result in cracking.

It should also be established if there are any piped drains or a water supply running adjacent to or below the wall which may have been damaged or burst, causing soil erosion beneath the foundation, causing it to fracture. Garden walls are predominantly of a single leaf/one brick thick (225mm/9”). Confirmation is required that the wall thickness to height ratio is correct or that support piers were constructed (if required).

Some climbing plants, such as ivy, can damage walls if growth is not controlled. Check also the rear face of the wall. Incomplete or poorly capped walls can lead to water penetration resulting in cracking due to frost damage.

Is the wall constructed in close proximity to a main road, which may have resulted in impact from traffic – is there an active farm to the rear using heavy machinery?

There may be issues within the adjoining lands which are contributing to the cracking; for example, is the ground behind higher or lower than the ground on your side? The original wall may not be adequately designed as a retaining wall.

If you are concerned that there is a risk of collapse, I would recommend that you obtain the services of a building surveyor. They will be able to advise if the cracking to the wall can be repaired or if more extensive work is required.

Andrew Ramsey is a Chartered Building Surveyor and Chartered Project Management Surveyor, scsi.ie