Chartered Building Surveyors, as part of a building or pre-purchase survey, frequently have to report on internal and external cracking and the cause or causes of such movement. Most internal and external cracking is minor and cosmetic and caused by normal uniform ‘settlement’ which is caused by the downward movement of the soil due to the weight of the building. Depending on the type of soil, this settlement can occur relatively quickly or be more gradual. Internal and external cracking may form, usually no more than 1.00mm in width, which is easily filled. Very few residential dwellings will present without minor cracking caused by natural settlement.
‘Subsidence’ is a more serious issue and occurs when the ground beneath the building is unable to support the foundations. This issue can be localised or more extensive and can have a number of causes. The cracking will be much more severe and noticeable. Most subsidence occurs as a result of damaged underground drains, particularly in older period properties where clay drain pipes have collapsed or joints in the pipes have opened causing fines in the soil beneath the foundation to be washed away or eroded. A detailed CCTV drainage survey is essential before contracts are signed to find out the condition of all underground pipes.Another factor is the drying out of soil due to the close proximity of trees draining water from the soil causing shrinkage. A lesser issue in Ireland would be tunnelling beneath a property causing disturbance to the ground beneath it. The Dublin Port Tunnel is an example of this.
Chartered Building Surveyors can assist their Clients by diagnosing whether the cracking is caused by settlement or subsidence based on an analysis of the location and pattern of the cracking. The survey report will clearly list where the movement has occurred and outline the next steps for the buyer. Foundation repairs to address subsidence are complex and costly to execute and will often result in the purchaser withdrawing their offer unless a substantial reduction in the asking price is obtained. In this scenario, expensive invasive investigations are essential at pre-contract stage to arrive at cause of the subsidence. Typically, these investigations will require excavations to be completed down to foundation level to establish the depth and width of the foundation and quality of the soil. Trial bore holes may be required to determine at what depth adequate bearing strata is present. A Structural Engineer will be needed to design for example underpinning works.