Underpinning & Subsidence

Underpinning is the process of strengthening and stabilizing the foundations of the home or property. It needs to be carried out if the initial foundation isn’t strong enough or an extension has meant the structure has changed due to subsidence.

Underpinning can often be avoided in cases of minor subsidence which is not uncommon in developments built on shrinkable clay. Heavy foliage, trees or even a hot summer can result in moisture loss in the soil causing shrinkage and thus subsidence. Soil and root samples will usually be taken to pinpoint the cause. Removal of trees can reverse the process as the moisture returns to the clay, called ‘heave’.

The process of underpinning can be messy and disruptive. It involves increasing the depth of the foundations by excavating the soil underneath the existing foundations and filling the space, usually with concrete.

If underpinning isn’t carried out to a professional standard it could lead to further damage and, in a worst case scenario, could cause the existing property to collapse. Underpinning existing foundations will usually require building regulations approval.

Subsidence and the classic tell tale signs

When a building’s foundations start to sink, subsidence occurs. Serious subsidence issues are rare but areas with clay soil are most vulnerable.

The main causes of subsidence are:

• Leaking or damaged pipes and drains.

• Certain soils – clay for instance – are particularly vulnerable.

• Shallow foundations.

• Vegetation – shrubs and trees – sucking moisture from the soil, which is more likely to be a problem during long periods of dry weather.

Fine and minor cracks in a new building’s plasterwork are quite common. Same with new extensions. While they are not necessarily indicative of the dreaded S-word, there are classic signs to look out for, which include:

• Diagonal cracks wider than a 10p coin suddenly appearing in plasterwork or in outside brickwork.

• Sticking windows and doors.

• Cracks appearing at weak points, notably doors and windows.

• The appearance of cracks at the join where an extension meets the main residence. This could be an indication that the extension is pulling away from the main house.

Underpinning is a technical process and Builder Guide has a number of building specialists listed capable of carrying out the work out.