Your bespoke home will have unique requirements, but so will the ground you’re building on. Tim Doherty looks at the different foundation solutions

Foundations spread the weight of a building over the ground to maintain stability and prevent unexpected movement in the superstructure. They are generally wider than the ground wall thickness and anchored deep down where the subsoils are more compressed and have a higher bearing capacity.

Key principles

Site levels, vegetation, sub-soil and building’s weight need to be considered to design the appropriate foundations. It is commonplace for contractors to include a standard type and depth in their quotations and to worry about the actual depths when the job starts. However, prudent self builders will evaluate site variables in advance to understand the most suitable foundation before work begins.

Remember that the foundations need to carry the load of the structure itself, as well as you, your furniture and the weather (ie snow on the roof). The weight of your building is important, but the bearing capacity of the subsoils is, too. Different makeups have different capabilities. With soft silty or sandy clay, you’re likely to need a reinforced and engineered solution. Compact sandy and granular soils should support most houses at modest depths and average widths, with a safe bearing capacity of circa 100kN/m² (a measure of force used to calculate loads).

Firm and stiff sandy clays are likely to call for extra depth and/or width, as their bearing capacity might be reduced to 70 or 80kN/m². Rock at shallow depths often means the base of the external wall will need no thickening. Vegetation on site will also have a bearing, with nearby trees influencing both foundation type and depth (see box, right). If you’re demolishing a building to start again, it’s unlikely that any existing foundations will be used. Consequently, these will need to be removed, which can churn up the site, affect sub-soil compaction and make it more difficult to excavate neat trenches.