foundations for an extension

Soil and Foundations


Without solid foundations, a building’s structure can weaken. The type of soil that foundations are based on gives you an idea of what might be happening below the surface. That’s why it’s important to know where you stand with your property.



Peat is great for gardens, but not for foundations

Soil with a high peat content holds water well until summer months when it dries considerably. The changing structure of peaty soil means that it doesn’t provide the support required by foundations. London has very little peaty soil; there are patches where soils with ‘peaty pockets’ are located in south-east London with a few more to be found outside the south-east perimeter of the M25.

London clay

The clay soils found throughout London will retain more water, and for longer, than other soil types. The very small particles in clay expand and shrink from wetter months through to summer. The effect of this on foundations is to cause upward and downward movement which may lead to cracking. This is why it is important to dig foundations until more stable gravel is found below the clay.

Sand and gravel

The large particles in sand and gravel mean that this soil type doesn’t hold moisture and drains easily. Although dry and gritty to the touch, and a good foundation soil, sand and gravel can be washed away resulting in unstable gaps.

Silt is good in loam

Silt is made up of small particles and doesn’t drain well. Because it retains moisture, and therefore expands, it can weaken foundations by pushing against them. Silty soil alone is not best for laying foundations, but when mixed with sand and clay soils, as loam, it will support foundations well.

What is your soil type?

If you are going to extend your property, or dig foundations for any other construction purposes, you should find out the soil type of your plot. The easiest way to do this is to contact your local Building Control Body – your council will signpost you. Once you know more about the soil type in your location, you will have a better idea about what type of foundation will best support your property.

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