If foundations are to be constructed on a shrinkable clay soil, then the depth of the foundations needs to be carefully considered if there are any trees close to the building. If the soil is a granular soil such as sand or gravel, then the minimum foundation depth required is not affected by trees.

Shrinkable clay soils change in volume as their moisture content varies. Where there are trees present the roots from the trees can take out enough water to cause building subsidence. In fact this is the most common reason for foundation subsidence in the UK.

In broad terms if a tree is closer to the building that its height in a shrinkable clay soil, then deeper foundations will be required in order to ensure the foundations are taken down below the zone of influence of the foundations.

Unless existing trees are to be removed at the time the building or extension is being built, then the foundation depth should be designed for the mature height of the tree.

Often this issue only raises its head when the work starts on site and 9 times out of 10 nobody has noticed the insignificant 2m high Leylandii tree in the neighbour's garden right next to the building. The best answer is to get you neighbour to remove the tree before you start work, but if they won't, the foundations will need to be designed to allow for the tree to mature.

Our designs minimise the foundation depth required wherever possible, by using stepped foundation depths as the distance from the trees increases.

We need the following information to design the foundation depths in a shrinkable clay soil:

a site plan showing the trees which are closer to the building than their mature hight.

details of the types of tree

the distances of each tree type from the building:

the approximate location of the site

the type of soil (low, medium or high shrinkage potential), your local Building Inspector will give you guidance with this.

Design information

All designs are carried out in accordance with BS 8110 (The structural use of concrete), NHBC Chapter 4.2 and BS 8004:1986 (The code of practice for foundations).

The calculations provided will be suitable for Building Regulation approval.

You should satisfy yourself that the excavation for the foundations will not undermine any adjacent foundations or neighbouring ground

Excavations may fall under the Party Wall Act Please seek specialist advice. click for party wall act information