When it comes to foundations near trees they need careful consideration. The likelihood is that the existing trees would have been established for a number of years and may continue to grow and demand moisture from the ground after your build is complete. Therefore, they need to be factored into the foundation design of any build.

Even if they are removed, issues can arise due to the ground, having become used to the moisture demand, swelling when that demand is removed.

The root of all problems

There is no doubt that the roots of trees, plants and other vegetation absorb water from soil. If there is clay present in soil then this will shrink or swell depending on the water content. This can cause problems if this occurs below the foundations causing them to move. The result may be damage to the building or structure above.

How do you avoid this?

One solution is to increase how deep the foundations are. Avoiding the risk associated with moisture change can implicate on the cost and therefore there needs to be a balance between the potential damage that may occur and depth of the foundations.

Heave precautions may also be necessary and reference should be made to our Technical Manual Chapter 5.2

Risk is determined by:

The soil

Soil can be broadly classified into cohesive, which contains mainly clay or fine silt and non-cohesive that is mainly sand with only a small amount of clay or silt. Clay particles will shrink or swell.  

Potential species of a tree, its size and proximity to the foundations

The amount of water that a tree may need to grow differs across species; Oak, Poplar and Eucalyptus are considered the highest ‘water demanding’.  The distance the tree is, dictates the depth that is required for the foundations. To simplify this, please use our Foundation Depth Calculator for a quick and easy answer.

Climate conditions

Hot sunny weather causes an increase in the amount of water that tree roots will take up therefore causing soil drying. Rainfall helps restore the water that has been lost in the soil via the roots of a tree. This risk is greater in South East England, reducing in the North and West of the country. 

Determining the depth of foundations

We have developed our Foundation Depth Calculator to allow you to establish the required foundation depth as simply and quickly as possible. You will just need to know; the site postcode, extent of any slopes on the site, the types of tree which are present near the planned building, distance of any trees to the nearest part of the foundations and if known, the volume change potential of the soil (Plasticity index).

Where raft or piled foundations are proposed additional measures will be required and it is recommended you contact us to discuss.

For more information or to speak to our technical team, contact us on technical.services@premierguarantee.co.uk or call 08444 120 888