Trees and foundations (foundation depth calculator)

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Digger with trees and foundation - LABC advice on trees and foundations - foundation depth calculator

Building on sites where trees are present can be tricky.

Trees and other vegetation can affect moisture content considerably, leading to soil shrinkage or swelling (commonly known as heave). This can cause cracking and movement of foundations and damage to whole structures.

And it is not just trees in close proximity to your site that pose danger - trees up to 30m away have been known to draw moisture from the soil at a proposed building site.

LABC Warranty Foundation Depth Calculator

Different trees have different water demands, so it is important to also consider and identify trees on adjacent sites.

Once you have identified your trees, you need to get the most appropriate foundation depth for your building or extension. For this you need the LABC Warranty Foundation Depth Calculator, which is designed to help determine the exact foundation depth required when building near trees.

The depth of foundation is determined by the:

  • Plasticity index of soil
  • Water demand of the tree
  • Mature height of the tree
  • Distance of relevant tree to nearest part of foundations and distances elsewhere if stepping foundations
  • Allowance for climatic conditions

Because the calculator allows you to select the actual plasticity index of the soil (if known from testing), it gives more accurate results for foundation depths, which can save you having to dig deeper than actually necessary.

The freely available Foundation Depth Calculator covers the vast majority of trees found in the UK. If a tree is not listed you should consult a structural engineer and an arboriculturist for advice. Consider also obtaining advice on a foundation design from a suitably qualified and experienced expert.

Found this article interesting? Read about Foundations on clay soil



Submitted 2 months 3 weeks ago

Will there be a fully downloadable foundation depth calculator which does not require internet connection?

App version

Submitted 2 months 3 weeks ago

Hi John

We are working on a downloadable app version which we are hoping to make available soon. If you want to sign up to our blog updates you will be informed as soon as the app goes live

I hope that helps.


LABC Warranty


Submitted 2 months 3 weeks ago

Why do Hawthorn trees cause so much trouble with subsidence?


Submitted 2 months 3 weeks ago

Hello there

You'll see from the Foundation Calculator mentioned in the article that hawthorns have a high water demand.

Julie, LABC

Extension issue

Submitted 2 months 3 weeks ago

Dear Sir, my neighbour has an extension to his house that has been built on the border of his land with mine. The extension has 600 mm foundations and is built 1 metre away from my 65 ft. high poplar. He is having problems with his extension in the form of cracks. Is this normal and what should I tell him?
Looking forward to your reply.



Submitted 2 months 3 weeks ago

Hello Alastair

Foundation design in relation to trees is subject to many considerations, including tree species, distance and ground conditions. Because of this you'll need to engage a private surveyor to advise.

Julie, LABC


Submitted 2 months 3 weeks ago

Very useful. What happens if tree species is not listed? Alder.


Submitted 2 months 3 weeks ago

Hi Tim - as above, a private surveyor will be able to help.

Thanks, Julie.

Mature hedgerow

Submitted 2 months 3 weeks ago

How is this affected by a mature hedgerow in place for at least 100 years before the property is built? trees include, primarily, hawthorn & elder?


Submitted 2 months 3 weeks ago

Hello there

Thanks for your comment.

Hedgerows can cause problems to foundations in a similar way to trees.

Although you've mentioned some type of trees you'll need to find out which species are contained in the hedge and then calculate depths accordingly. If the species isn't listed you will need to seek further advice.

John, LABC

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