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 (commonly known as subsidence) 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 may be able to use 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. Also think about obtaining advice on a foundation design from a suitably qualified and experienced structural or civil engineer.

Found this article interesting? Read more about working with foundations

• Foundations on clay soil
• How to get it right: Building a wall on foundations
• How to get it right: Using reinforcement in your foundations
• Offset foundations: For when you're tight to the boundary
• Underpinning of existing foundations
• Raft foundation basics

Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.

^{This article was updated on August 2024.}