Southwest Environmental Limited

Building Near Trees

If you would like us to undertake a building near trees assessment for you project please do get in touch for a fixed price quotation. We want to keep your project moving. 

This page explains the problems encountered when building near trees and how to get around them.

The Problem

If you intend to build near trees it is sometimes worth taking a few extra precautions. It is good to build near trees for many reasons, it increases property value, it provide seasonal shade to prevent summer overheating and in some cases the roots provide stability.

However, trees can also affect the soils on which you build in a way that may cause damage to proposed or existing buildings, for this to happen you need two ingredients:

1. You need a soil with shrink / swell potential - If you are building on pure gravels or sands then trees will have little affect on soil volume change. Unless of course the tree is very close and very large, and then the shear volume of the roots may cause problems.
2. You need a tree - You need an existing or proposed tree (removal or planting) within a certain distance of the building (whether planned or existing).  

The foundation of a typical house rest within the soil. The weight of the house is support by the soil. If the soil changes volume, then this will destabilise the building.

Shrink

The more common example is when an existing or new tree grows over time, and as it does the roots grow outwards and take moisture from a clay soil (see below for discussion on "clay soil"). The soils the house is resting upon shrink. Quite often this will happen on one side of a building rather then all of the way around it. As such one side of the house is supported and the other side is not. This causes "twisting" and crack may begin to appear in the wall of foundations.

Swell

A less common problem is heave or swell. When a tree is removed or a section of the rooted area, is cut off from the tree (digging a trench through roots). The water demand is removed, and the soil may begin to expand.

Perhaps the most dramatic example of this are when swelling soils are confined within the "box" of a new building. Surrounded by concrete, the swelling soils push outwards and the force is so strong it can push apart reinforced concrete.

A Word on Soils

Clay soils are not all made equal. Some clays such as kaolin, do not change shape much when there moisture content changes. Other clays such as montmorillonite, change volume dramatically dependent on whether they are wet or dry.

The other thing to note is that clay soils do not have to be pure clay. In fact you could potentially have 70% gravel and sand in your soil, but the remaining 30% clay dictates its behavior.

A quick (an not very accurate check) is to pick up a handful of soil, and squeeze it in your hand. If it stick together in a lump, this is an indication it behaves like a clay.

A Word on Trees

 Not all trees are made equal. Some trees are more thirsty than others.

High Water Demand Trees

• Cypress
• Elm
• Eucalyptus
• Oak
• Poplar
• Willow
• Hawthorn

Low Water Demand Trees

• Birch
• Elder
• Fig
• Hazel
• Holly
• Honey locust
• Hornbeam
• Laburnum
• Magnolia
• Mulberry

Solutions

If an existing tree is getting too big then pruning or removal are good ways to prevent damage to existing structure. But even in these circumstances care should be taken. The swell created by removing a tree, can be more rapid than the decades of shrink prior. So be careful.

If you are building something new then the following steps can help:

Foundation Depth

We can work out the required foundation depth for you.

As an example a Cypress, Leyland panted 3.00 meters from a proposed foundation, will mean that the foundation depth will be 2.4m. This is based on a soil with a plasticity index of 33%.

A typical footing depth might 1.5m. So you can see that building near trees can complicate things considerably.

Bear in mind the footing can be piled or trench footing. Sometime for sites with lots of trees, and shrink swell conditions it is better you use piled foundations.

Clay Board

Clay board is a squashy board that is placed on the outside and inside of any foundation structures. If swell occurs the board compresses and minimizes chance of damage.

Information from BGS

The British Geological Survey Provide the following advice on Shrink Swell Soils:

• Take specialist advice before starting major building work.
• Consider the effect of laying impermeable drives, paths or hard-standing on the rainfall reaching the soil below and changing its moisture content.
• Seek expert advice before planting trees near to a house. The safe planting distance will depend on the tree species, the type of foundation and soil composition.
• Ensure foundations of new constructions or extensions are designed for the shrinkable clay soil conditions that are present.
• Do not plant potentially large trees next to a house.
• Do not remove mature trees that pre-date the construction of the house before taking advice. Tree management by crown reduction or thinning may be better than removal because it will maintain a stable soil moisture profile.

What We Do

We have worked on many sites over the year where client require advice on building near trees. Are most popular service is providing foundation depths based on NHBC guidance for building near trees. We can arrange the soil sampling required to undertake this work, and carry of the reporting. This is then passed on to architect, building inspector, engineer. And your project moves forward.