Building near Trees and Shrubs
If there are trees or shrubs within 35 metres of your new conservatory they will impact on the design and cost of your foundations. Clay shrinks or swells according to seasonal changes in weather and by trees, hedges and shrubs extracting moisture from the clay. In very hot summers this can cause subsidence, so to prevent possible damage to the conservatory it is important to dig foundations to a depth where the moisture content of the clay will remain stable. British Standard 8004 recommends a minimum depth of one metre.
However, if trees or shrubs have been removed, are remaining, or are planned as part of any new landscaping, greater depths may be necessary. This is because if a tree has been removed or a new one is planted, it will alter the level of moisture within the soil causing it to swell or shrink.
When a tree is felled or dies, the clay soil will gradually regain the moisture that has been removed by the tree; the clay will then swell and push on the foundations. This can also occur when foundation excavations cut tree, shrub and hedge roots. With new landscaping, moisture will be removed from the soil and the clay will shrink.
Calculating Foundation Depth
Whether a new tree is being planted or an existing one is to remain, the mature height of the tree is used to calculate the foundation depth together with the distance between the centre of the trunk and the nearest face of the foundation.
With new landscaping you should choose your trees according to their water demands and the depth of your foundations. For example, Cypress, Eucalyptus, Oak Poplar and Willow have high water demands. Chestnut, Plane Pine and Yew are moderate users, while Beech and Holly have smaller water requirements.
A builder or a foundation specialist will have access to a variety of tools and references to calculate the foundation depth, including a foundation calculator and a list of mature tree heights.
Most Local Authorities will also take guidance from the recommendations in British Standard 5837 2005:“Guide for Trees in Relation to Construction”. This gives advice with regard to important issues relating to trees in relation to construction and how development can damage trees.