Foundations & Trees
There are a number of special considerations when building close to trees but inevitably the starting point is the effect on the foundations. Foundations are the first consideration anywhere but tree roots are a special factor to consider. Assuming you wish to retain it you need to consider both the welfare of the tree itself, and the effect of its presence on the foundations. Most want to keep any trees we have in our gardens but the more worthwhile the tree is to retain the more likely it is to have a detrimental effect of the foundations of anything built nearby. Tree roots can present hidden problems to the foundations for any structure. In broad terms the closer any tree is to a new building the further down the foundations need to go. The bare minimum depth for any foundations should be no less that 900mm but this might be substantially more in for a structure close to a well established tree.
The objective of the design of the foundations is to take them down to such a depth that any tree roots can’t pose any threat to the effectiveness of the foundations. Surprisingly the size of the tree is not always the determining factor. It’s easy to imagine that an Oak of a Beech may have detrimental effect on building foundations but even the presence of an Apple or a Plum tree can have serious consequences over time
Depth for Foundations
The following is a guide to depth in metres recommended for foundations depending on the type of tree but the figures should be viewed with caution. The type of soil will also have an affect the recommended foundation depth and distances recommended in the table are for a moderately clay based soil.
Go here for recommended Foundation distances and depths.
It should also be remembered that felling a tree already present in the garden affects existing foundations. ‘Heave’ can occur in some soils particularly clay. The soil retains moisture and swells after the tree is felled. The same thing can occur when the roots are severed. To avoid adverse effects of ‘heave’ the foundations and sub structure design have to be tailored to allow for it. In extreme cases a completely different type of foundation, like a raft, beam, or pile solution may be required. If so it’s not advisable to try to cope with these types of foundations yourself. If there’s any likelihood that you need something of the sort consult a structural engineer who will need to submit proposals to the Local Authority.
Most trees have substantial radial root systems which can affect structural foundations. The roots can extend out to a distance of 1 and a half times the height of the tree. Severing only one, of the roots whilst laying foundations can cause the loss of 20% of the root system. It can reduce the tree’s water-absorption capacity, and leave the tree unstable in high winds. So when laying pipes if possible, tunnel below the roots. Usually up to 90% of a tree’s roots are to be found in the top half metre of soil; and up to 99% within the top 1 metre. Some mature species like Oak and Pine have central tap roots which can extend down to 2 metres. The nuisance that root systems cause in sites and the potential problems they might cause to foundations however has to be balanced up against the benefit they make to soil stabilisation particularly on sloping ground. From the point of view of constructing an extension it’s really only the radial roots which need to be considered. But trees should not be ignored. Around £400 million of tree related insurance claims are made every year. The general rule is that construction should take place as far away from trees as possible. Damage directly caused by roots exerting pressure on actual structures and pipes are rare. The most common affects on foundations are caused by the effect that trees have on ground conditions and soil moisture content. You might need to liaise with your local authority when thinking about foundations to see if your locality has any specific requirements to deal with local ground conditions.
As a rule clay soils cause more problems than porous sandy soils because they retain more water and swell in heavy rain. Healthy trees however extract large amounts of water causing the soil to shrink particularly in droughts. And these ever changing soil conditions substantially affect the forces acting on the foundations located within the soil. Foundations can, over time, crack and subsidence can develop. 60% of the UK’s housing is currently constructed upon foundations dug into shrinkable clay. But removing trees will also affect ground conditions even where the tree is removed but the roots still remain. A recently felled tree will leave significant amounts of moisture in the soil which no longer has a way out. Over time ground conditions around any nearby foundations will be affected along with the foundations themselves. Clay soil will swell and heave placing pressure on slab foundations
And deciduous trees have an annual variable seasonal impact on foundations by virtue of their seasonal effect. Winter rainfall hydrates the which has dried out in Summer, and the effect is augmented by the presence of deciduous trees which in the Winter become dormant are no longer active in removing the water. So the presence of the trees causes magnification of the variation in the soil water content, which in turn affects the foundations. Mature elms, oaks, horse chestnuts, ashes and planes take up as much as 50,000 litres of water annually from the nearby soil, so soil water retention, can lead to significant heave. Reports of concrete slabs ‘humping’ as the soil expansion cause upward pressure on the floor slab foundations. Trench foundations crack with movement and affect the structure above them.