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The "Problem" with Trees

Trees can cause three main types of problem.

1. The most well known is subsidence. Trees need a great deal of water to survive. Clay soils change volume with changes of water content. This can lead to the soil near a tree shrinking as the tree grows. The amount this occurs depends upon:

• The size of the tree.
• The type of the tree, some trees are "thirstier" than others, the roots of some trees spread outwards while others spread downwards, this affects the area affected by the tree.
• The amount of rainfall.

The amount that any ground movement will affect a building will mainly depend upon the type and depth of foundation. Shallow strip footings are most vulnerable. Older houses tend to have shallower foundations and additions like bay windows may have foundations shallower than the rest of the house. A building on deep trench fill or piled foundations should not be affected by trees.

The shape and robustness of the building will also affect the location and severity of any damage.

It is important to remember that all this only affects clay soils. This unfortunately affects much of London and the South-East.

2. The potentially next most serious effect is interference with drains. A tree's roots will naturally seek out water and if a drain is leaking the roots will tend to concentrate in that area. The roots can actually get into the broken drain, make the break worse and/or block it. A small leak can become a major problem.

3. Thirdly, tree leaves will block gutters and downpipes if a tree is close enough to drop the leaves directly into the gutter.

Remember, trees are wonderful things.
There are various things that can be done to avoid or minimise damage. Cutting a tree down can actually cause more harm than it avoids. With the removal of the tree the ground will become wetter than it was and will swell. This can be more destructive than the shrinkage would have been.

Also remember, most cracks are nothing to do with subsidence.
There are many causes of cracks. That is why a proper structural survey is usually necessary to determine the cause(s) and any action to be taken.

If it is suspected that cracking is linked to a tree, there are a number of options other than those directly related to the tree.
If the cause is found to be movement in the clay soil and trees or other vegetation are withdrawing water from the soil, this will be aggravated if less water is getting into the soil in the first place. This can be caused by a number of factors including:

• particularly dry weather
• when a leaking water main or sewer is mended (this could be an issue in many places now that so many water mains are being replaced)
• a porous surface such as grass or gravel being replaced with concrete - e.g. if someone creates a parking area in their front garden

In such cases it may be possible to increase the porosity of the surface or even divert some of the surface water drainage into the ground using a soakaway or French drain.

There is often an assumption that when a tree is found to be causing subsidence, it must be removed. The need to remove it should be a matter for careful consideration. If the building is underpinned, it is likely that the tree will no longer be a problem as the new foundations will be so much deeper. In addition, when a tree is removed, the soil will expand as much less water is being removed from it. That expansion may cause new problems.