Tree Related Subsidence Reports

Arbtech are under contract to one the UK’s largest loss adjusters and undertake subsidence surveys and tree reports throughout the entire country, year round. The article that follows is designed as a quick ready reckoner for homeowners faced with the prospect of tree related subsidence and the requirement for an arboricultural consultancy report. If you have a suspected case of subsidence caused by trees and other vegetation, call us on 08450 176950 and we’ll talk to you about your individual case.

Subsidence then …

Tree related subsidence arises usually as a consequence of tree roots abstracting water from the soil with a high clay fraction. This process results in a reduction in soil moisture. The capacity of clay to contract and expand with variances in soil moisture content is known as plasticity and can have consequences for low-rise buildings constructed upon it. When clays are subjected to significant and persistent soil moisture deficits under and about the foundations of a structure leading to their—usually downward—movement: subsidence.

Can you make that a bit clearer?

The soil beneath your foundations is primarily made up of three components: sand, silt and clay. The former two components, so-called fractions, are inert. However, clay has different properties: it contracts and expands with changes in moisture content. Clay is said to be ‘highly shrinkable’ as prolonged desiccation makes it contract and so reduce in both volume and its ability to support the load above it, like say, your house. As you might expect, trees are capable of abstracting enormous quantities of water from the ground around them and especially so in summer, when broadleaved trees lose large quantities of water through transpiration, a process their coniferous contemporaries ‘manage’ much better. So in summer trees use more water precisely at the time there is less of it to go around!

Will my trees cause subsidence in the future?

Subsidence is something we can confirm once it exists. However, no matter what you are told by anyone, subsidence reports are not probabilistic tree assessments. You cannot predict subsidence. (Were that so, most buildings insurers would have wised up to it by now.) Of course, like the insurers, you could spend a lot of money on Met Office weather data and laboratory testing of your soils. But nothing can be said with certainty about the future and nowhere is this truer than with reference to tree related subsidence. Likewise with what has become known as heave.

Tell me more about heave…

Heave occurs where an already desiccated soil re-hydrates and expands, such as when for example a house is built on a soil desiccated by trees, which subsequently are cut down. The higher the clay fraction, the more shrinkable the soil is and so the greater the expansion. Again though, predicting heave is a futile exercise in all but a few instances. The only thing that can be said with some degree of confidence is that logic suggests the following: if your house was built before a soil was desiccated there is little chance that removing a tree will cause heave when the soil re-hydrates. The risk of heave is much greater where trees are being removed on soils that were desiccated before a property was built, suggesting that the sudden re-hydration of the soil could expand it beyond the capacity of the foundations.

The need to know….

It’s a pretty simple process. Provide us with whatever engineer’s reports you have, which we will review prior to attending your site. A survey of the trees will inform our report which will make practical and cost effective solutions to alleviate the problem. In the majority of instances the only practical solution is to remove the tree. However, if you don’t own the tree, or a Tree Preservation Order protects it you will almost certainly require some assistance in navigating the complex process of satisfying your insurer and negotiating with local planning authorities and neighboring landowners.

Our arboricultural consultant’s can visit your property; inclusive of traveling expenses; undertake a tree survey; review your engineer’s subsidence opinion; and provide a factual report from as little as £299 ex VAT. Or you can give us a call and chat about your site for free!