For homeowners, the presence of trees near a property raises concerns about subsidence. Subsidence occurs when the ground beneath a structure starts to sink, which can cause movement or damage to its foundations. In this brief guide, we’ll discuss the relationship between trees and subsidence, providing insights into how to maintain both the trees and the structural integrity of your home.
Understanding the risk
Trees, particularly those with large root systems, can be a contributing factor to subsidence. The roots of a tree seek moisture, and in doing so they can extract it from the soil beneath foundations. As some soils lose moisture they can shrink, leading to ground movement and potential movement or cracking to the building above.
However, not all trees pose the same risk. Various factors determine the level of threat they may pose to a property.
Tree Species
Different tree species have varying root structures and moisture requirements. Species with large root systems, such as Willows and Poplars, are more likely to contribute to subsidence compared to others like Beeches or Oaks.
High risk trees in the UK related to subsidence are:
· Willow
· Poplar
· Oak
· Elm
· Horse Chestnut
· Plane
· Ash
· Cypress
· Lime
· Maple
Distance from property
Trees planted too close to a structure can change the soil beneath its foundations, increasing the risk of subsidence. The safe distance between a tree and a property depends on the type of tree and its size; whilst a small fruit tree may cause little risk while only several metres away, a large willow or poplar tree can still interact with the foundations of a building if less than 40m away.
Preventative measures
If a high-risk tree is located close to your property and there is no damage to the property so far, there’s usually no cause for immediate concern. It’s also a bad idea to cut a large nearby tree down to prevent any future issues; a large oak tree can use over 350 litres of water a day, which will build up in the ground instead if the tree is removed. In clay soil, this will cause the ground to expand, causing ‘heave’ which can damage foundations and walls in the same way as subsidence.
If allowed to continue growing, however, a large tree’s root system will expand and remove more water from the soil, increasing the risk of subsidence; for this reason, the best course of action is to regularly maintain and cut back any trees to ensure they remain the same size. This will prevent any major changes to ground conditions, and reduce the risk of movement below any nearby buildings.
Building movement - what to do
If you’ve identified cracking or other movement to your property, it’s essential to ensure that the property is safe and whether there’s any ongoing movement. An inspection should be made by a surveyor or engineer, who can find the cause of the movement; they’ll also need to make further visits to check whether any movement is continuing.
If movement to the building is minor and there’s no ongoing movement, it’s often possible to make small repairs to cracks with no further risk. If movement is severe, however, more extensive works can be needed. This can include ‘stitching’ of cracks using steel bars, or ‘underpinning’ foundations with additional concrete. In either case, it’s always important to find and remove the source of the movement first; this often involves advice from an Arborist on whether nearby trees should be reduced in size or cut down entirely.
Other issues
It’s important to ensure that any trees close to your property are in good health. If not supplied with sufficient light or water a tree can die prematurely; it’s important to understand any specific issues that may affect specific trees, such as ‘Ash dieback’ which is a serious disease caused by fungus. Unhealthy or dying trees can be dangerous; the trunk of a mature sycamore tree can weigh up to four tons, with the leaves alone weighing up to 250kg. If you’re concerned about the health of a tree on or near your property, it’s important to seek professional advice.
Trees are a beautiful and essential part of the green spaces surrounding our homes. Whilst it’s important to understand the risks associated with nearby trees, they usually do not pose a risk to nearby buildings if maintained and cut back regularly.
If you have any concerns related to movement or cracking in your property, please contact us.