Understanding the Impact of Existing and New Trees on Building Foundations

Our focus on seasonal desiccation continues in this latest post which looks at the impact that trees and their roots have on development projects and more specifically building foundations.

Seasonal desiccation is where moisture is lost from clay rich soils which can then cause either subsidence or heave if the foundations are not deep enough. Trees can also cause soils to become desiccated, but whereas seasonal desiccation typically only affects the top 1m of soil, the influence of trees can be significantly deeper.

As with seasonal desiccation, foundations close to trees need to be deepened beyond the potential zone of desiccation; however, there are a few more factors to consider when designing the foundations. These include:

  • Soil type: Water take-up by tree roots impacts different soils in different ways. Of most importance are trees growing in clay rich soils, which are susceptible to volume change as water is removed by tree roots. Clay soils are classified as having either low, medium, or high-volume change potential.
  • Root water demand: Different tree types have different water demands, typically categorised into high, moderate, and low. For example, oak, willow, and poplar all have high water demand roots, while birch trees have a low water demand.
  • Tree height: larger trees will understandably have a greater root network. But it is also important to remember to forecast future growth and likely maximum tree height once they reach maturity too.
  • Density of trees: higher density means more roots, and therefore higher water demand in that specific area (even for lower water demand trees such as birch).
  • Distance from foundations: the closer the foundations are to trees the greater the amount of tree influence and therefore the deeper they need to be.

The impact of trees and roots on construction developments can often be underestimated, especially at the pre-planning stage. It is also important to remember that new trees can also have an impact and their projected growth needs to be accounted for.

If you have any development or construction projects in which there are existing or trees or planting is planned, then please get in touch to find out how we can help save both time and costs. Please use your main point of contact at GRM or for new enquiries email richard.upton@grm-uk.com or call 01283 551249.