To understand Root Protection Areas (RPAs), first we must understand how the RPA is calculated. The British Standard (BS5937: Trees in Relation to Construction and Demolition – Recommendations) sets out that the radius of an RPA from a tree is calculated as twelve times the diameter of that tree as measured at 1.5m high.
Inevitably this is a guide, and certainly doesn’t provide assurance that roots aren’t outside the RPA, that roots are all the way through the RPA, or indeed that the RPA is circular. The growth of roots will be influenced by site conditions such as foundations, compaction, water-logging, major level changes. Roots favour ground where conditions are better, which may beyond the basic RPA circle. Understanding the site and soil conditions can help determine where roots will likely be present or absent.
The loss of parts of the RPA area can be tolerated within a development. If you consider the roots system like the branch system, there are main structural roots close to the centre, with smaller roots at the outer edge. Smaller roots can easily be lost and replaced, but the loss of main roots is much less tolerable. Ultimately you are more likely to cause harm as you get closer in.
The tolerance to changes within the RPA can be specific to the species or the age of the tree. For example, most tree roots are in the top 600mm of soil, but for some species root growth may be even shallower, meaning any disturbance is potentially more significant for that species.
Sometimes root locations are obvious during the site survey, for example cracked or lifted paving and exposed surface roots. Where this isn’t possible, RPA data can be supported through careful excavation to locate roots, or in some cases, using tree root radar. These techniques are particularly useful for contentious schemes.
Tree roots hate compaction. They don’t grow in compacted soil, and may die if the soil is subsequently compacted. During development, compaction can be prevented using ground protection for pedestrians, or trackway mats for vehicles boards. Post-development, the use of typical reduced dig paving, with a three-dimensional cellular confinement system to spread loading, is often the most appropriate mitigation.
If you are excavating within an RPA, do so with care (typically by hand) to reduce risk of accidentally pulling out roots or causing damage. Cut smaller roots with a sharp saw to produce the smallest wound (as you would do with branches), but seek advice on larger branches as they may be critical to tree health or stability.
And remember, there are plenty of other mitigation options too, such as special techniques for foundation construction (like using piles or cantilever) to minimise root loss. These may enable development to take place within the RPA.
To understand more about trees in relation to construction, tree surveys and Arboricultural Impact Assessments, call Richard Parmee (Principal Arboricultural Consultant) on 01638 663226 (Newmarket office) or email firstname.lastname@example.org.