Tree Root Protection Zone – What is it and what does it mean?

Tree Root Protection Zone
Tree Root Protection Zone

One of the more common questions we receive is – what is a tree root protection zone? Followed by what does it mean to my building or construction project?

Firstly, Root Protection Zone or RPZ is generally an American term, in the UK the accepted phrase is Root Protection Area or RPA. However, both terms relate to the area surrounding a tree that cannot be excavated, compacted, surfaced, driven over or any other form of disturbance during any phase of the construction process. This area, although it may cause constraints within the construction phase, will ensure that the trees to be retained on or off site around the development project will not be affected (or only affected to a certain, acceptable level).

How is the RPA or RPZ calculated?

RPA sizes are based on a simple set of calculations that are produced within the British Standards – 5837:2012 Trees in relation to design, demolition and construction. These calculations are based on the diameter at breast height or DBH of a tree and are a standard set of calculations that are based on averages of all species, regardless of age, growing conditions, pruning etc.

For single trees, the following calculation is us used:

For single stem trees, the RPA (see 3.7) should be calculated as an area equivalent to a circle with a radius 12 times the stem diameter. For trees with more than one stem, one of the two calculation methods below should be used. In all cases, the stem diameter(s) should be measured in accordance with Annex C, and the RPA should be determined from Annex D. The calculated RPA for each tree should be capped to 707 m2.”

Why is the RPA or RPZ so important?

Many people understand that the roots of trees provide the stability for the tree, however they are also responsible for the uptake of all of the trees water and nutrients and therefore the effects of root damage may not be immediately obvious, but will show as a slow decline in health over a number of years. This slow decline may not even be attributed to the root damage that occurred many years before.

How can trees root damage and construction be managed?

As seen previously RPA’s are calculated using an average calculation that is based not on species or size but only stem diameter. Unfortunately, the RPA calculation often means that either tree roots are damaged, or the construction size or area is un-necessarily compromised. In order to mitigate this we can offer our clients a number of solutions to accurately plot the location and size of tree roots in order to ensure the trees remain healthy and provide a positive financial benefit to the project,(See Urban Tree Management Blog for more information on the value of trees within the urban environment.) as well as ensuring the maximum area available to the building project is achieved. We have a number of options for our clients in order to achieve this, including, airspade investigations and using Ground Penetrating Radar to scan for and plot the tree roots locations.

Contact us today to find out more

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