The root protection area
The root protection area is the minimum area around a tree that it is considered is necessary to protect its root system from damage such as construction or compaction.
Arborists calculate it either by a formula or by drawing it manually.
How is a root protection area calculated?
Under the British Standard BS5837 for each tree, the arborist calculates the RPA by multiplying the diameter breast height ( DBH ) of the tree in meters by 12. A simple circle is then drawn around the centre point of the trunk to this distance. A maximum RPA of a radius of 15m is assumed. There are more complex rules for calculating trees with multiple stems.
The arborist can hence provide a simple diagram of the root protection areas for trees, which is easy to calculate and understand.
How is a root protection area estimated manually ?
Although root protection areas are drawn simplistically as a circle, often in practice they are asymetrical or even irregular shapes.
Drawing a root protection area manually is more complicated than the formula approach and requires input from an experienced arborist. The estimation may also often involve further ground investigations. This might include digging of small test pits to look for roots or other specialist equipment. The arborist will allow for the factors like :
- hard surfaces,
- previous trenching works,
- water sources,
- soil conditions
- and the ecology of the tree.
Accurate manual drawing of root protection areas is particularly important on resticted sites or for large trees and important specimens.
How is a root protection area used ?
Architects and planners can use root protection area in site design for buildings and roads, but service trenches as well. The arborist will use it later for a detailed Arboricultural Method Statement to show how to protect tree roots in constuction.