As described in BS 5837:2005 an RPAis the minimum area around the base of the tree that contains enough rooting volume to ensure that a tree can survive and grow.
The RPA is a key tool in protecting trees when developing or building on a site as it sets out the minimum area within which existing soil conditions must remain undisturbed.
The radius (m) of the RPA for a single stemmed tree is calculated as follows:
- Stem diameter (m) measurement at 1.5m height (breast height) x 12
The radius (m) of the RPA for a multi-stemmed tree is calculated as follows:
- Diameter (m) measured at ground level x 10
m = metres
The RPA when calculated forms the basis for setting out a Construction Exclusion Zone (CEZ), which is the area around the retained tree that is fenced off to exclude construction work.
In certain circumstances, special ground protection systems - subsequent to advise by an arboriculturalist in conjunction with the engineer - may also be used to protect parts of the RPA that cannot be fenced off during construction.
A number of additional factors need to be taken fully into account when setting out the RPA and the CEZ. Please refer to the BS 5837:2005 for more details.