Root Protection Area
RPA

The RPA radius is calculated as the stem diameter (measured at 1.5m above ground in BS5837, a different height than the 1.3m DBH convention used elsewhere) multiplied by 12. Multiply that radius by itself and by π to get the protected area in square meters. A 500mm stem diameter gives a 6m radius and roughly 113m² of protected ground — for how that circle gets drawn onto an actual site plan, see the BS5837 survey guide.

BS5837 caps the calculated area at 707m² per tree (equivalent to a 15m radius) unless there's specific arboricultural justification for going larger — in practice this cap rarely bites except on very large, old specimens.

For a multi-stem tree, you don't run the RPA formula on each stem separately or average them — BS5837 uses an aggregate stem diameter derived from all the stems together (the square root of the sum of each stem diameter squared), then applies the same 12x multiplier to that single aggregate figure. This is one more reason multi-stem DBH needs to be recorded per-stem in the field, not lumped into one number at the time of measurement — you need the individual readings to calculate the aggregate correctly later.

Ground inside the RPA is normally fenced off before any site works begin, kept free of material storage, vehicle movement, and excavation. Where access or works genuinely can't avoid the RPA, no-dig construction methods (cellular confinement systems, ground-protection matting, hand-dug trenching instead of machine excavation) are used to reduce root damage — the RPA doesn't automatically forbid all activity, it forbids activity that would damage the roots without mitigation.

The full walkthrough is in the BS5837 tree survey guide. LogLog doesn't draw RPA circles on the plan today — record accurate per-stem DBH via the tree data fields and run the RPA calculation with your usual method or CAD tech afterward.