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If there are trees on or next to a development site, the local planning authority will usually expect them to be assessed to BS 5837:2012, Trees in relation to design, demolition and construction. The root protection area (RPA) is the part of that assessment that most affects a layout: it is the minimum area around a tree that should be left undisturbed so the tree stays healthy, and it decides how close new buildings, drives and services can go.
The arboriculturist plots each tree’s RPA on the site’s topographical survey for planning, so the tree positions, stem sizes and canopy spreads on that drawing need to be right. This calculator lets you check an RPA from the stem diameter for single and multi-stemmed trees.
Root protection area calculator
One stem, or up to 30 stems separated by commas. Measure each stem at 1.5 m above ground (see below for forks, swellings and slopes).
What a root protection area is
BS 5837 describes the RPA as a layout design tool. It shows the minimum area around a tree that is assumed to contain enough roots and rooting volume to keep the tree viable, and where protecting the roots and the soil structure is treated as the priority. During construction the RPA is normally fenced off as part of a construction exclusion zone, and any work inside it, such as a new path or a foundation, has to be justified and designed so the tree can survive it.
How the RPA is calculated
For a tree with a single stem, BS 5837:2012 (clause 4.6.1) sets the RPA as a circle with a radius of 12 times the stem diameter. The area is π × radius². The RPA is capped at 707 m², which is a circle with a radius of 15 m, so any stem of 1,250 mm or more gets the maximum.
| Stem diameter | RPA radius | RPA area |
|---|---|---|
| 75 mm | 0.90 m | 3 m² |
| 150 mm | 1.80 m | 10 m² |
| 300 mm | 3.60 m | 41 m² |
| 450 mm | 5.40 m | 92 m² |
| 600 mm | 7.20 m | 163 m² |
| 900 mm | 10.80 m | 366 m² |
| 1,250 mm or more | 15.00 m | 707 m² |
The RPA is a planning and design figure, not a measurement of where the roots actually are. Roots often spread well beyond it, and can be absent from parts of it.
Trees with more than one stem
Multi-stemmed trees are converted into a single equivalent stem diameter, then the 12 times rule is applied as normal.
Two to five stems. Square each stem diameter, add them up and take the square root. A tree with stems of 220 mm, 180 mm and 150 mm works out as √(220² + 180² + 150²) = √103,300 = 321 mm, giving an RPA radius of 3.86 m and an area of 46.7 m².
More than five stems. Take the mean (average) stem diameter, square it, multiply by the number of stems and take the square root. Six stems with a mean diameter of 120 mm work out as √(120² × 6) = 294 mm, giving a radius of 3.53 m and an area of 39.1 m².
The calculator picks the right method from the number of stems you enter.
Measuring the stem diameter
BS 5837 measures stems at 1.5 m above ground level, and its Annex C shows how to deal with awkward trees. In summary:
On sloping ground, measure at 1.5 m above the highest adjacent ground level, which is the uphill side of the trunk.
Where the trunk forks, or has a swelling such as a burr, below 1.5 m, measure at the narrowest point below the fork or swelling.
For multi-stemmed trees, measure each stem separately, then combine them as shown above.
Diameter tapes read the diameter directly. If you only have a girth (the circumference), divide it by π. BS 5837 asks for trees with a stem diameter of 75 mm or more to be plotted on the topographical survey, together with trees off site whose canopies overhang the boundary or whose RPA could reach into the site.
Changing the shape of the RPA
The RPA is first drawn as a circle centred on the base of the stem. Where there is reason to expect roots to have grown unevenly, for example along an existing road, a building, a buried service, a change in level, poor drainage or different soil, the arboriculturist can change it to a polygon of the same area. The shape has to come from an assessment of where the roots are likely to be, not from where the design would like the line to go.
Ancient and veteran trees
BS 5837 is not the only guidance that applies to the oldest trees. Natural England and the Forestry Commission’s standing advice for planning decisions asks for a buffer zone around an ancient or veteran tree of at least 15 times the trunk diameter, or 5 m beyond the edge of the canopy if that is larger. Ancient woodland should have a buffer of at least 15 m. The calculator shows the 15 times figure for single-stemmed trees, and uses the canopy radius if you enter one.
Tree preservation orders and conservation areas
A tree with a tree preservation order (TPO) needs the council’s consent before it is cut down, lopped or topped, and in a conservation area you must give the council six weeks’ notice of most work to trees. The RPA does not change any of this. Our free planning constraints checker shows tree preservation orders and conservation areas for any address in England, using the government’s planning data, although coverage depends on what each council has published.
Getting the trees on the drawing
The arboriculturist’s tree survey, constraints plan and tree protection plan are all drawn over the topographical survey. Our surveys record each tree’s position, stem diameter at 1.5 m, canopy spread and height, tied to the Ordnance Survey grid, so the RPAs land in the right place relative to the boundaries, buildings and levels. Read what a topographical survey shows and how much a topographical survey costs, or ask us for a quote.
Sources
BS 5837:2012 Trees in relation to design, demolition and construction: Recommendations, published by BSI (clause 4.6, Annex C and Annex D), and Natural England and the Forestry Commission’s standing advice, Ancient woodland, ancient trees and veteran trees: advice for making planning decisions.