Root protection areas for multi-stem and coppiced trees

A root protection area multi stem tree calculation starts by turning several stems into one number. BS 5837:2012 sets the root protection area for a single stem tree as a circle with a radius of 12 times the stem diameter measured at 1.5 m above ground level, capped at 707 m2 per tree. For a multi-stem tree you first derive a combined stem diameter, then feed that into the same formula.

The method for combining depends on how many stems there are, and the thresholds are two to five stems and more than five. Get that part wrong and every downstream number on the tree constraints plan is wrong with it.

The single stem calculation as the baseline

Measure the stem diameter at 1.5 m above adjacent ground level. Multiply by 12 to get a radius, and treat the result as an area rather than as a fixed circle, because the shape can be adjusted later while the area cannot.

Two measurement points cause most of the errors. Where the ground slopes, follow the method in the standard rather than averaging by eye, because a difference of half a metre in the reference level on a steep bank changes the diameter noticeably. And where the stem swells, flutes or carries basal growth at 1.5 m, take the reading at a point typical of the stem rather than the swelling, and record where you took it.

Record the diameter to the nearest 10 mm and note the measurement height on the schedule. An arboricultural report that gives diameters without saying where they were taken invites the council to ask, and that question costs a week.

How multi-stem diameters are combined

BS 5837:2012 gives two methods, separated by stem count.

For a tree with two to five stems, the combined diameter is the square root of the sum of the squares of the individual stem diameters. So three stems of 200 mm, 250 mm and 150 mm combine as the square root of 40,000 plus 62,500 plus 22,500, which is the square root of 125,000, or approximately 354 mm. The root protection area radius is then 12 times 354 mm, roughly 4.2 m, giving an area of about 56 m2. That worked example is illustrative.

For a tree with more than five stems, the combined diameter is the square root of the mean stem diameter squared multiplied by the number of stems. Arithmetically that is the mean diameter multiplied by the square root of the stem count, which is a useful check on your own spreadsheet.

Why two methods? Because summing squares across many small stems overstates the rooting resource, and the mean-based approach damps that. The practical consequence is that a coppice stool with 12 stems does not generate an enormous protection area simply because it has many stems.

One judgement sits before all of this. Decide whether you are looking at one multi-stem tree or several trees growing close together, because the answer changes the calculation and the categorisation. Stems arising from a single root plate or stool are one tree. Separate individuals that have grown together are not.

Coppice stools and veteran forms

Coppice stools are the commonest multi-stem form on development sites in England, particularly on old field boundaries and in relict woodland. Measure each stem above the stool but below the point where it starts to flare, and record the stool diameter separately, because the physical extent of the stool matters for the tree protection fencing line even where it does not enter the calculation.

Veteran and ancient trees are handled differently, and more generously. Standing advice for ancient and veteran trees points to a root protection area based on 15 times the stem diameter, or an area extending 5 m beyond the canopy, whichever is larger. These trees also attract policy protection in their own right, and the buffer around ancient woodland is a separate question again, covered in our note on ancient woodland buffers.

Pollards, layered stems and trees on banks need judgement rather than arithmetic. Where the form does not fit either method cleanly, say so in the report, explain the approach taken and why, and give the council the numbers behind it.

The area cap and when it applies

The 707 m2 cap applies per tree, and it corresponds to a circle of 15 m radius. Any single stem diameter above about 1,178 mm produces a calculated radius beyond 15 m, so the cap takes over.

The cap catches large mature trees and, occasionally, a multi-stem calculation on a big old stool. Apply it, and say in the schedule that you have applied it, because a reviewer comparing your radius against 12 times the diameter will otherwise think you have made an error.

Modifying the shape without reducing the area

The root protection area is an area, and BS 5837 allows the shape to be adjusted where there is evidence that roots are unlikely to occupy part of the notional circle. The total area must not be reduced, and the retained area has to remain a sensible, contiguous shape that includes the ground immediately around the stem.

Situations where modification is defensible include an existing building or retaining wall that already truncates the rooting volume, a road or other long-established compacted surface, a change in soil or geology, a watercourse or a steep drop in ground level.

Situations where it is not defensible are just as common in submitted reports. Offsetting the area because the proposed building is in the way. Modifying to a shape that leaves a thin tail wrapped around an obstruction. Reducing the area and calling it a modification. Reviewers see all three regularly, and an arboricultural officer who finds one stops trusting the rest of the report.

Offset away from the constraint and towards the ground the roots are most likely to occupy, keeping the shape compact.

Evidence needed to justify a modification

Assertion will not carry it. What works is physical or documentary evidence about the rooting environment:

1. Root investigation by trial trenching or air spade excavation at agreed locations, with photographs and a record of root diameters and depths found.
2. Historic mapping and construction records showing when the adjacent surface or structure was built, and what depth it was formed to.
3. Soil profiling or a borehole log showing a rooting restriction such as rock, made ground or a high water table.
4. Levels information where the ground falls away sharply within the notional circle.
5. A species-specific rationale, because rooting habit varies and a shallow-rooting species on a compacted urban soil behaves differently from an oak on free-draining sand.

Agree the investigation scope with the council's tree officer first. Digging exploratory trenches inside a root protection area without agreement is itself the kind of act a protection condition is written to prevent, and our note on discharging a tree protection condition covers the sequencing.

Plotting the result on a drawing

Put the modified areas on the tree constraints plan as polygons with their areas annotated, alongside the canopy spread and the BS 5837 category for each tree. Show the notional circle as a dashed line where you have modified the shape, so the reviewer can see both and check that the areas match.

Then carry the same polygons through to the tree protection plan, with the fencing line, ground protection, any no-dig construction and service routes shown against them. A Tree Survey and Arboricultural Impact Assessment that shows one set of areas in the constraints drawing and a different set in the protection drawing is the single most common reason these documents come back for revision. Councils usually expect a suitably qualified author for arboricultural work and professional review is strongly advised.

Service runs and drainage deserve their own check at this stage, because a soakaway or a new sewer inside a protection area is a conflict that is far harder to solve on site, as our piece on trees and drainage explains.

This is general guidance rather than legal advice. Root protection areas are applied by the local planning authority's tree officer and depend on site conditions, so check the approach with them or with a qualified arboriculturist before relying on it.

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