Root protection area calculations in an AIA

The root protection area calculation in BS 5837:2012 is simple arithmetic that carries a lot of weight: a circle with a radius of 12 times the stem diameter, measured at 1.5 metres above ground level, capped at 707 square metres. Every tree officer checks it, and a layout that ignores it usually comes back with a request to move the building.

What follows is how the calculation behaves in a real arboricultural impact assessment, including multi-stem trees, reshaping, and the point at which encroachment stops being defensible.

Where the root protection area sits in a BS 5837 report

The assessment has an order, and the root protection area appears twice in it.

First in the tree survey schedule, which records each tree by reference number, species, height, stem diameter, branch spread at four points, crown clearance, age class, condition, remaining contribution in years and category grading under the cascade chart. The stem diameter in that schedule is the input to the calculation.

Then on the tree constraints plan, where the root protection areas are drawn to scale alongside crown spreads and, for trees to the south of proposed windows and gardens, shade patterns. The constraints plan is a design tool. Produced after the layout is fixed, it becomes a record of what went wrong rather than a way of getting it right.

An Arboricultural Impact Assessment then sets out the effects of the proposal on retained trees, and the tree protection plan and method statement describe how those trees will survive construction.

The 12 times stem diameter calculation

Measure the stem diameter at 1.5 metres above ground level. On a sloping site, measure on the upslope side. Where the tree forks below 1.5 metres, the standard treats it as multi-stemmed.

Radius equals 12 times that diameter, in the same units. A 400mm stem gives a 4.8 metre radius. Area is then the ordinary circle formula, and for that tree it comes out at about 72 square metres.

The 707 square metre cap corresponds to a radius of 15 metres, which is a stem diameter of 1.25 metres. Above that the standard stops scaling the circle and expects site-specific investigation of the rooting environment instead.

Multi-stem trees and the combined diameter

Multi-stem trees trip up more reports than any other part of the calculation, because the fix is not to add the diameters together.

For a tree with two to five stems, take the square root of the sum of the squares of the individual stem diameters. Three stems of 200mm, 250mm and 150mm give a combined diameter of about 357mm, and a radius of 4.28 metres.

For a tree with more than five stems, multiply the mean stem diameter by the square root of the number of stems. Both methods approximate the cross-sectional area of an equivalent single stem, which is why simple addition overstates the result badly.

Record the individual diameters in the schedule as well as the combined figure. A tree officer who cannot reproduce your arithmetic will ask for the raw data, and that request adds a fortnight.

Modifying the shape without changing the area

The circle is a default, not a survey of where roots actually are. BS 5837:2012 allows the shape to be modified where soil conditions, existing structures, hard surfacing or topography indicate that rooting is directional.

Three rules keep a modified shape defensible. The area must stay the same. The shape must remain contiguous with the stem rather than becoming a detached lobe. And the reasoning must be recorded, ideally with evidence from trial pits or a ground penetrating survey rather than assertion.

A common and legitimate case is a street tree with a road on one side. Roots are unlikely to extend far under a compacted sub-base, so the area is offset into the verge and garden. A modification that happens to move the circle exactly clear of the proposed building, with no site evidence offered, reads as what it is.

An illustrative calculation for a rear extension

Take a mature oak in a rear garden with a stem diameter of 600mm, in good condition, graded category B. This example is illustrative and the figures are worked from the standard rather than from a real site.

The radius is 12 times 0.6 metres, or 7.2 metres. The area is about 163 square metres. The proposed single-storey extension clips the circle at its nearest corner and takes 24 square metres of it, which is roughly 15 per cent of the root protection area.

That sits under the one fifth threshold the standard uses as a working limit, so the argument becomes whether the remaining 139 square metres of soil can support the tree. On an open lawn with uncompacted loam, probably. On a plot where two thirds of the remaining area is already patio and driveway, the honest answer is no, and the extension needs to move.

Compensate the lost area where you can, in soil that is contiguous with what remains, and say clearly in the report how the compensation was located.

Foundations where encroachment cannot be avoided

Once encroachment is agreed in principle, the method statement carries the weight. The usual options, in rough order of how often they are accepted:

1. No-dig construction using a cellular confinement system laid on a geotextile over the existing grade, for paths, drives and terraces.
2. A pile and beam foundation with a suspended slab and a void beneath, so that the excavation footprint is small and dispersed.
3. Screw piles, which avoid spoil and vibration and can be positioned around larger roots identified in trial pits.
4. A cantilevered slab spanning from outside the root protection area, where the geometry allows it.
5. Hand digging under arboricultural supervision, with roots over a set diameter retained and severed cleanly only where unavoidable.

Whichever route is chosen, the report should state the fencing specification, the ground protection during works, the position of site facilities and material storage, and who supervises. Barriers go up before anything else arrives on site, which is why the condition is almost always pre-commencement.

What the tree officer checks first

In practice the officer opens the constraints plan, measures a couple of circles against the scale bar, and looks for the building line. Then the schedule, to see whether the categories match the condition described. Then the method statement, for the words "no-dig" and "supervision".

Three failures repeat. Root protection areas drawn at the wrong scale. Multi-stem diameters added rather than combined. And a tree protection plan that shows fencing exactly on the building line, leaving no working space, which means the fence will be moved in week one.

Trees also shape the wider case, from shading studies for habitable rooms to the planting proposals in a Landscape Strategy. Where a tree has a preservation order or stands in a conservation area, separate consent or notice is needed for the works themselves, and permission for the building does not carry it.

Councils expect a suitably qualified arboriculturist to prepare and sign this work. A structured draft can save the survey write-up time, and it still needs professional review before it goes to the council. If you are scoping a submission, browse the report types a scheme usually needs or enter your postcode on PlanScope to see what your site is likely to trigger.

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