Root protection areas: calculating and drawing the RPA

A root protection area is the ground around a tree that has to stay undisturbed if the tree is to survive construction. Under BS 5837:2012 you calculate it from stem diameter alone: measure the stem at 1.5 metres above ground level, multiply that diameter by 12, and use the result as the radius of a circle centred on the stem. The area is then capped at 707 square metres, which is what a stem of 750 millimetres produces.

It is a crude proxy and everyone in the field knows it. Roots do not grow in circles, they grow where water, air and nutrients are, and a tree on a made-ground site with a service trench along one side has an entirely asymmetric root plate. The standard uses a circle because a circle is checkable, and it then allows the shape to be adjusted where there is a reason.

What the area is protecting

Fine absorbing roots sit in the top few hundred millimetres of soil and extend well beyond the canopy edge. They need air-filled porosity, which is what compaction destroys. Most construction damage to trees is not severance by a digger bucket. It is a season of vehicle movements and material storage over the rooting zone that leaves the soil unable to hold air.

That is why the protected area is expressed as ground rather than as roots, and why the practical measures that follow are about keeping loads off the surface rather than about avoiding cutting.

The calculation in practice

For a single stem tree the sequence is short. Measure the diameter over bark at 1.5 metres above ground level. Multiply by 12. That is the radius in the same units. Draw the circle, calculate the area, and cap it at 707 square metres if it exceeds that.

Multi stem trees need a combined diameter first. Where there are two to five stems, take the square root of the sum of the squares of the individual stem diameters. Where there are more than five, the standard uses the mean stem diameter and the number of stems to reach the same kind of combined figure. Then apply the multiplier as normal.

Trees on sloping ground are measured differently, with the measurement taken on the upslope side or averaged, and a tree that forks below 1.5 metres is measured below the swelling. These are the points where two surveys of the same tree diverge, so the survey schedule should record the height at which each diameter was taken.

The output belongs in the tree survey schedule alongside category, height, canopy spread and life expectancy, and it feeds the Arboricultural Impact Assessment that follows.

Reshaping the circle

BS 5837 permits the notional circle to be modified where soil conditions, topography, the presence of existing structures or observed root distribution justify it, provided the area of the modified shape equals the area of the circle.

Two rules keep this honest. The shape has to remain plausible for a tree, so a long thin tail running down a garden to avoid a proposed house is not a root protection area, it is a wish. And the reshaping is the arboriculturist's judgement, informed by what is visible on site, not the designer's.

Where reshaping is proposed, say what drove it. A road on one side and open lawn on the other is a reason. A proposed extension is not.

Existing surfaces and structures

A building, a road or a hard standing inside the notional circle raises a question the standard leaves to judgement. Where a structure truly precludes rooting, the area beneath it can be discounted, and the equivalent area found on the unobstructed side.

Be careful with this. A block paved drive on a sand bed is not impermeable, and roots commonly run under and along the edges of older surfaces. Trial holes will settle the point where a large discount is being claimed. A survey which discounts a third of the area on the strength of an aerial photograph will not survive a tree officer's site visit.

The same logic applies to buildings. A basement with a tanked retaining wall is a real barrier. A Victorian house on shallow strip footings, with a garden that has been cultivated for a century, is not, and roots are commonly found under and around such foundations. State which type of structure you are discounting and why.

Levels matter as well. Where the ground inside the protection area has been raised or cut since the tree established, the rooting depth is no longer where the standard assumes. A survey that notes a retaining wall and a change of level across the area gives the designer something to work with. One that records only a diameter does not.

Recording incursions

Almost every constrained site has some incursion. The standard treats incursion of less than 20 per cent of the area as potentially acceptable where the tree is in good vigour and compensatory measures are provided elsewhere in the area, typically no dig construction with a cellular confinement system and a geotextile over the existing grade.

Above that threshold, the case has to be made with evidence rather than assertion. Trial excavation by air spade or hand digging, photographed and recorded, shows what roots are actually present. Sometimes the answer is that the tree can take it. Sometimes the answer is that the design has to move.

Record every incursion in a schedule: tree number, area of the notional protection area, area lost, percentage, and the compensatory measure. And give the percentage as a number rather than describing it as minor, because minor is not a unit.

Drawing it on the constraints plan

The tree constraints plan is the drawing that turns the calculation into a design input, and it should exist before the layout does.

It carries the surveyed position of every tree with its reference number, the canopy spread as surveyed, the root protection area as a hatched or dashed outline, and shade patterns where they matter. Trees are shown in their retention categories, usually by colour, so that the designer can see at a glance which trees are worth designing around.

Neighbouring trees go on the same plan. A protection area does not stop at the red line, and an offsite tree with a Tree Preservation Order can be the binding constraint on a plot even though you have no control over it. Check the council's order records and whether the site is in a conservation area, where a notice requirement applies to works to trees.

The plan should be at a scale where the outlines can be measured, cross-referenced to the survey schedule by tree number, and issued to the design team as a base drawing rather than as an appendix nobody opens. Planting proposals in a Landscape Strategy then have something to respond to, including replacement planting where a removal is unavoidable.

Councils usually expect a suitably qualified author for arboricultural work, so use an AI-assisted draft for the policy framing and the report structure and have the survey, the calculations and the method statement produced or checked by an arboriculturist. You can see which reports a constrained site needs before you brief one, and Enter your postcode on PlanScope to check whether tree constraints are flagged on your site.

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