Root protection area: how the BS 5837 figure is derived

The root protection area is a calculation, not a measurement. Nobody has excavated to find out where the roots of the oak on your boundary actually go. What BS 5837:2012 does is give a default area, derived from the size of the stem, that should be left undisturbed so the tree has a reasonable prospect of surviving development.

Understanding that it is a default matters, because it tells you where there is scope to reason with a tree officer and where there is not. The area is negotiable in shape. It is much less negotiable in size.

What the figure represents

For a single stemmed tree, the root protection area is the area of a circle whose radius is twelve times the diameter of the stem, measured at 1.5 metres above ground level. A tree with a stem diameter of 500 millimetres therefore produces a radius of 6 metres.

The area is capped. Beyond a certain stem size the calculation stops growing, which prevents a very large veteran tree from generating a protection area covering an entire site. That cap is one of the more useful features of the standard and one of the least known.

Multi stemmed trees need a combined figure. Where a tree has several stems, the diameter used is derived from the individual stem diameters taken together rather than from the largest one, with a different approach where the number of stems is high. The report should show the working, because two arboriculturists using different methods on the same coppiced hazel will produce noticeably different areas.

Measurement itself has rules. The reading is taken at 1.5 metres, and where the tree is on a slope, leaning, or has a pronounced butt flare, the standard sets out how to adjust. A survey that records diameters without noting these conditions is doing half the job.

Why the area is expressed as an area

The circle is a convenience. What the standard is protecting is a quantity of undisturbed soil, and the assumption behind the calculation is that a tree of a given size needs roughly that much ground to draw water and nutrients from.

So the shape can change where there is a reason. A tree growing hard against a building, a road or a retaining wall has not rooted uniformly in all directions, and offsetting the area away from the obstruction reflects reality better than a circle drawn through a Victorian foundation.

Three rules govern that adjustment. The total area stays the same. The modified shape remains contiguous with the stem rather than becoming a detached blob elsewhere on site. And the reason for the modification is stated, drawing on soil conditions, existing hard surfacing, evidence of root distribution or the tree's history.

The commonest misuse is reshaping the area to fit the proposed building rather than to fit the tree. An officer will spot it, because the resulting shape looks like the development rather than like a rooting zone.

The category grading that decides which trees count

Before any of this is relevant, the survey grades each tree. The standard uses categories running from those in such poor condition that they should be removed regardless of development, through to high quality specimens that would be an important constraint on layout, with subdivisions reflecting arboricultural, landscape and cultural value.

Trees in the lowest category do not generate a root protection area for design purposes, because they are not being retained. Trees in the higher categories do, and the higher the grade the harder it is to justify an incursion.

This is where the cost of getting the survey wrong shows up. A layout designed around an optimistic grading, which the council's tree officer then disputes, has to be redrawn. Where the change needed is small, a non-material amendment under section 96A may cover shifting a building by a metre, but the limits of that route are narrow and it cannot rescue a layout that conflicts outright.

Fencing and the construction exclusion zone

Once the areas are fixed, the protection has to exist physically. The default barrier in the standard is a scaffold framework carrying welded mesh panels, braced so that it cannot be pushed over or moved, with signage identifying the exclusion zone. Where the ground itself must not be disturbed, a free standing barrier system is used.

The enclosed area is a construction exclusion zone, and the word exclusion is meant literally. Nothing happens inside it. No excavation, no storage of materials, no cement or mortar mixing, no fuel or chemical storage, no fires, no level changes in either direction, no vehicle movement, no site cabin, no skip.

Every one of those is a real breach a tree officer has recorded on a site visit. The one that causes most argument is level changes, because importing topsoil inside the zone feels helpful and is not.

Where the zone has to be entered for a specific operation, that operation is described in the method statement with its own protection measures. The detail of the barrier, its position and the signage belongs on the drawing, and the practicalities of setting out the fencing specification on a tree protection plan are worth getting right first time.

Working inside the area where it cannot be avoided

Incursions are dealt with by design rather than by argument. Where part of the area must be used, the measures available include load spreading surfaces laid above existing ground level, foundations that bridge rather than bear, trenchless service routes, and temporary ground protection for access.

Each of those needs a specification and a supervision schedule. A note on a drawing saying "no dig construction" without a build up detail, a level and a named supervisor will be conditioned into something more specific or refused.

Compensation is sometimes offered, meaning an equivalent area of ground elsewhere within the likely rooting zone is improved and protected. It helps at the margins. It does not license a large incursion.

Recording compliance during construction

The paperwork chain runs survey, tree constraints plan, impact assessment, tree protection plan, method statement, supervision schedule. Councils normally condition the last three and require written confirmation at defined points.

Photograph the fencing when it goes up, with something in shot to establish the position. Keep the supervision records. Note any variation agreed on site, in writing, at the time.

The reason is practical rather than bureaucratic. Discharging a tree protection condition eighteen months later, with no record of what was installed and when, is slow at best. And where damage is alleged, contemporaneous photographs are the only evidence that will settle it.

If you are scoping a site with mature trees, the arboricultural work sits alongside ecology, drainage and landscape, and those disciplines argue with each other over the same ground. Looking at the documents a site with retained trees usually needs together, rather than one at a time, tends to surface the conflicts while the layout can still change.

Enter the site postcode on PlanScope to see which reports your site is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Councils expect tree surveys and method statements to be prepared by a suitably qualified arboriculturist, and this article is general guidance rather than legal advice.