Reshaping a root protection area without reducing it
The root protection area is an area, not a shape. What BS 5837:2012 allows when the default circle is wrong, the evidence that supports a modification, and why reviewers reject most of them.
The root protection area in BS 5837:2012 is an area, not a shape. That distinction is the whole of the reshaping argument. Where the default circle is demonstrably wrong, the standard allows the area to be reshaped to reflect the likely root distribution, provided the total area is not reduced and the shape remains contiguous with the stem. Reviewers reject far more modified root protection areas for lost area than for bad shape.
The default calculation is a circle with a radius of twelve times the stem diameter, capped at 707 m². For a single stem that is the starting point and nothing more.
Getting the default area right first
Reshaping a wrongly calculated area produces a wrong answer with extra steps, so the arithmetic comes first.
Single stemmed trees take the diameter measured at 1.5m above ground. Where a tree forks below that height, the standard derives a combined diameter: for two to five stems, the square root of the sum of the squared diameters, and for more than five stems, the square root of the mean stem diameter squared multiplied by the number of stems. Those two derivations produce meaningfully different radii, and a schedule that applies the wrong one for a six stemmed hazel coppice will under-protect the tree.
The 707 m² cap corresponds to a 15m radius, reached at a stem diameter of 1,250mm. Veteran and ancient trees are a separate matter: national policy treats them as irreplaceable habitat, and a 15m circle is rarely the right protective distance for a 1,500mm oak.
Check the survey date and the measurement method before accepting any of it. Diameters taped over ivy or over a burr are wrong by enough to matter.
What the standard permits and what it does not
Three things can be modified: the shape of the area, the position of the area relative to the stem, and the assumptions about what lies within it.
What cannot be modified without a separate justification is the quantity. If the default calculation gives 152 m², the modified area should also give 152 m², redistributed. Any proposal that reshapes and shrinks in the same move is asking for two things while evidencing one, and it should be treated as two separate questions.
Nor can the area be reshaped into a form that no longer functions. A root protection area drawn as a long thin corridor running away from the tree protects nothing, because the roots that keep the tree standing and fed are concentrated near the stem and in the upper soil horizons. The shape has to stay contiguous with the stem and has to remain plausible as a rooting volume rather than as a plan geometry exercise.
Depth is part of the same judgement. Most structural and absorbing roots sit in the top 600mm of soil, so a modification that assumes rooting at depth beneath an existing slab needs to say why.
Evidence that supports a modification
The standard expects the modification to reflect soil conditions, the tree's morphology and any existing structures or surfaces that already preclude rooting. Each of those needs something behind it.
Existing hard surfaces and structures are the easiest to evidence. An adopted carriageway with a compacted sub-base, a building with strip foundations to 1m, a lined watercourse, a basement wall: these physically exclude rooting, and photographs plus a section usually settle the point. Be careful with driveways and patios, because a thin bitumen layer over uncompacted subsoil does not exclude roots at all, and roots frequently run beneath paving where moisture collects.
Ground conditions come next. A shallow soil over rock, a high water table, or a made-ground layer with high stone content will all skew rooting, and a soil description from trial pits gives the arboriculturist something to argue from.
Direct investigation is the strongest evidence and the least used. Trial trenching excavated by hand or with an air spade, dug perpendicular to the stem and recorded by depth and root diameter, shows what is actually there. Ground-penetrating radar and electrical resistivity tomography can map rooting without excavation, with the caveat that interpretation in heterogeneous urban fill is difficult and results should be presented with their limitations stated.
Setting out the modification
An illustrative example makes the arithmetic visible. A mature lime with a 620mm stem gives a radius of 7.44m and an area of about 174 m². The tree stands 2m from an adopted footway with a full depth sub-base running along its northern side. The arboriculturist takes the northern segment out, evidences the exclusion with a section through the footway construction, and adds the equivalent area to the south and east as an offset lobe that remains contiguous with the stem. The figure stays at roughly 174 m², and both numbers appear on the drawing. Treat that as illustrative rather than as a template. The real answer depends on what the trial pits show.
Record the modification where a reviewer will find it:
- Show the default area and the modified area on the tree constraints plan, one dashed and one solid, with both areas stated in square metres.
- Annotate the reason for each segment removed, referenced to the evidence that supports it.
- Repeat the modified area on the tree protection plan so the construction exclusion zone matches.
- Carry the justification into the arboricultural impact assessment as text, not only as a drawing note.
- Reference the investigation record, including the date, the method and who carried it out.
An arboricultural impact assessment and tree survey that shows the working is a much easier document to approve than one that presents a modified shape as a given.
Where reviewers push back
The common failures are predictable.
A modified area whose outline matches the proposed building footprint exactly. That is design-led rather than evidence-led, and it reads that way to anyone who has seen it before. A modification supported by a single sentence asserting that roots are unlikely to extend in a given direction. An area that has quietly lost 15 per cent in the reshaping. A crown shape used as a proxy for root distribution, when asymmetric crowns usually reflect light competition and past pruning rather than rooting.
And modifications applied to veteran trees, where the whole framework of twelve times the diameter is the wrong tool and a root protection area based on canopy extent plus a buffer is more appropriate.
Where an authority rejects a modification, the usual remedy is not more argument but more evidence. A day of trial trenching costs less than a redesign and far less than a refusal.
Carrying the decision into construction
A modified root protection area only works if the site respects it. The construction exclusion zone fencing has to follow the modified line, the arboricultural method statement has to describe any works inside it, and supervision has to be conditioned rather than assumed.
Where services must cross the area, trenchless techniques and hand digging are the normal answer, and the detail matters as much for street trees affected by highway works and utility trenching as it does on a development site. Keeping the tree alive through the construction programme depends on what happens after the drawings are approved.
If facilitative pruning forms part of the scheme, specify it properly rather than leaving it to the contractor, because a pruning specification written properly is what the council will condition against.
Browse the PlanScope report types to see which arboricultural documents a scheme is likely to need before submission.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
Can a root protection area be made smaller?
Reshaping and reducing are separate questions. BS 5837:2012 allows the shape and position to be modified where evidence supports it, but the total area should be maintained. A proposal that reshapes and shrinks at the same time is asking for two things while evidencing one, and reviewers commonly reject it on that basis alone.
How is the root protection area calculated for a multi-stemmed tree?
Where stems fork below 1.5 metres, a combined diameter is derived. For two to five stems it is the square root of the sum of the squared diameters. For more than five stems it is the square root of the mean stem diameter squared multiplied by the number of stems. The two derivations give different radii.
Does an existing driveway exclude rooting?
Not reliably. A thin bitumen or block layer over uncompacted subsoil offers little barrier, and roots often run beneath paving where moisture collects. A full depth compacted sub-base under an adopted carriageway is a different matter. Evidence the construction with a section and photographs rather than assuming the surface excludes roots.
What investigation evidence do councils accept?
Trial trenching excavated by hand or with an air spade, recorded by depth and root diameter, carries the most weight. Ground-penetrating radar and resistivity surveys can map rooting without excavation, though interpretation in mixed urban fill is difficult and limitations should be stated. Soil descriptions from trial pits support arguments about rooting depth.
Is the twelve times diameter rule right for veteran trees?
Usually not. Ancient and veteran trees are treated in national policy as irreplaceable habitat, and the standard circle derived from stem diameter rarely reflects their rooting extent. A protection zone based on canopy extent plus a buffer is the more common approach, and specialist advice should be taken before any works are designed.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.