Root protection areas in a BS 5837 tree report

The root protection area in a BS 5837 tree report is a number before it is a shape. For a single stem tree it is calculated as a circle whose radius is twelve times the stem diameter measured at 1.5 m above ground, with the resulting area capped at 707 m². Everything else in the report, from the constraints plan to the fencing specification, follows from that figure.

Architects lose layouts to root protection areas that were never properly interrogated, and occasionally gain space by asking one question the arboriculturist was waiting to be asked.

How the root protection area is calculated

Twelve times the stem diameter gives the radius. A tree with a 400 mm stem has a 4.8 m radius and an area of about 72 m². A tree with an 800 mm stem has a 9.6 m radius and an area of about 290 m². The relationship is not linear, which is why one mature oak in the middle of a plot changes the scheme more than four semi-mature limes along a boundary.

Multi-stem trees use the combined stem diameter method in the standard's annex, with different treatment for trees with two to five stems and for those with more. Coppiced and multi-stemmed trees are frequently mismeasured, so check that the schedule records the number of stems and their individual diameters rather than a single figure.

The 707 m² cap applies to the largest trees, equivalent to a radius of 15 m. Beyond that point the calculation stops growing, which does not mean the roots do.

Modifying the shape, and when that is allowed

The default is a circle centred on the stem. The standard allows the shape to be modified where the likely root distribution justifies it, for example where a building, a road construction or a river has already prevented rooting on one side.

Three conditions apply to any modification. The total area has to be retained, so pushing the boundary in on one side means pushing it out on another. The modified shape has to remain adjacent to the stem and reflect a credible rooting pattern, rather than becoming a thin arm reaching into open ground. And the justification has to be written down, with the evidence for it, which usually means trial holes or exposed root observations rather than an assumption.

This is the question worth asking on a constrained site. Where a tree stands 2 m from an existing 1950s extension with a deep foundation, the roots on that side are already absent, and a properly justified offset can release usable space. Ask the arboriculturist to investigate rather than to redraw.

The tree constraints plan and the shade it maps

A Tree Survey produces the schedule, and the tree constraints plan turns it into a drawing an architect can design against: stem positions, canopy spread as surveyed, root protection areas, heights, and the quality categories that indicate which trees are worth designing around.

Shade is the part most often ignored. The plan should show shadow patterns cast by retained trees, because a layout that satisfies the root protection areas can still place a habitable room behind a mature beech to the south. Future growth matters too, since a semi-mature tree retained today will be considerably larger over the lifetime of the buildings, and the pressure to fell it will come from the occupants rather than from the developer.

Category U trees, those in such condition that they should be removed irrespective of the development, should be identified early and separately. Losing them is not a cost of the scheme. Presenting them as retained and then applying to fell them after permission is how an authority's tree officer loses confidence in a report. The way the plan constrains a layout is covered in more detail in the tree constraints plan and the layout it forces.

Incursions and no dig construction

Some encroachment into a root protection area can be acceptable. The standard contemplates encroachment of up to about a fifth of the area, where the tree is vigorous and healthy and the remainder of the area can be protected effectively, with compensatory area provided elsewhere next to the retained area.

Where an access road, a path or a parking bay has to cross the area, no dig construction is the technique that makes it possible:

• existing ground level retained, with no excavation and no stripping of topsoil within the area
• a geotextile or cellular confinement layer laid on the existing grade
• a load bearing granular layer built up above ground, with the finished level raised rather than the ground reduced
• a permeable surface, to keep water and gas exchange available to the soil below
• hand excavation only for any service that cannot be routed elsewhere, under arboricultural supervision

Services are the recurring failure. A drainage run, a gas main or a duct set out later by a subcontractor cuts straight through an area the drawings protected, so the service strategy has to be resolved at the same time as the layout, not at technical design stage.

Foundations near trees raise a separate question that has nothing to do with root protection: clay heave and shrinkage, which drives foundation depth under the guidance house builders work to. The two constraints are often confused. One protects the tree from the building, the other protects the building from the tree.

Protective fencing and ground protection

The default specification is welded mesh panels mounted on a scaffold framework, at the height and construction shown in the standard, forming a construction exclusion zone. Chestnut paling and plastic mesh on stakes are not the default, and a tree officer will say so.

Where access is unavoidable, ground protection replaces fencing for that part of the area: scaffold boards on a scaffold frame, or proprietary panels laid over a geotextile and a mulch layer, designed for the loading actually expected rather than for pedestrians when a telehandler will use it.

Two details make the difference between a specification and a site that complies. Fencing installed before any materials, plant or spoil arrive on site, with a photograph dated and kept. And signage on the fence stating that it is a construction exclusion zone and must not be moved without written arboricultural approval.

Sequencing tree work with construction

The order matters, and it is short enough to remember:

1. Obtain consent for any tree works needing it, whether a Tree Preservation Order application or a section 211 notice for a tree in a conservation area, which requires six weeks' notice.
2. Carry out felling and pruning outside the bird nesting season, or after a check by a suitably qualified person.
3. Install fencing and ground protection, and have it inspected and signed off before work starts.
4. Hold a pre-start briefing with the contractor, including the service routing and the hand digging requirements.
5. Carry out monitoring visits at the frequency stated in the method statement, with written reports to the authority.
6. Remove protection only at the stage the method statement allows, which is usually after hard landscaping.

What the arboricultural method statement adds

The report identifies the constraint. The method statement says how the constraint will be respected during construction, task by task: demolition, site clearance, fencing, access construction, foundations, services, level changes, storage areas, concrete washout locations, and landscaping.

That document, with the tree protection plan, is what a condition normally requires before development starts. Councils usually expect a suitably qualified author for arboricultural work, and professional review is strongly advised.

Illustrative example. A plot with a category B oak of 620 mm stem diameter, giving a root protection area of roughly 174 m² at a radius of 7.4 m. The first layout put a house corner 5 m from the stem, taking nearly a third of the area with a trench foundation. The revised layout moved the house 1.5 m, used a pile and beam foundation on the nearest elevation, took the driveway across the outer edge of the area on a no dig build-up, and provided compensatory area in the rear garden. Same number of units, permission granted with tree protection conditions.

Where construction access and routing are the binding constraint, the arboricultural requirements have to agree with the site logistics, which is the point where the routing section of a construction management plan and the tree protection plan need to be drawn by people who have spoken to each other. A Construction Management Plan that contradicts the method statement is a condition discharge refused.

To see which reports a site with retained trees is likely to need, enter the postcode on PlanScope.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.