Tree protection fencing calculator for BS 5837 (UK)

The RPA. A circle centred on the stem, with a radius of 12 times the stem diameter at 1.5 m, capped at 15 m (BS 5837:2012 clause 4.6). Multi-stem trees use a combined diameter. The RPA calculator shows the formulas.

Panels stay outside the circle. A straight panel of width w that touches the RPA circle covers an angle of 2 × atan(w ÷ 2r). To go round an arc without cutting into the RPA you need n = ⌈(arc angle ÷ 2) ÷ atan(w ÷ 2r)⌉ panels. For a small tree, length ÷ panel width gives too few: a 150 mm stem has a 1.80 m radius, and four 3.5 m panels would cut inside it. Five are needed.

Feet, couplers and uprights. A closed loop needs one foot per panel. An open run needs one more foot than panels. We show two couplers at each joint, which is common practice. For a scaffold frame, the same formula with 3 m spans counts the spans between uprights. A mesh panel cannot bend round a corner, so each span gets its own panel (two per span if your panels are narrower than 3 m).

A summary in our own words. We do not quote the standard. We took the clause numbers from council and consultant documents that cite it; check them against your copy of BS 5837:2012, which you can buy from BSI.

• Purpose (clause 6.2.2.1). The barrier keeps construction activity out of the protected area. It suits how close and how heavy the work is, and it stays firm and unbroken.
• Timing. The barrier goes up before work, materials or machinery arrive on site, and stays until the development is finished. It is moved or taken down only with the project arboriculturist's agreement (and the council's, where needed).
• The construction exclusion zone. The RPA, fenced and/or covered with ground protection, is the construction exclusion zone. No construction activity takes place inside it.
• The default barrier (Figure 2). A braced scaffold framework, with uprights driven into the ground (the figure shows them about 3 m apart) and weldmesh panels fixed to it. Place poles and braces so they miss roots and underground services.
• An alternative (Figure 3). Where the site risk allows, welded mesh panels on rubber or concrete feet, joined with anti-tamper couplers and braced with stabiliser struts, whose bases are pinned into soft ground or set on block trays on hard ground.
• Signs. Weatherproof notices on the barrier mark the construction exclusion zone and say there is no access.
• A barrier set back inside the RPA (clause 6.2.3). Where the project arboriculturist agrees, the barrier can be set back inside the RPA, with the exposed part of the RPA covered by ground protection. Temporary ground protection carries the expected loads without compacting the soil.
• The RPA shape. The arboriculturist's assessment of the site, such as roads, buildings, slope and soil, can change the RPA's final shape, so a real fence often follows straight site lines, not a circle.

These are what UK suppliers and council advice notes describe, not requirements of the standard.

• Heras-type mesh panels are about 2 m high and about 3.5 m wide. Some suppliers list 3.4 m or 3.45 m, so enter your supplier's width.
• Each panel stands on a rubber or concrete foot, and panels are joined with couplers. One council advice note asks for at least two anti-tamper couplers at each joint, about 1 m apart.
• Drawings vary on stabiliser struts, from one on every panel to one on every third panel. Follow the method statement for the site.
• Ready-made scaffold-frame kits are sold, with posts no more than 3 m apart and driven at least 600 mm into the ground.
• Some councils accept a lighter fence on a low-risk, pedestrian-only site when agreed in writing. Ask the council and the project arboriculturist first.

• It is an estimate for ordering and quoting. It is not a tree protection plan, and it does not decide where the barrier goes.
• It fences each tree on its own. It does not merge the overlapping RPAs of a group or of neighbouring trees.
• The ends of an open run must close against a building, wall or another fence. Gates and returns are not counted.
• Struts, braces, ground pins, block trays, ground protection and signs are not counted.