A root protection area, usually shortened to RPA, is a calculated zone around the stem of a tree in which the roots, and the soil they grow through, must be protected from damage. The idea comes from BS5837, the British Standard that deals with trees on and around construction sites, and in practice it works as a line on a drawing: inside that line, nothing should be built, dug, stored or driven over without proper justification and precautions. If you are planning a development near trees, the RPAs on your site will shape where buildings, drives and services can sensibly go, so it pays to understand what the zone is and why it is drawn the way it is.
How the root protection area is worked out
The RPA is calculated from the size of the tree, not guessed at. Our surveyors measure the diameter of the stem at a set height above ground, and the standard converts that measurement into an area of ground that should be treated as off limits. In broad terms, the bigger the stem, the bigger the zone, and a mature tree with a substantial trunk can command a surprisingly large circle of protected ground.
By default the RPA is drawn as a circle centred on the stem, but it does not have to stay that way. Where there is good evidence that roots have been constrained on one side, for example by an existing building, a wall or a road, the shape can be adjusted so that the protected area reflects where the roots actually are. Any change of shape has to be justified by an arboriculturist, and the overall area stays the same, so reshaping is never a way of shrinking the zone.
Why roots need this much room
Tree roots do not mirror the canopy and they do not plunge straight down. Most of the fine roots that keep a tree alive sit in the upper layers of soil, spreading well beyond the branch line, and they depend on that soil staying uncompacted, aerated and able to take in water.
That is why the activities builders take for granted are exactly the ones the RPA guards against:
- Paving and hard surfacing, which compact the ground and seal it against air and water.
- Changes of level, because raising the ground smothers roots and lowering it removes them altogether.
- Trenches for drains, ducts and other services, which sever roots outright.
- Site traffic, storage and concrete washout, which compact soil just as effectively as a finished surface.
The damage these cause is usually invisible at first. A tree can take years to decline after its roots are harmed, long after the site is finished and handed over, which is exactly why planning officers want the protection settled on paper before any work starts.
What the RPA means for your layout
On a live project, the RPA drives decisions. Wherever possible, buildings, extensions, driveways and service runs are kept outside it, and the earlier the RPAs are known, the easier that is to achieve. A layout designed before anyone has measured the trees often needs reworking; a layout designed around accurate RPAs rarely does.
Work inside an RPA is not automatically forbidden, but it has to be justified and carefully specified. Depending on the situation, that can mean no-dig construction for paths and drives, hand digging instead of machine excavation, routing services around the zone or boring beneath it, and protective fencing to keep plant and materials off the root zone during the build. These measures are set out in an arboricultural method statement and tree protection plan, which the council will often require as a condition of planning permission.
Where the RPA fits in a BS5837 survey
An RPA is not a standalone product. It is calculated for every relevant tree as part of a BS5837 tree survey, which records each tree’s species, size, condition and quality, then plots the RPAs on a constraints plan so that designers can see exactly what they are working around. If the trees on or near your site are covered by a Tree Preservation Order or stand in a conservation area, that adds a separate layer of control: the council’s register is the definitive record, and our guide to checking whether a tree is protected explains how to find out where you stand.
Our in-house team carries out these surveys across Preston, Lancashire and the North West. The report is independent and stands on its own: we also carry out tree works, but the survey never depends on them, and you are free to take the report to any contractor you choose. We quote within one working day, survey visits are booked within days, and the report follows days after the visit, not weeks.
If you are not sure whether your project needs a BS5837 survey, or what the council will expect to see, use the free scope check form on our tree surveys page or call 01772 369694. We will tell you what you need, and just as readily what you do not.
