2 November 2011

RPAs - Planning and Building Near Trees with Root Protection Areas

If you are planning to build near to existing trees, you will need to protect their roots throughout construction and occupation of the building. This is simply done by designating a Root Protection Area around the tree into which the new building does not intrude and contractors do not enter during construction. This article introduces the rules of thumb behind Root Protection Areas (RPAs), how they can be estimated at an early stage.

Most trees’ roots spread across the surface like in the picture opposite, with all the most important roots in a zone between 250mm and 500mm below the surface. Even shallow excavations can damage the root system and ultimately kill the tree. Damage might not be immediately obvious as tree dies very slowly, sometimes occurring up to 15 years after damage from construction.

Oxygen is just as important to root systems as water, so concreting the ground above a root system can have the same effect as cutting the roots themselves. If you really need a hard landscape within an RPA, you can specify permeable paving, laid with a no-dig construction.

Root Protection Areas
RPAs are often enforced by local planning authorities as planning conditions attached to permissions. They are used to protect TPO trees (subject to a Tree Preservation Order) and trees in conservation areas. The RPA is calculated as 12 times the diameter of the tree trunk, 1.5m off the ground. So, if a tree has a trunk that is 500mm in diameter, measured 1.5m above the ground, the RPA will have a radius of 6m (500mm x 12 = 6,000mm). Note that the RPA is a radius, relative to the tree trunk but the calculation is based in the trunk diameter.

RPA radius = 12 x (trunk diameter @1.5m above ground)

Beware the survey!
A large diameter tree trunk can quickly eat up a lot of site so it is important to know both where the existing trees are and how thick their trunks are. A full topographic survey should pick up this information. If, however, you’re proposal is getting particularly close to a tree's probable RPA then it’s a good idea to double check the survey information yourself. If the diameter has not been measured 1.5m off the ground, or the stem location is slightly out, you could gain planning permission, only to find it impossible to comply with the planning conditions and RPAs. This is a headache that no-one wants.

Using an Arboriculturalist
If you are in any doubt, it’s best to ask a qualified arboriculturalist. There are a number of variations and loopholes where RPAs and tree protection are concerned so speaking to the right person early on can pay dividends later. Either way, you are likely to need an arboriculturalist to prepare a tree report for your planning application or working method statement to discharge your tree-related planning conditions.

Finally, if you are looking for an arboriculturalist for a project in the South East of England, then we’d be happy to suggest a couple of names to approach.  Just email us at studio425@london.com or you can tweet John, @john_goes_boom