Excluding construction activity from the root protection area

Calculation the root protection area

If works are necessary near to the tree’s root protection area (RPA), a construction exclusion zone (CEZ) must be identified from the base of the tree’s trunk as per  S5837:2012 Trees in relation to design, demolition and construction– Recommendations. The CEZ should be protected with substantial fencing (see Figure 42 below) and be excluded from the construction site. If works are necessary within the CEZ, the council’s Forestry team must be contacted at the design and implementation stages to identify specific measures to minimise damage. 

The RPA is normally calculated by measuring the trunk diameter at 1.5 metres above ground level, multiplying this distance by 12 and converting the result into a radius centred on the tree or in other words. If the diameter of a tree is 500mm, this amount is multiplied by 12 to give a distance of 6.0 metres (equivalent to 113m2). This 6.0 metres then becomes the radius of the RPA. 

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Diagram showing how to calculate the root protection area that will exclude construction activity

Figure 41: Calculating the root protection area

Protective fencing

On larger schemes where trees are to be retained within the site area, protective fencing should be erected, as specified in Figure 42 below, before any works begin, including ground preparation. The fencing must stay intact for the entirety of the construction process. It is essential that the protection zone is excluded from the construction site and all associated activities including excavations and changes of soil level. Areas where machinery, materials, chemicals and waste material are stored should also be excluded from the protection zone.

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Figure showing the Figure 42: default protection barrier BS5837 - This is the standard for all situations unless a justified variation is agreed with the council’s Forestry Team.

Figure 42: Default protection barrier BS5837 - This is the standard for all situations unless a justified variation is agreed with the council’s Forestry Team.

All weather notices should be attached to the barrier with words such as: “construction exclusion zone - no access”.

At this stage the potential effects of the works on privately-owned trees next to the site must also be considered.

Installation of services 

The installation of equipment within a tree’s potential rooting area and within its crown or potential growing space should be avoided. Site equipment such as road signs, street lights etc. should be sited to avoid damage to trees during installation and to avoid the need for regular pruning to maintain visibility.

Any service runs, highway equipment or street furniture within a tree’s protection area should be installed using hand-digging methods to ensure that all roots with a diameter of more than 25mm are retained. The recommendations contained in National Joint Utilities Group's (NJUG) Volume 4: Street Works UK Guidelines for the Planning, Installation and Maintenance of Utility Apparatus in Proximity to Trees should be adhered to.

When activities within the root protection area cannot be prohibited

Due to the linear nature of many highway maintenance operations (for example, kerbing) and the fact that most highway corridors are narrow, it is not always possible to exclude a tree’s root protection area from the working area. In such cases, please adopt the following specific measures to minimise the effects of the works:

  • Refer to NJUG Guidelines for the Planning, Installation and Maintenance of Utility Apparatus in Proximity to Trees.
  • Before starting the work, a survey team will identify the protection zone with marker paint or pegs. This will ensure that the construction gang know the precise area where special precautions are required to avoid or minimise damage.
  • The trunk of each tree must be protected with substantial hoarding, supported on scaffolding, to a height of at least two metres, to protect its bark from mechanical damage. This protection should be reusable so that as work progresses, it can be transferred from tree to tree.
  • If the work needs surfaces to be removed or excavated, this should be carried out as follows:
  • Footways – Slabs or paving should be manually removed and initial layers of concrete or tarmacadam should be removed using a handheld breaker. All sub-base material should then be removed using hand tools only.
  • Carriageways – Tree roots are not likely to penetrate the road base, base course or wearing course and may only be present in the sub-base material below the carriageway. This means the road base, base course and wearing course can be removed by machine. The compacted sub-base material below may initially be broken using a hand-held breaker but all material, once broken, should be excavated carefully using hand tools only.
  • All excavations using hand tools need to be carried out with great care to avoid damage to as many roots as possible. There must be close supervision during hand-digging operations and all site staff should understand what is required. All tree roots over 25mm in diameter should ideally be worked round and retained. If, however, it is necessary to remove these roots, a request for advice should be made forestry officer. 
  • Any root cutting should be done with a sharp handsaw or secateurs and the size of the wound should be kept to a minimum. Individual roots of less than 25mm may be severed, but mats of smaller roots (including fibrous roots) should be retained. Smaller roots can easily desiccate (dry out) and die when exposed, particularly in warm or windy conditions. These should be covered and protected with damp hessian until the excavation is back filled.
  • Where kerbs are being installed or refitted through a tree’s protection zone and roots with a diameter of more than 25mm obstruct the work, the forestry should be consulted officer before severing the roots. If the root cannot be severed without putting the stability of the tree in danger, the following should be considered:
    • reducing the section of kerb;
    • creating a gap in the kerb to allow for the root;
    • bridging the kerb over the root; or
    • constructing a kerbside build-out.
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