Protection of Trees on Development Sites
Introduction
Existing trees and shrubs on a development site should not be ignored or regarded as obstacles. They can be valuable assets, allowing the new building to blend into its surroundings, giving maturity and continuity to the site while new planting becomes established. If they are carefully protected during the building process they will enhance the site. If they are neglected or damaged they may blight it or cause problems in the future.
The purpose of this guide is to describe the survey and planning required as part of the design process, explore some of the problems that can occur on site, and describe the precautions that should be taken.
The Law Relating to Trees
Check with your Local District Council Planning Office before starting on any design work. Is the site in a Conservation Areas? Are any trees covered by Tree Preservation Orders? Are any conditions attached to any current planning consent? Are there any restrictive covenants?
It is a criminal offence to cut down, uproot or wilfully damage a protected tree. Damage includes the cutting of branches and roots. The latter are usually closer to the surface, and spread considerably further, than most people think. The fine on summary conviction may be up to £20,000, or an unlimited fine on indictment. Local Authority Planning teams are always happy to give advice on any proposals relating to protected trees.
National Guidance
The British Standard BS. 5837:2005: Trees in Relation to Construction gives recommendations and guidance on the principles to be applied to achieve a satisfactory juxtaposition of trees, including shrubs, hedges and hedgerows, with structures. It follows, in sequence, the stages of planning and implementing the provisions which are essential to allow development to be integrated with trees. The standard recognizes that there can be problems of development close to existing trees which are to be retained, and of planting trees close to existing structures. It is written to assist those concerned with trees in relation to construction to form balanced judgements and provides guidance on how to decide which trees are appropriate for retention, on the means of protecting these trees during development, including demolition and construction work, and on the means of incorporating trees into the developed landscape. Some key elements in this process are outlined below.Site Survey, Analysis and Design
Before design work begins, a detailed survey must be carried out. The survey should show the exact location of all trees, hedges and shrubs. Details recorded in the survey should include species, height and spread, girth, physical condition and existing ground levels.
A Tree Constraints Plan (TCP), based on the tree survey should be prepared as a design tool. This will show the growth potential of individual trees, plus any above and below ground constraints. The plan should also show root protection areas (RPAs). These are equivalent in size to a circle with a radius 12 times the stem diameter of those single stem trees that are scheduled for retention. They should extend at least as far as the crown spread. As a rule of thumb, an estimate of the extent of significant root spread can be made by calculating half the height of the tree in question.
The TCP should inform the design process. The aim should be to retain as much healthy mature vegetation as possible, and in such a way as to accommodate its future growth and management. Once the layout proposals have been finalized, a Tree Protection Plan (TPP) should be produced which shows which vegetation is scheduled for retention and removal, and the precise location of temporary protective fencing - within which digging, tipping, site storage or vehicles will not be permitted.
You may need to engage a landscape architect or arboriculturalist to assist in this process. Local Authority Planning teams can also offer advice. Your detailed survey, TCP and TPP should accompany your Outline Planning Application.
Always consider the proximity of existing trees to any proposed development with the user in mind. The temptation to site dwellings too near to trees scheduled to be retained should be resisted. Future applications to fell TPO protected trees that shade the garden, drop untidy leaves or create concern on stormy nights, may be rejected by the Planning Authority. A rule of thumb suggests that dwellings should be sited no nearer than a distance equal to two thirds of the predicted mature height of the tree; on the assumption that most structural damage is caused by lower major limbs and trunk. However the rooting characteristics of trees will probably dictate that a greater distance should be maintained. Roots extend well beyond the canopy. This has serious implications for both trees and development.
Design of Foundations, Drains and Other Services
Building inspectors will expect that the proximity of trees and their existing and future rooting zones will be taken into account in the design of foundations, and in the provision of root barriers for the protection of drains. Similarly the 'shrinkability' of any clay soils should be taken into account as a design factor. This will avoid the unnecessary sacrifice of valuable trees and/or underpinning of foundations and the replacement of drains in years to come. Services should be grouped and restricted to passing existing trees on one side only. Consider allowing for ducting with draw strings to accommodate future expansion without more disturbance.Site levels and Changes to the Water Table
Most tree roots are within 600mm of ground level. Any change in ground levels can kill trees by cutting or burying roots, reducing oxygen levels in the soils, or altering the level of the water table and 'drowning' roots. Mature trees are particularly susceptible. A tree may take several years to succumb. Covering the roots with an impervious driveway or path upsets the root systems and can have a similar fatal effect.Compaction
Trees need well drained, aerated top soil within which to develop healthy roots. Compaction caused by heavy machinery cuts off oxygen and causes a build up of toxic carbon dioxide. This is one of the greatest dangers to a tree's continued well-being.Exposure
The removal of individual trees within a group can result in exposure to direct sunlight and cause the scorching of foliage or bark split. Wind-throw and root destabilisation is a more common result. Reducing the canopy of the tree, to an agreed schedule, can sometimes improve its long-term prospects.Pruning and Felling
Having obtained Detailed Planning Permission, and before building work starts, a qualified tree surgeon, holding adequate professional indemnity insurance, should be employed to carry out the agreed work. The extent of this work must be strictly in accordance with the terms of the permission. The Local Authority planning team should be contacted so that arrangements can be made to agree the detailed extent of any tree surgery on site.Protective Fencing
Before any plant or machinery is allowed on site, or any demolition is started, trees and hedges to be retained should be protected behind stout fencing, at least 1.5m high to an approved detail. This should be erected in accordance with the TPP. No site traffic, site cabins, temporary latrines or their drains, concrete mixing, stored building materials, chemicals including diesel, oil, petrol or creosote should be allowed within the protected zone. Burning of paint tins, washing out and draining of engine oil must be avoided in the vicinity. Notices and cables should not be attached to trees, nor should a tree be used as an anchorage point for equipment. Where vehicle tracking is agreed as being unavoidable, roots should be bridged over using heavy timbers. Any damage to the fencing should be quickly repaired.Trench Excavations for Services
Where the exact route of trenching work within the curtilage of root systems containing roots over 50mm in diameter has been agreed, excavations should be by hand digging. Tables 1 - 3 show minimum distances between tree and excavation. Where possible, roots should be left intact and protected against desiccation by immediate wrapping or sheeting.
Backfilling should be carefully undertaken as soon as possible to avoid drying out or flooding. Sharp sand should be carefully compacted around the roots and trenches backfilled by hand with topsoil which had been previously separately excavated and heaped. This work should be undertaken during the dormant period and not during wet weather. Damaged roots or those requiring pruning should be cleanly cut and treated with a fungicidal sealant and surrounded by top soil enriched with a phosphate fertilizer (BS. 3998:1989). Roots left exposed in trenches to be filled with concrete should be protected with several layers of hessian.
How to work out Tree Size Class
Convert girth, height and spread into points using the following tables:Stem Circumference | |
| (cm) | (points) |
| 0 - 30 | 1 |
| 31 - 60 | 2 |
| 61 - 100 | 3 |
| 101 - 160 | 4 |
| 161 - 220 | 5 |
| 221 - 300 | 6 |
| over 300 | 7 |
Tree Height | |
| (m) | (points) |
| 0 - 5 | 1 |
| 5 - 7 | 2 |
| 7 - 10 | 3 |
| 10 - 14 | 4 |
| 14 - 17 | 5 |
| 17 - 21 | 6 |
| over 21 | 7 |
Crown Diameter | |
| (m) | (points) |
| 0 - 4 | 1 |
| 4 - 7 | 2 |
| 7 - 10 | 3 |
| 10 - 13 | 4 |
| 13 - 16 | 5 |
| 16 - 19 | 6 |
| over 19 | 7 |
Add up the points for girth, height and spread to give the Tree Size Class
| Total points | Tree Size Classes |
| 3 - 6 | 1 |
| 7 - 9 | 2 |
| 10 - 14 | 3 |
| 15 - 17 | 4 |
| 18 - 20 | 5 |
| 21 + | 6 |
Minimum distances between tree and excavations for services (one side of tree only)
| Tree Size Classes | Oak and Beech | Other species |
| 1 | 1.2m | 1.2m |
| 2 | 2.5m | 2.0m |
| 3 | 4.0m | 3.0m |
| 4 | 5.0m | 4.0m |
| 5 | 6.0m | 5.0m |
| 6 | 7.0m | 6.0m |
Tables developed by Norwich City Council and reproduced with their permission.
Further Information
- BS. 5837:2005 Trees in Relation to Construction can be obtained from British Standards Online
- For guidance on planting new trees on development sites see Trees and Buildings.