Tree Root Protection Areas (RPAs) in Construction

Root Protection Areas (RPAs) are a means of ensuring that the root systems of trees are not damaged or disturbed by construction or other activities. Prior to construction work commencing, RPAs should be established around each tree that is to be retained and protected.

Ground disturbance must be avoided within RPAs. Construction activity, access roads, and working platforms should be routed around the RPAs. Where this is not possible, precautions must be taken to prevent compaction of the soil and damage to the tree root system.

This is a broad topic with plenty of ground to cover. Use the links below to find the information you're looking for:

• What is a root protection area? 
• Who needs to use root protection areas?
• How can construction projects affect trees?
• How to calculate a root protection area
• Can you build on a root protection area?
• The British Standard for tree root protection: BS 5837 (2012)
• Protecting tree roots from compaction
• The use of geogrids in root protection areas
• Mechanical Stabilised Layer for root protection

 

Ready to discuss how Tensar solutions can protect tree root systems for your project? Speak to our team today.

What is a root protection area?

A root protection area is designed to safeguard a tree’s root system. Within this area, protection of the root system and soil is a priority. The area is calculated to contain sufficient roots to ensure the viability of the tree. 

Under what circumstances are root protection areas a requirement?

Root protection areas (RPA) should be calculated during the planning stages of a project before any construction work begins to avoid loss or damage to trees within proximity of the construction site. A tree survey should be conducted by a qualified Arboriculturist or Tree Consultant, in accordance with BS5837:2012.

Trees to be potentially retained or removed will be identified from the tree survey. Input from the planning authorities, the client and design team will influence what trees are to be retained. Tree protection requirements will then be finalised, and RPAs established before detailed design commences any construction activity takes place.

In the UK, planning decisions must take all existing trees and woodland into account. In addition to this, Local Authorities can establish a Tree Preservation Order (TPO) to protect specific trees, groups of trees or woodlands. TPOs prohibit the removal, or wilful destruction of trees without local planning authority consent. TPOs are enforceable by law.

In many cases, clients and architects will also wish to retain established trees wherever possible. Trees identified for retention will need to be protected by the establishment of RPAs.

Construction activities that may require establishment of RPAs will be:

• Major landscaping
• Utility installations – both above and below ground
• Trenching
• Building work
• Below ground construction
• New roads and hardstanding’s
• Temporary access road
• Demolition work

 

How can construction projects affect trees?

For a tree to survive and thrive its root system must be able to absorb sufficient water to sustain the foliage. That could be over 1,000 litres per day for a large tree. The network of roots must also be capable of anchoring the tree to withstand massive wind forces. If the root system is damaged or restricted the tree may become unviable. Construction activity or development taking place near a tree can therefore put that tree in danger.

It is a common misconception that tree roots grow vertically down to anchor the tree and draw water and nutrients from deep below the surface. That is not the case. Tree roots are shallow growing, almost parallel to the ground, mostly within a zone of soil less than 1.0m deep. The roots radiate outward from the trunk and extend beyond the diameter of foliage to a distance equal to the height of the tree. Beyond the first few metres the roots subdivide into multiple roots less than 2-3cm in diameter.

Note: Illustration not to scale

Excavation - Any excavation close to a tree, even topsoil stripping, will potentially sever roots and may impact water supply to the tree, immediately affecting the foliage. While minor root loss may not affect the tree, greater loss will result in dieback. If the excavation is over 1.0m deep, the anchorage and stability of the tree may be affected. 

Compaction - Soil compaction can arise from construction traffic operating directly over the ground or as a result of surcharge from new construction. Tree roots need oxygen to survive. Without it, the roots cannot function. If starved of oxygen for several days the roots will begin to die back. The supply of oxygen to the root system relies on pores between soil particles. Compaction of the soil from construction activity will reduce porosity of the soil and restrict the oxygen supply. Any activity causing soil compaction within an RPA must therefore be avoided or mitigated.

Permeability - Any impermeable construction within an RPA will restrict water replenishing the soil. As we have learned from above, trees consume a large volume of water each day. Any restriction in water getting to the root system will quickly affect the tree. Any paved areas within an RPA should therefore be permeable.

How to calculate a root protection area

Guidance for calculation of a RPA is provided in British Standard BS 5837:2012.

In most instances the RPA will be circular centred on the base of the stem. Exceptions to this will occur where existing structures or other factors have restricted root growth in one or more directions. In these cases, expert arboreal advice should be sought to properly define the perimeter of an RPA.

The radius of the RPA will be 12 times the stem diameter. For a single stem tree that is measured 1.5m above ground level, or at the narrowest point below a fork or swelling that occurs within the first 1.5m height
It is a little more complicated for trees with multiple stems, where a notional combined stem diameter is used. 

For trees with two to five stems the combined stem diameter is:

√ ((stem diameter 1) ^₂ + Stem diameter 2) ^₂   +....(stem diameter 5) ^₂ )     

For trees with more than five stems the combined stem diameter is:

√ ((mean stem diameter) ^₂ x number of stems))    

While the above method adequately covers most trees, arboriculturists suggest that for ancient trees, the RPA should be larger than that calculated using the BS 5837:2012 approach. The Woodland Trust and the Ancient Tree Forum support the standing guidance from the Government, developed by Natural England and the Forestry Commission, which suggests introducing a buffer zone to provide an extra layer of protection for ancient woodland. The guidance, provided for the assessing of planning applications, proposes that an RPA and buffer zone for ancient trees should have a radius of 15 times the stem diameter or 5m beyond the crown (foliage growth), whichever is greater. The buffer zone should consist of a mix of scrub, grassland, or other semi-natural habitat, and be part of the green infrastructure of the area.

Can you build on a root protection area?

There will be competing needs for any proposed development. The retention of trees is just one constraint that must be considered throughout the planning and design process. All those involved with design and construction of a project need to be made aware of any tree retention requirements. An arboriculturist should be retained throughout the project to review all drawings and assess and advise on any impacts on tree retention.

While it may be desirable that no construction activity takes place within RPAs, and this should remain the default position, this may be impractical. BS 5837:2012 provides recommendations for how such incursions may be accommodated and managed.

Where it has been agreed that building on an RPA is necessary, an assessment must be made of the probable impacts from construction and future building operations on the tree, and of the potential impact of the tree on the building, with full consideration of future growth and maintenance requirements. The project arboriculturist must prepare an impact statement and where necessary propose mitigation measures. These might include the use of piled or cantilevered foundations.

The British Standard for tree root protection; BS 5837:2012 

British Standard BS 5837:2012 Trees in relation to design demolition and construction, sets out guidance and recommendations. It is not a specification document. Users claiming compliance with the standard can deviate from its recommendations, provided that any such action can be justified.

The standard sets a process for design and construction that places tree care at its heart. It is applicable whether or not planning permission is required.

The standard proposes the use of root protection areas (RPAs) to safeguard trees identified for retention on construction projects, demolition sites or utilities installations. Methods for calculating the size of root protection areas are given, alongside advice on barriers and ground protection methods to secure RPAs.

Design recommendations for permanent hard surfacing, engineered foundations, demolition, and utilities incursions, within RPAs are also provided.

How do you protect tree roots from soil compaction?

To avoid soil compaction and protect tree root systems, we need to keep vehicles, construction machinery and heavy loads away from the area to be protected. When this is not practical, some means of distributing load to reduce ground pressure and avoid surface deformation should be employed.

As we learned above, tree roots need oxygen to survive, and that requires soil porosity. Any compaction of the soil arising from construction activities or surcharge loading must be avoided within RPAs.

Where it has been agreed that construction activity needs to take place within an RPA, some form of ground protection must be put in place to prevent soil compaction. There are a range of approaches available.

For temporary storage, scaffold platforms can be constructed that bridge over the RPA. For temporary access, metal, concrete or plywood plates can be used to spread the load. Geocells can be used to create a stiff platform. The use of geogrids can be considered, provided they are designed to create mechanically stabilised layers capable of distributing load without deformation. More on this later. 

For permanent hard surfaced areas, where these are unavoidable, site-specific arboreal advice is essential. The design should require no excavation or soil removal and loading needs to be distributed to avoid localised compaction. Drainage, designed to avoid rood damage within the RPA, should be considered where waterlogging may occur as a result of the construction. Appropriate and effective subbase support options may be used within the RPA to distribute loads, reducing load concentration that would result in localised soil compaction.

The use of geogrids in root protection areas (RPAs)

When it is absolutely necessary to construct permanent access roads, cycle paths, or hard surfacing over an RPA, it is essential that soil compaction is minimised. The design needs to incorporate a stiff base capable of distributing load and preventing localised deformation or soil surface rutting.

One approach suggested in BS 5837:2012 is to use geocell systems to improve the subbase by providing confinement. This is a perfectly valid approach. The use of ‘two-dimensional load suspension systems’ is not recommended. Here the standard is referring to geosynthetic products such as geogrids and geotextiles that provide a reinforcement function in a ‘load suspension’ mechanism. Let’s look at this further.

When a reinforcement product is incorporated below a subbase layer, it will need to deform into a rutted profile to provide ‘load suspension.’ This is referred to as a tensioned membrane mechanism. This is completely unsuitable for tree root protection as the soil will be compacted below the rutted profile. There is a completely different mechanism applicable to geogrids that are designed to provide a stabilisation function, where ground surface deformation can be negligible and soil compaction prevented. 

Note: Illustration not to scale

Mechanical Stabilised Layer for root protection

Stabilisation geogrids, such as Tensar InterAx, are designed to allow subbase aggregate to fully interlock within the apertures of the geogrid and to provide confinement to the aggregate particles, preventing them from moving. This creates a mechanically stabilised layer that is much stiffer and resistant to rutting compared to subbase alone. 

Tensar has developed design methods (Lees-Hans Method) for mechanically stabilised layers, validated by extensive laboratory and field testing, which enables engineers to decide on the magnitude of acceptable ground surface deformation.  For root protection areas, engineers can specify a subgrade deformation less than 6mm using Tensar+ design software - a free cloud based software that allows engineers to design mechanically stabilised layers for access roads, cycle paths and permanent hard standings. This design approach, with control over the allowable ground deformation, was not available at the time of the drafting of BS 5837:2012. It does offer engineers an effective economical solution for constructing access over RPAs.

BS 5837:2012 does suggest the incorporation of a geotextile at the base of any construction to prevent contamination of the rooting area below. A suitable separation geotextile can be placed over the ground surface before the stabilisation geogrid is laid out. Or alternatively, Tensar InterAx is available as a geocomposite version, with a separator geotextile bonded to the underside of the geogrid. This simplifies installation and enhances root protection.

Installation – Construction of mechanically stabilised layers within RPAs must be undertaken with protection of the root system as the highest priority. 

• Removal of surface vegetation including turf can be removed using hand tools.
• No topsoil should be excavated or removed.
• The stabilising geogrid or geocomposite should be laid directly over the soil.
• Edge boards may be used to contain the subbase and provide edge restraint. These will be pegged in place with wooden stakes.
• The subbase material should be placed by hand or by lightweight tracked plant. The subbase should be cascaded forward onto the geogrid with the tracked plant operating only on top of the placed subbase. The initial layer thickness will be normally 150mm.
• Tracking over by the lightweight plant will facilitate the aggregate interlock with the geogrid to establish the stabilised layer without compacting the soil below.
• A final roll with a small non-vibrating roller will firm the surface.