Whilst I have been constructing gardens, including patios and driveways for over fifty years, mainly in the South East of England, during the past ten years or more, I have been dealing with failed projects that have become the source of Court disputes and claims. I act as a consultant and expert witness in such cases, recommended by various legal practices and hard landscape materials suppliers.
Although a hard landscaper by trade, working with paving and walling materials, both natural and man-made, I have a strong awareness of horticulture and ‘soft’ landscaping elements including soils and base materials. I have worked in many different counties in the UK, and across Europe, building on a wide variety of soils and under different conditions.
I was the ‘hands-on’ practical demonstrating member of the Traditional Paving Development Group based at the University of The West of England at Bristol (Frenchay Campus) during the 90s, working with a team of specialists, including Michael Heap of CED, Mike Astin of Marshalls and Richard Guise of the UWE, raising awareness of the value of traditional paving materials when gauged over a long period of time and compared with cheaper, short term solutions, and introducing a new generation of specifiers into the uses of natural stone.
I have worked on many older properties, including various ‘stately homes’ requiring larger scale paving solutions, and in many towns and cities as a paving contractor, specifying and laying a wide range of natural and man-made paving materials including sandstone, ironstone, Portland stone, bricks/tiles and smaller element products, including Guildford (where I built several ‘roads’ and pavements in a range of materials for the Guildford Borough Council), Alton (granite setts in fan patterns), Farnborough (sawn York-stone) Petworth (reclaimed sandstone setts) and others, both new and reclaimed.
Training ‘sessions’ developed from the Traditional Paving Development Group where I took to conducting practical demonstration works showing laying techniques in towns and cities across England including Nottingham, Plymouth and Sheffield at the request of local authorities and landscape architects.
All of these works included specifying the whole scheme, including base and laying beds, laying techniques and ‘finishes’ suited to each site and application. Working with Michael Heap in particular was very enlightening, as he was instrumental in developing the British Standards for Paving (BS7533) in those days, and was similarly involved in the latest BS7533 2021, as a leading member of The Stone Federation.
It is against this background, that I am able to undertake expert witness commissions for Court cases, the marriage of both hard landscaping and horticulture enabling me to be aware of the environmental elements of hard landscaping projects, and the need for care and attention when surveying, planning and specifying landscape projects, looking at the wider picture, not simply at the works involved in actually laying hard paving materials.
Tree root awareness.
Simply being aware of the needs of individual sites, not only the practical requirements of the project in hand – heavyweight traffic, light traffic, pedestrian use/likely footfall, falls and levels etc, as demanded by the client, but also the surrounding areas and soil conditions is an essential part of a contract.
A major cause of failures in paving projects, and driveways in particular, is the effects of tree roots, primarily due to a lack of care and attention by the designer/specifier when conducting a site survey prior to planning the project.
I have long been a keen advocate of the works of D F Cutler and I B K Richardson and their book Tree Roots and Buildings 1981 (ISBN 0-582-03410-8) being my principal guide when assessing sites for potential problems with trees and their roots in particular. Whilst not directly quoting from their book, I have taken their findings into account at every stage, although research is continually being updated as more examples and cases are added to the knowledge on this subject all the time.
I would also draw attention to the changes in building density since the book was first published over forty years ago. There are thousands more ‘new build’ sites and millions more houses constructed since 1981, and the first matter I take into consideration when undertaking either producing specification or a Court survey, is to ascertain the history of a site. This is much easier done today, with the advent of Google Earth providing us with accurate images of sites and surrounding terrain, nowadays allowing us to follow a site progress over a period of years, usually at two-year intervals.
Having identified the location using a post code, it is a simple matter to print off an image of the site. Go back through time to the previous date and print off a second image, and so on back as far as you can. Most sites will have images from 2010, i.e. seven or eight pictures. Take careful note of the amount of tree cover there is today when compared with previous photos. Any substantial reduction in tree cover will indicate a strong likelihood that drainage issues may be a problem on the site, as the ‘missing’ trees will no longer be taking up water from the ground, especially on clay ground (See Landscape Library article ‘Hard Landscaping on Clay Soil’)
This exercise is particularly valuable on ‘new-build’ sites, which may have been completely stripped of trees and vegetation. In these circumstances, it is advisable to undertake or commission a LIDAR (Light Detection and Ranging) survey of the land, which will show you how ground levels may have changed, and allow you to ascertain how water movements across the site may have altered by the development. This information may have an affect on the site survey and specification for a project involving capping off a large area of a site with hard landscaping materials, which will mean changes in the way the ground reacts to rainwater and ground water.
Note too, that capping off an area may create shrinkage issues under the paving, and potential ‘heave’ where the water is discharged over non-paved areas, especially on clay ground.
It is crucial to the success of a paving project that potential problems with tree roots are taken into consideration. Too many site surveys only consider the immediate measuring of the area to be paved for estimating purposes, without considering the surrounding area which may affected by the works. The proximity of trees, their species and sizes are fundamental factors that are so often missing from site plans. Their current size should be noted, along with their location and distances recorded at the time of the survey. Young trees may not be considered a problem at the time, but a few years in the future, they may cause problems, and their ultimate size should be taken into account.
Site orientation is another vital factor, as tree roots may favour one particular direction i.e. may have more potential roots on one side or another. Factors to be considered are shade and proximity to other trees (competition for water) and prevailing winds which may incline roots to grow towards the wind pressure to stablise the tree. Trees growing on a bank will be more likely to have ‘anchor’ roots growing outwards on the upper level, to support the tree against slippage and wind pressure.
It is important to look carefully at each potentially problematic tree, identify the specie to establish the likely growing requirements and nature of the tree, their proximity to the site and potential to cause underground damage to the area to be paved. Equally important is to establish the current surface of the ground to be covered. For example – if the existing site is a shingle driveway, whereby all rainwater currently is passing through the surface, and the plan is to cover the area with non-permeable paving, it is important to consider the effect such capping off is going to have, not only on the ground itself (leading to potential ground movement/shrinkage/heave as the site dries out or becomes saturated) but also the effect on any tree roots by removing the source of water to the tree.
For the purposes of this article, I have included various notes regarding tree species and their various habits and requirements in respect of landscaping projects. The following list is by no means comprehensive, and merely highlights the essential need to take their needs into consideration when planning a hard surface. Comments regarding distance include damage to drains and foundations as well as paving schemes.
Tree varieties and recorded problems.
Ash (Fraxinus excelsior) Maximum recorded tree-to -damage distance – 21 metres. Minimum 6 metres, with most damage on shrinkable clay.
Beech (Fagus sylvatica) Maximum 15 metres, minimum 6 metres on shrinkable clay.
Birch (Betula pendula) Maximum 10 metres, minimum 4 metres on shrinkable clay.
Cherry (Prunus varieties) Maximum 10 metres, minimum 3 metres on shrinkable clay
Hawthorn (Cratageus varieties) Maximum 12 metres, minimum 5 metres on shrinkable clay.
Horse Chestnut (Aesculus species) Maximum 23m, minimum 8 metres on shrinkable clay.
Lime (Tilia species) Maximum 20 metres, minimum 6 metres on shrinkable clay.
Oak (Quercus species) Maximum 30 metres, minimum 9 metres over shrinkable clay.
Plane (Platanus species) Maximum 15 metres, minimum 5 metres over shrinkable clay.
Poplar (Populus species) Maximum 30 metres, minimum 11 metres over shrinkable clay.
Rowan (Sorbus varieties) Maximum 11 metres, minimum 5 metres over shrinkable clay.
Sycamore (Acer species) Maximum 20 metres, minimum 6 metres over shrinkable clay.
Willow (Salix species) Maximum 40 metres, minimum 5 metres over shrinkable clay.
NOTE: These figures are shown as maximum and minimums as recorded by Cutler and Richardson, and do not take into account various factors. They are simply research figures, based on their original brief. References to shrinkable clays is made in recognition of the brief undertaken by Cutler & Richardson, where the majority of their research was made.
Attention should be given to the tree canopy – how far out do the branches extend? As a general guide, the canopy will indicate the spread of tree roots underground. None of the above measurements show any indication of the size and age of the trees examined, although the maximum/minimum may be taken as a guide.
I would aver that another factor should be borne in mind: where trees are located in relation to any hard standing areas in the vicinity. For example, trees that are planted into tree pits within a landscaping scheme will have been sited and selected to suit that site. As the trees are growing into their positions, they will naturally take up nutrients and water from wherever they can, automatically and from the start of their final-position lives. Therefore, London Planes will grow happily to become ancient trees, finding everything they need to survive – even if they do occasionally lift the tarmac with their localised roots – not seeking moisture, but simply with their girth, without the requirement for any soil space around their trunks.
Mechanical evidence of tree root damage.
From personal experience and observations, certain tree species react against being capped off by hard non-permeable paving, not only by soil shrinkage, which may be minimal, but by lifting the paving from their roots by mechanical action. This manifests itself by creating callouses within the surface roots – often where they have been damaged albeit slightly by scraping the bark – growing large callouses and attempting to produce new shoots. Ash, birch, cherries, poplars and a number of large shrubs (including cotoneaster and pyracantha) will also react strongly in this manner.
The effects of such mechanical damage cannot be underestimated. Dependent on the structure and construction of the paving, whole rafts of material weighing tons may become cracked and broken, or lifted in sections e.g. block paving as the tree attempts to lift the offending material and locate water. These areas are generally restricted to the branch canopy (which coincides with the tree rooting main system), although some trees i.e. willows, may be over a much larger area, dependent on the amount of water available elsewhere.
Alleviating root problems
Alleviation may be possible, depending on the type of tree involved. Better by far, to prevent any problems in the first instance by taking time and trouble to fully investigate the likely effects existing tree may have on a project. Identify potential troubles and try and design them out of the scheme by allowing sufficient room for the trees to receive water above ground and prevent any reaction in the first place. By piping water under the hard standing areas using perforated flexible drains, thereby providing the tree sufficient moisture, not paving too close to existing tree, avoiding damage to the surface roots and taking care to protect it at all times will avoid many of the aforementioned issues.
Summary
All of the comments above are merely advisory notes, and it is important to understand the need of trees – especially existing trees that may have been on site for many decades, and carefully note the species and identify their likely needs, given all of the criterion I have mentioned.
Better by far, to identify potential problems, and design them out of a project, rather than have to try and resolve them – which can be extremely expensive – at a later date. Bear in mind too, that failure to identify and deal with resolutions may result in a claim against the designer/specifier for neglect as well as for any remedial works to be undertaken. Failure to at least conduct a professional survey when working near to trees and designing hard landscaping projects can prove a very expensive matter.
Ensure you keep a dated record of all notes made and site photographs taken during your site survey, as circumstances may change from the date you made them, and sometime in the future, especially if there are any alterations to the local buildings, roadways or drainage channels or waterways, should any claim arise.
(See also Hard Landscaping on Clay Ground – under Contractor’s Section in The Landscape Library)
Alan Sargent FCIHort MPGCA