You are here: Does it really matter if there's a tree near a building?

Investigating large trees growing close to buildings

1. Introduction

It is only when trees are mature that their amenity value is maximised. Young trees can be planted but it can take at least 50 years to achieve the beauty of the one it replaced.

Mature trees are valuable and irreplaceable features, particularly to built up areas.

We are losing a significant proportion of trees because of alleged association with structural damage or concern about future possible structural damage.

This paper aims to highlight some of the flaws on which these assessments may be made and discuss the variables that should be considered when assessing a tree’s potential role in direct and indirect damage to a property with particular emphasis on indirect damage

Though an arboriculturist is best qualified to make such a judgment other property professionals can make more thoughtful decisions based on some of the considerations raised in this paper.
Suggestions for quite radical changes of approach to the whole question of trees and property are made.


2. Summary

Whether a tree is causing direct damage to a property or not is generally obvious. We briefly outline the ways this can occur.

Whether a tree is likely to be bearing or bear any indirect influence on a property in the future is far less clear but is dependent on two fundamentals:

  •     whether roots are under or can get under foundations and cause subsidence
  •     the capacity of those roots to abstract moisture.

A realistic assessment of this is fundamental should it be deemed necessary to consider whether a tree is a threat to property or not.

The NHBC has attempted to attribute trees with certain moisture uptake capacities in order to determine foundation depth.

This and the tables of appropriate distances between trees and property used by the ISE and the Loss Prevention Council is based on P.G. Biddle’s water demand classification of tree species. This in turn was based on a combination of his own case studies, the findings of the Kew Root Survey, (significantly updated in 1989), the experience of the BRE digest and several other older studies.

Figure Findings of the Key Report 1989 (Cutler & Richardson, 1989)

All species of trees are simply categorized as, “high”, “moderate” or “low” water demanders. Species in the high group are generally considered to extend their influence on soil moisture levels over a distance of 125% the height of the tree. Moderate water demanders such as sycamore and cherry extend their influence over 75% of their height and low water demanders such as holly and beech extend their influence over 50% of their height.

Figure NHBC classification of water demand for genera

Problem with is, as Dr Biddle has stated is:

  •     it is too simplistic to be an accurate tool to assess a tree’s potential to affect a property
  •     there are many flaws in the data used, linked to the accuracy of the data recorded over the years

Most significantly, so much other relevant criteria that influences tree root growth and moisture uptake is excluded thus rendering it almost meaningless in assessing a tree’s potential to affect a property.

Since the amount of suction exerted by all species of trees in a temperate climate is similar this potential is largely determined by site factors. The distances and depth a root spreads is a function of:

  •     arboricultural characteristics
  •     site conditions
  •     structural features.

All must be considered holistically before determining the likelihood of a tree bearing an influence on a structure. These determinants will be explored and hopefully the flaws of any current basis of assessment highlighted and illustrated.

The following illustration shows the typical structure of tree roots and it is important to be aware of this as it will prove fundamental to much of the content of this paper.

Figure 1 - Popular conception (a) and realistic representation (b) of tree root systems. (Helliwell D. R., 1989)

3. Indirect and Direct Damage or Potential Damage

1) Direct damage is damage caused by the mechanical action of any part of a tree in direct contact with part of a structure.

  • This contact may be constant such as a stem of a tree abutting a wall or fleeting, such as a branch hitting a structure as it moves in wind or as it falls. The latter is rare and usually avoidable by light trimming back. It rarely justifies removal of a tree. The former is rare simply because trees are seldom allowed to develop and mature when in contact with a structure (other than boundaries walls).
  • Roots encountering a solid object will divert and follow the course of least resistance, thereby causing no damage. The continuous radial expansion of trunks and structural roots in contact with a structure and in a restricted space, however, can exert sufficient pressure to displace heavy structures.

2) Indirect Damage is caused by the influence of a tree on soil moisture levels of a substrate prone to shrinkage and expansion. The ground is de-hydrated through the transpiration of leaves abstracting moisture from the ground and, less significantly, by the interception of rainfall by the crown. As a result the ground contracts and settlement occurs. Usually winter rainfall results in full recovery so that the process is seasonal. Conversely removal of tree results in long-term recovery and often significant expansion of the ground.
When structural damage has occured you should consider documenting this by getting a Chartered Structural Engineer to carry our a Structural Inspection.

4. Arboricultural Factors that Determine the Likelihood of Tree-Related Damage

1) Variation amongst Tree Species

Identifying a tree accurately is fundamental to the assessment of its moisture or potential moisture uptake capacity.

Currently most data and reference tables refer to simply a genus such as "Willow". This does not account for the huge variation of species within a genus. There are, for example, over 2000 species within the pine family and these will vary dramatically in terms of size and ability to grow in a given situation. A white willow will grow into a tree exceeding 30m whilst an eared willow will rarely grow beyond 3-4m. The wild cherry may become a large 25m x 14m tree whilst the sour cherry is unlikely to grow much beyond 5m.

Reference to “willow” or “cherry”, therefore, is meaningless without qualification.

Similarly, where a species or genus is referred to it often takes no account of cultivars within that species. A Lawson cypress cultivar may be referred to as “cypress”. Notwithstanding the fact that we do not even know what species of cypress it is, it fails to account for the fact that there are hundreds of Lawson cypress cultivars with a huge variance in size. “Pembury Blue” may not get much taller than 3m whilst “Triumph of Boskoop” can exceed 25m.

Conclusions about the appropriateness of one should be dramatically different to the appropriateness of the other. Removal should not be recommended simply because it is a cypress since the tree may be perfectly innocuous.

2) Intra specific differentials

Differences between two species is little known and rarely accounted for. There can be numerous different genetic clones of a species of tree.

Despite being, nominally, the same tree, two say common horse chestnuts may have quite different physiological processes and, therefore, rates and patterns of moisture uptake. This is demonstrated by P.G Biddle in a case cited in his book “Tree Root Damage to Buildings” (1998) whereby there are considerable fluctuations in soil moisture deficit at 1m depth between 4 similarly sized horse chestnuts growing in London clay.

This may explain (as may many other factors) why a mature tree growing close to a building founded on clay has never caused damage (and would suggest that leaving the tree in-situ and not disturbing the equilibrium may be the best course of action).

Figure 2 - Seasonal fluctuations in SMD at 1.0m for 3 horse chestnuts on similar clay (PG Biddle, “Tree Root Damage to Buildings” 1998)

3) Phenotypic variation

This is a fundamental determinant of soil moisture abstraction capacity.

Trees of the same clonal origin (i.e. identical genetically) will show quite different patterns of soil drying on different sites. In other words, site or environmental factors are having a major influence on a tree’s growth pattern and growth rate.

A silver birch may thrive in an open well-drained site but the identical clone will struggle and show very different growth patterns in a poorly drained shadier site. Hence a tree that is clearly mature but substantially smaller than its expected size may not need to be removed.

4) Graft

Many trees are grafted onto a rootstock and it is that rootstock and not the scion that will determine rates of growth and moisture uptake.

This is clearly illustrated where scions taken from the same apple tree can be grafted onto numerous different rootstocks that will determine whether a tree remains a small fastigiate tree ultimately growing to 2m×0.5m or 9m×9m.

It is known that roots of the same species will graft. One must assume, therefore, that there is a theoretical possibility of a property falling within the root zone of influence of a tree whose roots could not extend as far as the property if there is a nearer tree of the same species and root graft has occurred.

Figure 3 - Plan of a single lateral root of red maple about 60 year of age with circles showing other red maples (Lyford W.H. & Wilson B. F. 1964)

5) Vitality in relation to moisture abstraction capacity.

This term describes how vigorous a particular tree is, as opposed to the species in general. Sycamore, for example, is a vigorous species but a particular sycamore specimen may be old, suppressed or diseased and lack vitality.

Trees of poor vitality, regardless of NHBC “water demand” classification or the vigour of the species, may be having very little influence on soil moisture levels.

Crown shape, extension growth of lateral and terminal buds, apical dominance are some of the characteristics to look out for when trying to establish the health and stage of development of a tree. Poor health, poor tree-work, competition, inappropriate ground or climatic conditions or senescence are likely to result in low vitality.

This may explain why a property located close to a large old tree may show signs of historic cracking but that current movement is either negligible relative to the former cracking or does not occur. The tree has simply grown old and is in decline and a very gradual process of re-hydration is occurring. Indeed where cracks on old properties have been filled in over many years, any minor heave related movement resulting from removal may be compounded by a structure’s inability to re-close the cracks.

Thus assessment of a tree’s crown is far more important than its height. Current formulas used consider tree height (or ultimate tree height) as a key element of the equation that is supposed to indicate root zones of influence. A tree may, however, have an exceptionally tall crown but a low rate of moisture abstraction and root distribution because the actual TLA (Total Leaf Area) is very small.

6) Competition in relation to moisture abstraction capacity

Groups of trees do not necessarily bear more influence on a property than single trees. Where trees grow in close proximity to one another resources are distributed among the trees based on physiological capacity are and they are likely to adapt growth rates in order to survive on reduced resources

Trees may have spindly, etiolated crowns and insignificant moisture abstraction capacity. When attempting to date a tree based on girth, an arboriculturist will factor in the fact that the tree may be in competition with other trees and that growth will have been slower.

This modified growth rate clearly has an impact on rates of transpiration and moisture uptake. If four trees grow in a tight group adjacent a wall, the zone of influence will be localized and the wall may be unaffected.

Figure 4 - Classification of types of tree crown as a result of competition (Practical Forestry, Hart, 1991)

7) Significance of Crown Type

Trees abstract moisture at a rate largely determined by their TLA through a process of transpiration. This is the principle mechanism by which tree dehydrate ground. Essentially the larger and healthier the crown of a tree, the greater the moisture abstraction capacity.

Canopies of trees are, however, also relevant in terms of aggravating any ongoing de- hydration process through the interception of rainfall. The ground is recharged through rainfall. It is estimated that about a quarter of rainfall is intercepted by the crown and a third of that evaporates in the tree, Binns (1980).

This may be why Leyland cypresses are implicated in so many subsidence cases where they are in close proximity. Cypresses have dense evergreen crowns, are commonly grown and are usually grown as hedges close to a wall. Distance is usually within 3-4m and the ground will be exceptionally dry within this zone due to this dual process. Because of this high incidence of damage involving cypresses they are defined as high water demanding trees but I would suggest that much of the process that resulted in damage to a property was the de-hydration of ground close to a structure because of rainfall interception.

Figure 5 - Indication of high incidence of possible damage associated with cypresses in close proximity to the tree. (Cutler & Richardson, 1989)

8) Shrubs

Shrubs are often overlooked because they are not trees. This is wrong. Notwithstanding the rather dubious distinction between a tree and a shrub (essentially they have similar patterns of root development though not so extensive) many can grow into formidable plants as large as many small trees. Elderberry – (curiously considered by some as a shrub), buddleja, cotoneaster, laurel, pyracantha, ivy, wisteria are just a few examples of commonly grown shrubs that have the potential for significant moisture abstraction though little research appears to have been carried out on this.

They are significant, not simply because of potential moisture abstraction capacity but because of relevant characteristics associated with shrubs:

  • They are often grown adjacent walls and buildings to soften the structure. Severe localised drying can occur and such close proximity renders root trespass upon foundations, as likely.
  • Shrubs are commonly grown together or close to one another and develop into clusters of very dense foliage cover. Investigation of the ground below these shrubs can often show highly desiccated ground in the middle of winter where surrounding ground may be saturated.
  • Because shrubs are regarded as somewhat innocuous they are routinely allowed to grow into significant specimens. It is not unusual to see mature ivy or wisteria covering whole walls if not houses.

Before decisions are made about the presence of a tree, therefore, it may be more appropriate to consider the effect of nearby shrubs. Ultimately, however, the effect of most shrubs is likely to be seasonal and the extent of pruning regarded as effective and often inappropriate for trees can be carried out on most species. Transplantation or containerisation are often solutions that are generally exclusive to shrubs.

Photo 1 - Large pyracantha shrub growing against wall

5 Relevant Site Factors

1) Soil Type

This is complex but highly relevant. Vegetation cannot cause indirect damage to a property unless there is potential for volumetric change and so it must be established that the structure bears upon a shrinkable substrate. This is usually clay though peat is also shrinkable.

Photo 2 – Example of how trees can grow very close to property without harming then if the soil is not shrinkable

Photo 3 – Example of how trees can grow very close to property without harming then if the soil is not shrinkable

Degrees of shrinkability must be considered when considering tree influence or management. In the NHBC's revision of Practise Note 3 in 1985 this was simplified into three categories:

This was further refined in BRE Digest 240 in 1993 to add a category of "Very High" for a P.I. in excess of 60% Though an improvement, it remains somewhat crude. A root system abstracting high levels of soil moisture is likely to have rather less impact on the volume of a founding clay substrate with a P.I. of 20% than that of a clay with a P.I. of 40%.

2) Soil Structure

This is key in trying to make a reasonable assessment of how far a tree’s roots may have extended. Root growth requires oxygen levels in excess of 15%. Root initiation requires oxygen in excess of 12% and growth of existing root tips requires in excess of 5-10%. Optimum oxygen levels, however, are rare in the field. Damage to the soil structure through compaction, particularly in clay soil, inhibits oxygen diffusion. This is, by default, common near many structures.

Compaction plays a further role in assessing root extension because of the effect of mechanically impeding penetration. Impedance must be less than the pressure exerted by a root tip. Because this is a partially a function of the texture of a soil, course soil particles can be pushed aside by a root tip. In fine soils such as clay, where pore size and porosity is far less, bulk density increases and root extension is compromised. It will be dramatically reduced by bulk densities of more than 1.2g/cm2 and effectively cease above 1.8g/cm2. Many clay soils have a bulk density in excess of this.

An understanding of the ground conditions of a site should, therefore, suggest to the trained eye whether tree roots have extended as far, further or less than the expected root spread of a given species at a given stage of growth.

Thus on sites where bands of a more aerobic substrate such as hardcore, stone, sand, gravel etc. occur within a clay, it is reasonable to assume that this would facilitate root extension along these channels. Similarly roots commonly follow cracks and crevices in the soil, including pipelines (though this is also related to them exploiting the condensation on a pipe or additional soil moisture if leaks are presents).

Such opportunism is typical of root growth and it is this characteristic that must be considered when assessing the likelihood of root trespass.

Figure 6 - Interaction between tree roots and soil water with aeration (Manion 1981)

3) Soil Moisture Content

Root’s search for water is the dominant factor in determining the scope of its root system and is the major factor that limits tree growth. Reduced moisture levels encourage roots to grow in search of water.

A common perception is that roots find water. They do not. Roots are opportunistic and if they happen across it they will proliferate. Hence leaking drains, condensation along underground pipes, natural aquifers etc. may all result in greater root presence in that area.

Most of the subsidence investigations that I have been involved with seem to involve damaged drains and, notwithstanding settlement associated with compaction of soil particles, the increased soil moisture content may have encouraged greater if not an entirely new root presence from nearby trees that would not normally have been the case.

Diffusion rates through water are even lower than in compacted soils. Saturated soils are likely to be particularly effective in inhibiting root growth. Ditches/streams or drains leaking over the long term may result in sufficiently saturated ground to prevent root growth.

Since impedance is also a function of the turgor of a soil, resistance encountered by root tips is further increased as clay soils dry out.

Trees have very different tolerances to flooding. Condition of the tree is relevant but there are clear inherent differences between species, Thomas (1980). Some species such as Taxodium or Salix can survive flooding for several months or even permanently. One may observe saturated clay soil conditions but if the species of tree is one that thrives in the prevailing conditions, there is no reason to dismiss the likelihood of root trespass upon a property.

This represents a minority of tree species and in many other situations it may be reasonable to make such an assertion.

Figure 7 - Illustration of how root spread may be influenced by ground conditions

4) Site History

Knowledge of what may have occurred in the past may help explain what is happening on a site.

Sites cleared of vegetation may be resulting in gradual re-hydration and swelling of the ground. This is common where people have cut down or even significantly reduced vegetation on the site where the structure is built or extended.

Such awareness may encourage professionals investigating movement to consider heave rather than subsidence as a cause of movement with clearly very different solutions.

Wrong diagnosis may result in an arboriculturist, under the impression that subsidence it the problem, exacerbating the problem by suggesting further removal.

I saw a site where the garden was in an almost permanent state of flood and the owners were mystified since this never used to be the case. It turned out that they had clear- felled the whole garden of established vegetation in an area with a high water table. In conjunction with an adjoining development, where woodland had been felled for development, all sources of moisture abstractions had been removed and yet this property was subject of a subsidence investigation.

On another site, an elderly man who had refused to sell up, lived in a 1950s bungalow that was isolated for many miles by a large housing development. Recently cracks had sprung up everywhere and, notwithstanding the possible effects of vibration from the pile drivers, he said that the stream bordering his garden had completely dried out for the first time in the 40 years he had lived there. It would seem apparent that underground watercourses and ground levels had been changed dramatically resulting in this drying out. The trees in his garden were mature and had clearly co-existed with the property for many decades and were, indeed, declining. The trees, however, were removed.

5) Grass

Lawns abutting damaged property may be more a causal factor of subsidence than a distant tree - particularly if the lawn is recently established.

Grass is evergreen and starts transpiring much earlier than deciduous trees. It can create soil moisture deficits to 0.5m by early summer and competes fiercely with trees, even causing greater (though highly localised) soil suctions than most tree species. (Biddle – Tree root damage to buildings, 1998).

Research has shown root density to be 113% more in bare soil than in grassed areas. The dehydration process is a dual one with rainfall being intercepted by a dense network of roots and significant levels of moisture abstraction.

It is also noteworthy that herbicides are regularly applied to lawn and this will damage and kill tree roots if persistent or residual.

Since depth of drying can extend to 1.2m by late summer, it is reasonable to assume that this process could be pertinent in the many situations where footings are rather less.

6) Juxtaposition of trees and structure

Based on observations and perhaps common sense, I have noted over the years how trees adjacent corners can bear a greater influence on a structure than those adjacent the middle of an elevation.

Roots trespass the property on two fronts creating more intense localised desiccation. De-hydration below the centre of an elevation may not, within reason, result in cracking if either side of the wall is unaffected by this process and continues to bridge the middle of the wall.

Figure 8 - Illustration of how juxtaposition of property of trees to structure may be of relevance

7) Non Porous surfaces

This can be relevant in investigating existing damage or attempting to predict it for two reasons:
a) It may be the reason why the ground has become dryer.

  • Where there is an extensive area of impermeable hard-standing, natural recharge is prevented. Few surfaces are entirely impermeable and they greatly reduce or eradicate evaporation from the soil surface. Foundations, however, are usually significantly deeper than the level effected by evaporation and if roots manage to encroach upon part of the area, moisture is removed without being replaced and a persistent soil moisture deficit may occur.
  • I surveyed a site where desiccation shrinkage had only recently affected an end of terrace property despite the existence of established trees and vegetation. The only change in circumstances (notwithstanding any climatic effect) had been the recent laying of hard-standing. Since the property was surrounded by hard-standing, the limited area where natural recharge could occur was lost.
  • Removal of the trees and shrubs along with some of the hard-standing will have precipitated the recovery process. Retention of the trees but removal of the nearer shrubs and removal of the hard-standing, however, is also likely to have allowed for sufficient recovery, albeit slightly more slowly.

b) Impermeable hard-standing creates an extremely hostile rooting environment.

  • It prevents gaseous interchange thus depriving roots of oxygen and trapping carbon dioxide
  • It prevents the entry of nutrients into the ground (through the natural decomposition of organic matter)
  • Most importantly, it intercepts rainfall and deprives roots of moisture.

If the surface is impermeable and roots will not thrive nor are likely to survive. Large asphalt drives, highways etc. are, therefore, likely to act, in effect, as root barriers deterring root extension.

Porous surfaces, on the other hand, allow the penetration of rain as well as reduces surface evaporation. They are extremely effective in maintaining field capacity and can represent environments are conducive to root growth.

Figure 9 - Illustration of how impermeable hard-standing may be contributing to desiccation shrinkage

6. Relevant Structural Factors

1) Foundations

Foundation detail is critical in preventing root-related damage. BRE Guidelines are more sophisticated than in the past in so far as plasticity of the clay - albeit crudely -, species - also crudely - and distance of tree is considered. Unfortunately I see numerous examples where full NHBC compliance has not occurred. I would suggest that, flawed as it, if these guidelines were fully enforced there would be significantly less incidence of tree related damage.

Many structures are built without Building Control approval. The table below shows what structures are exempt from building control approval and I would suggest that, excluding the properties built pre the NHBC guidelines, these usually poorly constructed structures represent a significant percentage of investigations where trees are implicated in damage.

Figure 10 - The significant structures that can be built without BC approval

Buildings built on rafts (or with cellars) are usually safe from the effect of roots. Like anything, however, this is conditional on correct installation.

Poorly founded structures will be vulnerable to volumetric changes of the clay regardless of the presence of trees. Thus cutting down a tree may simply achieve a reduced magnitude in seasonal movement but the movement continues. It would make more long term sense to rebuild or underpin the structure so it is not vulnerable to any seasonal drying out and ensure that the new foundations account for the tree(s).

Photo 4 - Poorly founded extension and position of very old trees in decline 0314S

Figure 10 - Plan showing layout of above

2) Drains

Perhaps the most widespread problem caused by tree roots is the penetration and clogging of drains and sewers. The conditions within are perfect and roots proliferate, slowing flow rates and ultimately blocking the drains.

One willow root taken from a storm sewer in Utah was 41m long.

This, however, must not be a reason to remove trees. In view of the dense network of underground pipes and roots, it is physically impossible and clearly unacceptable to remove trees whose roots may encompass a pipe. We would have no trees left in any town or, indeed, village.

Roots cause damage only once they have gained ingress to drains. They cannot enter a pipe unless it is already damaged or sufficiently deteriorated.

The recommendation that should be made where trees are present should be to inspect drains and where roots have gained entry, re-line them or replace with UPVC pipes so that roots cannot penetrate in the future.

3) Structural Solution

As already noted many subsidence claims relate to poorly constructed, often DIY structures. Others relate to a section of a structure that is part of the original structure but is less well founded, such as bay windows or the section of a house that has no cellar below it. The result is differential movement.

A solution for such situations should be a structural one, particularly where an arboricultural one requires the removal of a fine tree with high amenity, environmental and ecological value. The cost is not necessarily more where large tree removal(s) are involved.

There are situations too where tree removal is not a solution due to the possibility of heave. Large trees significantly predating and near a property – subject to species, damage history, soil conditions, structural type – may cause more or new damage to a property if removed.

Where a large mature tree is noted near a property and no damage has occurred, why recommend removal when the likelihood is:

  • a) If is was going to cause damage it will have already done so
  • b) It may create a problem though excessive re-hydration that did not exist.

Photo 5 – Close proximity of numerous 100-170 year old trees including oak to a poorly founded bungalow PI in late 30s/early 40s. Removals will have resulted in heave.

Figure 10 - Plan showing layout of above

7. Perspective

Average rainfall levels through the late 1980s and early 1990s were approximately 400mm, allowing for seasonal moisture deficits close to trees to recharge.

Even in a dry year like 1989 the seasonal deficit did not exceed 200mm below 30cm and 50mm nearer the surface. In other words a total seasonal deficit of 250mm was substantially less than the 400mm annual rainfall.

The worse that will have happened subject to the shrinkage potential of the clay was that cracks would open during summer and early autumn but re-close.

Figure 11 - Seasonal fluctuations in SMD between 0-460mm caused by a mature Poplar. (PG Biddle, “Tree Root Damage to Buildings” 1998)

Since subsidence became an insurable item in the early 1970s, however, cracks suddenly become a source of concern for house-owners.  These are often highlighted after a Building Survey from a Chartered Surveyor.

There has, consequently, been a cultural shift from one where seasonal hairline cracks were taken for granted to one where it is viewed with dread. It also, dare I say it, provided a means of getting a makeover for no more than the excess on the policy and engendered the idea that removing the inconvenience of wall paper splitting every summer was more important than the existence of a 200 year old oak tree.

In one case in Lincolnshire, a couple were horrified when I recommended removal of a knarled, twisted 200 year old wisteria because they were suffering seasonal movement to a 17th century cottage. They negotiated with their insurance company and agreed to remove the insurance cover from their policy and live with the seasonal movement and the wisteria.

8. Discussion

Several years ago the Arboricultural Association attempted to establish a Subsidence Risk Factor (SRF) based on a significantly more detailed set of criteria than that used by the NHBC. It:

  • categorized trees into six rather than 3 moisture demand categories (See table)
  • It took account of actual plasticity rather than just the P.I. category it is within
  • and it took account of a tree's crown shape rather than simply height. Crown shapes are grouped as one of four potential shapes, semi-circular, ellipse, circular or triangular and height and spread is applied to the relevant shape.
  • Finally the climate is factored in based on location in the UK.

Despite this considerable refinement it is still not reliable and “considerable foundation movement has been shown to occur where the SRF indicated insignificant risk” Biddle, “Tree Root Damage to Buildings” (1998).

Figure 12 - PG Biddle’s tentative classification of Water Demand

Essentially this is because there are so many variables for each site. These relate to tree species and health, ground conditions and history, climatic conditions and structural detail.

Thus if an “apple tree” is observed we do not know what graft it is. If a pine is noted we do not know what species it is. We rarely know anything about the ground other than perhaps reference to a site being within a geological area identified by a 1:50,000 geological drift map (is it saturated, compacted etc.), do drains leak, is there a history of mining, do foundations depths vary, what, indeed, are the foundation depths, is the sufficient area to allow for natural recharge, have trees been removed in the past etc. Where cracks are noted, are they actually as a result of desiccation shrinkage.

The picture below shows the front of my house. Approximately 3m from the front elevation is a 50-year-old holly tree. It will not grow much larger, holly is a slow growing tree considered to have a low moisture abstraction capacity and the house is founded on a sandy, gravelly loam. This tree represents no threat at all to the property yet the surveyors report advised removal.

Photo 6 - Holly tree that is no threat to property because of species and ground conditions but was recommended for removal

There is an excessively over cautious attitude. This is understandable since we all live in a litigious society and we all have P.I. policies that need to be protected. The point is, however, that basic criteria are often not applied let alone the detailed assessment of factors summarised in this paper that would be required if a realistic conclusion about the potential threat from a tree is to be made.

At present the existing data, by default, describes situations where trees have caused damage. It does not include comparable situations where trees have caused no damage. Indeed, according to Dr Biddle, the few cases he has been involved with where damage has not occurred is indicative of how low the actual risk factor associated with actually appears to be. He considers this to be less than 1%.

Photo 7 – Case with numerous mature “high water demanding” trees to property founded on a desiccated clay of PI 39% but with no subsidence related damage 0314s (See photo in appendices).

Figure 13 Plan showing layout of above

Many of the claims or circumstances where damage has occurred (and what much of the available data is based on relate to poorly constructed and founded structures. The extensions, conservatories, bay windows and so on.

Perhaps we should be asking:

  • How serious is the damage or is it merely inconvenient?
  • Do some of these structures really warrant insurance cover?
  • What is the cost of underpinning these structure or, frankly, re-building them?

If trees are considered to pose a threat or to have caused damage, more attention needs to be given to the implications of tree removal. At present decisions are based on a purely monetary one. A cost benefit analysis is applied whether it be to the cost of foundations, structural repair or tree removal. These are quantifiable items and, therefore, the decision is black and white.

The value of trees, however, is far more intangible. We all know how important they are in:

  • moderating climates – particularly in urban environments,
  • how they filter out significant proportions of atmospheric pollutants,
  • how they suppress noise such as traffic,
  • how they provide a “feel good factor” (studies have conclusively shown that patients convalescing in hospital, recover far more quickly in rooms overlooking mature trees than rooms overlooking the back of a building or car park-apply),
  • how they are a key source of shelter and food for wildlife and promote biodiversity,
  • but finally, and perhaps most relevantly in this context, how mature trees increase the value of properties.

Though attempts have been made to value trees, I suspect this will remain elusive. One must try and consider whether £2000 to underpin (or re-build) a conservatory is a more sensible option than £800 to remove a large tree and the possible consequent loss of value to the house (perhaps up to 20%) and, of all it’s benefits to the area.

9. Conclusions

Hopefully it has been established that because there are so many variables on any site relating to environmental conditions, tree status and structural characteristics that, without this information, predicting whether a tree is a threat or potential future threat is rarely realistically possible.

Where there is some doubt, noting the presence of a tree(s) is necessary but this must always be accompanied with a clear statement that certain minimum criteria must be substantiated before taking further action over the tree(s).

In an ideal world this would include a detailed assessment to gather as much of the requisite information to draw a reasoned conclusion. Since this is not realistic this certain minimum criteria should include foundation depths on all parts of the property, analysis of the founding soils and a survey from an arboriculturist, (not a tree surgeon!). Thus trial pits would be the norm.

Where movement has occurred the cause of the movement must be determined beyond reasonable doubt. The above requirements in addition to level monitoring over a suitable period of time is the minimum that should be expected.

Though tree surgery and management has been increasingly rejected this should remain an option. There are many circumstances where it is effective but careful assessment of the site, distance, history (if any) of damage, species and how it will react to particular type of tree surgery must be undertaken.

Figure 12 - Indication of effectiveness of pollarding through measurement of annual radial growth (Tree Root Damage to Buildings, Biddle, 1998)

Photo 8 - A pollarded tree

By way of concluding remarks, the implementation of certain changes in the longer term could greatly improve the likelihood of peaceful co-existence of tree and structure or remove the doubt that prevails. This could include:

1) A new method of determining foundation depths for new build. Rather than following a set formula, penetrometers should be used for each site to determine at what depth the bulk density increase to the extent that root growth cannot or is unlikely to occur. If excessive, a raft should be constructed.

2) Planning legislation must be changed so that building control regulations are required for all brick built structures, regardless of size.

3) Some form of cost benefit analysis adopted that takes into consideration all the benefits that a tree or trees provide, the cost of structural repair or root barrier installation and the cost of tree surgery.

4) The insurance industry needs to evaluate properties (exempting houses built pre- NHBC guidelines) and how they have been constructed before providing subsidence related cover.

Chris Ovebeke of OMC Associates can be contacted on:
Tel: 01223 842253

For a list of suppliers who can provide an arboricultural survey in your local are click here.

For a list of suppliers who can provide a structural inspection of a property click here.