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Does it really matter if there is a tree near a building?
Chris Overbeke - Senior Partner OMC Associates
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:
o whether roots are under or can get under foundations
o 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
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findings of the Kew Root Survey, (significantly updated in 1989), the experience of the
BRE digest and several other older studies.
Common Name
Latin Name Max. tree-toDistance within
Distance within
damage which 90% of damage which 75% of damage
distance (m) cases were found (m) cases were found (m)
Figure

Willow
Salix
40
18
11
Oak
Quercus
30
18
13
Poplar
Populus
30
20
15
Elm
Ulmus
25
19
12
Horse chestnut
Aesculus
23
15
10
Ash
Fraxinus
21
13
10
Lime
Tilia
20
11
8
Maple
Acer
20
12
9
Cypresses
Cupressus &
20
5
3.5
Chamaecyparis
17
-
Hornbeam
Carpinus
15
10
-
Plane
Platanus
12
11
7.5
Beech
Fagus
12
11
9
Hawthorn
Crataegus
11
9
7
Rowan & whitebeam Sorbus
11
10
7
Cherries
Prunus
11
8
6
Birch
Betula
10
8
7
Elder
Sambucus
8
-
-
Walnut
Juglans
8
-
-
Laburnum
Laburnum
7
-
-
Fig
Ficus
5
-
-
Lilac
Syringia
4
-
-
False Acacia
Robinia
13.5
10
8.5
Apple
Malus
10
8
6
Pear
Pyrus
10
8
6
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.
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High Water Demand
Broad-leafed genera
Elm
Eucalyptus
Oak
Poplar
Willow
Coniferous genera
Cypress
Figure
Moderate Water Demand
All Other genera
Low Water Demand
Beech
Birch
Holly
Magnolia
Mulberry
All 0thers
NHBC classification of water demand for genera

Problem with is, as Dr Biddle has stated is:
o it is too simplistic to be an accurate tool to assess a tree’s potential to affect a
property
o 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:
o arboricultural characteristics
o site conditions
o 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.
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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.
2)
o
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).
o
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.
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.
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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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).
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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
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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.
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
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
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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 dehydration 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
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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:
o 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.
o 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.
o 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
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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
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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:
Plasticity Index (%)
Shrinkage Potential
10 to 20
20 to 40
40 to 60
60+
Low
Intermediate
High
Very High

2)
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%.
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.
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
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.
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
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.

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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 clearfelled 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.
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
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
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

b)
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.
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.
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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.
Building Control Exemptions
Sheds and detached buildings less than 30 sq.m
Garages less than 30 sq.m (garages attached to houses are classed as extensions)
Conservatories if floor area less than 30 sq.m
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.
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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
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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
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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.
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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.
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1981 (17m)
1982
Distance from tree: Red 4.2m
1983 (18m)
Violet 8.5m
1984
1985
Dark Blue 17.0m
1986/87/88
Light Blue 25.5m
1989 (20m)
Green 42.5m
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.

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:
o
categorized trees into six rather than 3 moisture demand categories (See table)
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o It took account of actual plasticity rather than just the P.I. category it is within
o 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.
o 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).
Highest Water Demand
Lowest Water Demand
(deepest/furthest)
(Shallowest/least extent)
Broad-leafed genera
Eucalyptus
Crataegus
Aesculus
Acer
Alilanthus
Catalpa
Populus
Salix
Fraxinus
Castanea
Alnus
Corylus
Qurecus
Sorbus
Platanus
Fagus
Betula
Ficus
(simple-leaf)
Ulmus
Tilia
Malus
Carpinus
Liquidamber
Prunus
Gleditsia
Liriodendron
Pyrus
Ilex
Magnolia
Pyrus
Juglans
Morus
Laburnum
Sambucus
Robinia
Sorbus
(compoud-leaf)
Coniferous genera
Cupressus
Chaemacyparis
Sequoiadendron
Cedrus
x Cupressus
Juniperus
Abies
Taxus
Araucaria
Tsuga
Ginkco
Larix
Picea
Pinus
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
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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%.
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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
Plan showing layout of above
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
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:
o How serious is the damage or is it merely inconvenient?
o Do some of these structures really warrant insurance cover?
o 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
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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
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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 preNHBC guidelines) and how they have been constructed before providing subsidence
related cover.
Chris Ovebeke can be contacted on:
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