trees draft 060227

8/10/2019 Trees Draft 060227

1/29

OMC

OMC ASSOCIATES www.omc-associates.co.uk Tel: 01223 842253 Email: [email protected] 1

DDooeessiittrreeaallllyymmaatttteerriifftthheerreeiissaattrreeeenneeaarraabbuuiillddiinngg??

Chris Overbeke - Senior Partner OMC Associates

1 Introduction

It is only when trees are mature that their amenity value is maximised. Young trees canbe 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 withstructural damage or concern about future possible structural damage.

This paper aims to highlight some of the flaws on which these assessments may bemade and discuss the variables that should be considered when assessing a trees

potential role in direct and indirect damage to a property with particular emphasis onindirect damage.

Though an arboriculturist is best qualified to make such a judgment other propertyprofessionals can make more thoughtful decisions based on some of the considerationsraised in this paper.

Suggestions for quite radical changes of approach to the whole question of trees andproperty are made.

2 Summary

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

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

o whether roots are under or can get under foundationso the capacity of those roots to abstract moisture.

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

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

This and the tables of appropriate distances between trees and property used by the ISEand the Loss Prevention Council is based on P.G. Biddles water demand classification oftree species. This in turn was based on a combination of his own case studies, the

8/10/2019 Trees Draft 060227

2/29

OMC



findings of the Kew Root Survey, (significantly updated in 1989), the experience of theBRE digest and several other older studies.

Common Name Latin Name Max. tree-to- Distance within Distance within

damage which 90% of damage which 75% of damage

distance (m) cases were found (m) cases were found (m)

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

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

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

their height.

8/10/2019 Trees Draft 060227

3/29

OMC



High Water Demand Moderate Water Demand Low Water Demand

Broad-leafed genera

Elm All Other genera BeechEucalyptus Birch

Oak Holly

Poplar Magnolia

Willow Mulberry

Coniferous genera

Cypress All 0thers

Figure 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 trees potential to affect a

propertyo 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 andmoisture uptake is excluded thus rendering it almost meaningless in assessing a treespotential to affect a property.

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

spreads is a function of:o arboricultural characteristicso site conditionso structural features.

All must be considered holistically before determining the likelihood of a tree bearing aninfluence on a structure. These determinants will be explored and hopefully the flaws ofany 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.

8/10/2019 Trees Draft 060227

4/29

OMC



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 directcontact with part of a structure.

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 rareand usually avoidable by light trimming back. It rarely justifies removal of a tree. Theformer is rare simply because trees are seldom allowed to develop and mature whenin 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 exertsufficient pressure to displace heavy structures.

22)) IInnddiirreeccttDDaammaaggeeiissccaauusseeddbbyytthheeiinnfflluueenncceeooffaattrreeeeoonnssooiillmmooiissttuurreelleevveellssooffaassuubbssttrraatteepprroonneettoosshhrriinnkkaaggeeaannddeexxppaannssiioonn..TThheeggrroouunnddiissddee--hhyyddrraatteedd tthhrroouugghhtthhee ttrraannssppiirraattiioonnoofflleeaavveessaabbssttrraaccttiinnggmmooiissttuurreeffrroommtthheeggrroouunnddaanndd,,lleessssssiiggnniiffiiccaannttllyy,,bbyytthheeiinntteerrcceeppttiioonnooffrraaiinnffaallllbbyytthheeccrroowwnn..AAssaarreessuulltttthheeggrroouunnddccoonnttrraaccttssaannddsseettttlleemmeennttooccccuurrss..UUssuuaallllyywwiinntteerrrraaiinnffaallllrreessuullttssiinnffuullllrreeccoovveerryyssootthhaatttthheepprroocceessssiisssseeaassoonnaall..CCoonnvveerrsseellyyrreemmoovvaallooffttrreeeerreessuullttssiinnlloonngg--tteerrmmrreeccoovveerryyaannddoofftteennssiiggnniiffiiccaanntteexxppaannssiioonnoofftthheeggrroouunndd..

8/10/2019 Trees Draft 060227

5/29

OMC



44 AArrbboorriiccuullttuurraallFFaaccttoorrsstthhaattDDeetteerrmmiinneetthheeLLiikkeelliihhooooddooffTTrreeee--RReellaatteeddDDaammaaggee

1) Variation amongst Tree Species

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

Currently most data and reference tables refer to simply a genus such as "Willow". Thisdoes not account for the huge variation of species within a genus. There are, forexample, over 2000 species within the pine family and these will vary dramatically interms of size and ability to grow in a given situation. A white willow will grow into a treeexceeding 30m whilst an eared willow will rarely grow beyond 3-4m. The wild cherry may

become a large 25m14m tree whilst the sour cherry is unlikely to grow much beyond5m.

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 cultivarswithin 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 toaccount for the fact that there are hundreds of Lawson cypress cultivars with a hugevariance in size. Pembury Blue may not get much taller than 3m whilst Triumph ofBoskoop 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 isa 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 benumerous different genetic clones of a species of tree.

Despite being, nominally, the same tree, two say common horse chestnuts may havequite different physiological processes and, therefore, rates and patterns of moistureuptake. This is demonstrated by P.G Biddle in a case cited in his book Tree RootDamage 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 abuilding founded on clay has never caused damage (and would suggest that leaving thetree in-situ and not disturbing the equilibrium may be the best course of action).

8/10/2019 Trees Draft 060227

6/29

OMC



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 patternsof soil drying on different sites. In other words, site or environmental factors are having amajor influence on a trees growth pattern and growth rate.

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

4) Graft

Many trees are grafted onto a rootstock and it is that rootstock and not the scion that will

8/10/2019 Trees Draft 060227

7/29

OMC



determine rates of growth and moisture uptake.

This is clearly illustrated where scions taken from the same apple tree can be graftedonto numerous different rootstocks that will determine whether a tree remains a smallfastigiate tree ultimately growing to 2m0.5m or 9m9m.

It is known that roots of the same species will graft. One must assume, therefore, thatthere is a theoretical possibility of a property falling within the root zone of influence of atree whose roots could not extend as far as the property ifthere is a nearer tree of thesame 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 ingeneral. Sycamore, for example, is a vigorous species but a particular sycamorespecimen may be old, suppressed or diseased and lack vitality.

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

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

8/10/2019 Trees Draft 060227

8/29

OMC



This may explain why a property located close to a large old tree may show signs ofhistoric cracking but that current movement is either negligible relative to the formercracking or does not occur. The tree has simply grown old and is in decline and a verygradual process of re-hydration is occurring. Indeed where cracks on old properties havebeen filled in over many years, any minor heave related movement resulting fromremoval may be compounded by a structures inability to re-close the cracks.

Thus assessment of a trees crown is far more important than its height. Current formulasused consider tree height (or ultimate tree height) as a key element of the equation thatis supposed to indicate root zones of influence. A tree may, however, have anexceptionally tall crown but a low rate of moisture abstraction and root distributionbecause 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 thetrees based on physiological capacity are and they are likely to adapt growth rates inorder 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 thatthe 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 moistureuptake. If four trees grow in a tight group adjacent a wall, the zone of influence will belocalized 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

8/10/2019 Trees Draft 060227

9/29

OMC



transpiration. This is the principle mechanism by which tree dehydrate ground.Essentially the larger and healthier the crown of a tree, the greater the moistureabstraction 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 throughrainfall. IItt iisseessttiimmaatteedd tthhaattaabboouuttaaqquuaarrtteerrooff rraaiinnffaallll iiss iinntteerrcceepptteeddbbyytthheeccrroowwnnaannddaatthhiirrddoofftthhaatteevvaappoorraatteessiinntthheettrreeee,,BBiinnnnss((11998800))..

This may be why Leyland cypresses are implicated in so many subsidence cases wherethey are in close proximity. Cypresses have dense evergreen crowns, are commonlygrown and are usually grown as hedges close to a wall. Distance is usually within 3-4mand 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 highwater demanding trees but I would suggest that much of the process that resulted indamage to a property was the de-hydration of ground close to a structure because ofrainfall 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. Notwithstandingthe rather dubious distinction between a tree and a shrub (essentially they have similar

8/10/2019 Trees Draft 060227

10/29

OMC



patterns of root development though not so extensive) many can grow into formidableplants as large as many small trees. Elderberry (curiously considered by some as ashrub), buddleja, cotoneaster, laurel, pyracantha, ivy, wisteria are just a few examples ofcommonly grown shrubs that have the potential for significant moisture abstractionthough little research appears to have been carried out on this.

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

o They are often grown adjacent walls and buildings to soften the structure. Severelocalised drying can occur and such close proximity renders root trespass uponfoundations, as likely.

o Shrubs are commonly grown together or close to one another and develop intoclusters of very dense foliage cover. Investigation of the ground below theseshrubs can often show highly desiccated ground in the middle of winter wheresurrounding ground may be saturated.

o Because shrubs are regarded as somewhat innocuous they are routinely allowedto grow into significant specimens. It is not unusual to see mature ivy or wisteriacovering whole walls if not houses.

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

Photo 1 - Large pyracantha shrub growing against wall

8/10/2019 Trees Draft 060227

11/29

OMC



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 establishedthat the structure bears upon a shrinkable substrate. This is usually clay though peat isalso shrinkable.

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

8/10/2019 Trees Draft 060227

12/29

OMC



Photo 3Example 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 ormanagement. In the NHBC's revision of Practise Note 3 in 1985 this was simplified intothree categories:

Plasticity Index (%) Shrinkage Potential

10 to 20 Low

20 to 40 Intermediate

40 to 60 High

60+ Very High

This was further refined in BRE Digest 240 in 1993 to add a category of "Very High" for aP.I. in excess of 60% Though an improvement, it remains somewhat crude. A rootsystem abstracting high levels of soil moisture is likely to have rather less impact on thevolume of a founding clay substrate with a P.I. of 20% than that of a clay with a P.I. of40%.

2) Soil Structure

This is key in trying to make a reasonable assessment of how far a trees roots mayhave extended. Root growth requires oxygen levels in excess of 15%. Root initiationrequires oxygen in excess of 12% and growth of existing root tips requires in excess of5-10%. Optimum oxygen levels, however, are rare in the field. Damage to the soilstructure through compaction, particularly in clay soil, inhibits oxygen diffusion. This is,by default, common near many structures.

8/10/2019 Trees Draft 060227

13/29

OMC



Compaction plays a further role in assessing root extension because of the effect ofmechanically impeding penetration. Impedance must be less than the pressure exertedby a root tip. Because this is a partially a function of the texture of a soil, course soilparticles can be pushed aside by a root tip. In fine soils such as clay, where pore sizeand 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 ceaseabove 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 thetrained eye whether tree roots have extended as far, further or less than the expectedroot 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 rootextension along these channels. Similarly roots commonly follow cracks and crevices inthe 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 beconsidered when assessing the likelihood of root trespass.

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

3) Soil Moisture Content

Roots search for wateris the dominant factor in determining the scope of its root systemand is the major factor that limits tree growth. Reduced moisture levels encourage rootsto grow in search of water.

8/10/2019 Trees Draft 060227

14/29

OMC



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

Most of the subsidence investigations that I have been involved with seem to involvedamaged drains and, notwithstanding settlement associated with compaction of soilparticles, the increased soil moisture content may have encouraged greater if not anentirely new root presence from nearby trees that would not normally have been thecase.

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

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 thereare clear inherent differences between species, Thomas (1980). Some species such asTaxodium or Salix can survive flooding for several months or even permanently. Onemay observe saturated clay soil conditions but if the species of tree is one that thrives inthe prevailing conditions, there is no reason to dismiss the likelihood of root trespassupon a property.

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

8/10/2019 Trees Draft 060227

15/29

OMC



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 happeningon a site.

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

Such awareness may encourage professionals investigating movement to considerheave rather than subsidence as a cause of movement with clearly very differentsolutions.

Wrong diagnosis may result in an arboriculturist, under the impression that subsidence itthe 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 ownerswere 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. Inconjunction with an adjoining development, where woodland had been felled fordevelopment, all sources of moisture abstractions had been removed and yet thisproperty was subject of a subsidence investigation.

On another site, an elderly man who had refused to sell up, lived in a 1950s bungalowthat was isolated for many miles by a large housing development. Recently cracks hadsprung 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 firsttime in the 40 years he had lived there. It would seem apparent that undergroundwatercourses and ground levels had been changed dramatically resulting in this dryingout. 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 adistant tree - particularly if the lawn is recently established.

Grass is evergreen and starts transpiring much earlier than deciduous trees. It can createsoil moisture deficits to 0.5m by early summer and competes fiercely with trees, evencausing 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 networkof roots and significant levels of moisture abstraction.

8/10/2019 Trees Draft 060227

16/29

OMC



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

Since depth of drying can extend to 1.2m by late summer, it is reasonable to assume thatthis 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 howtrees adjacent corners can bear a greater influence on a structure than those adjacentthe middle of an elevation.

Roots trespass the property on two fronts creating more intense localised desiccation. De-hydration belowthe centre of an elevation may not, within reason, result in cracking if either side of the wall is unaffected bythis 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

8/10/2019 Trees Draft 060227

17/29

OMC



prevented. Few surfaces are entirely impermeable and they greatly reduce oreradicate evaporation from the soil surface. Foundations, however, are usuallysignificantly deeper than the level effected by evaporation and if roots manage toencroach upon part of the area, moisture is removed without being replaced and apersistent soil moisture deficit may occur.

I surveyed a site where desiccation shrinkage had only recently affected an end ofterrace property despite the existence of established trees and vegetation. The onlychange in circumstances (notwithstanding any climatic effect) had been the recentlaying of hard-standing. Since the property was surrounded by hard-standing, thelimited area where natural recharge could occur was lost.

Removal of the trees and shrubs along with some of the hard-standing will haveprecipitated the recovery process. Retention of the trees but removal of the nearershrubs and removal of the hard-standing, however, is also likely to have allowed forsufficient 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 carbondioxide

It prevents the entry of nutrients into the ground (through the natural decomposition oforganic 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. Largeasphalt drives, highways etc. are, therefore, likely to act, in effect, as root barriersdeterring root extension.

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

8/10/2019 Trees Draft 060227

18/29

OMC



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

66 RReelleevvaannttSSttrruuccttuurraallFFaaccttoorrss

1) Foundations

Foundation detail is critical in preventing root-related damage. BRE Guidelines are moresophisticated 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 theseguidelines were fully enforced there would be significantly less incidence of tree relateddamage.

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

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. Likeanything, however, this is conditional on correct installation.

8/10/2019 Trees Draft 060227

19/29

OMC



Poorly founded structures will be vulnerable to volumetric changes of the clay regardlessof the presence of trees. Thus cutting down a tree may simply achieve a reducedmagnitude in seasonal movement but the movement continues. It would make more longterm sense to rebuild or underpin the structure so it is not vulnerable to any seasonaldrying 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

8/10/2019 Trees Draft 060227

20/29

OMC



Figure 10 - Plan showing layout of above

2) Drains

Perhaps the most widespread problem caused by tree roots is the penetration andclogging 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 toremove trees whose roots may encompass a pipe. We would have no trees left in anytown or, indeed, village.

Roots cause damage only once they have gained ingress to drains. They cannot enter apipe unless it is already damagedor sufficiently deteriorated.

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

3) Structural Solution

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

A solution for such situations should be a structural one, particularly where anarboricultural one requires the removal of a fine tree with high amenity, environmental

8/10/2019 Trees Draft 060227

21/29

OMC



and ecological value. The cost is not necessarily more where large tree removal(s) areinvolved.

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

Where a large mature tree is noted near a property and no damage has occurred, whyrecommend 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 inlate 30s/early 40s. Removals will have resulted in heave.

8/10/2019 Trees Draft 060227

22/29

OMC



Figure 10 - Plan showing layout of above

7) Perspective

Average rainfall levels through the late 1980s and early 1990s were approximately400mm, 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 and50mm nearer the surface. In other words a total seasonal deficit of 250mm wassubstantially 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.

8/10/2019 Trees Draft 060227

23/29

OMC



1981 (17m) 1982 1983 (18m) 1984 1985 1986/87/88 1989 (20m)

Distance from tree: Red 4.2m Violet 8.5m Dark Blue 17.0m Light Blue 25.5m Green 42.5m

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

Since subsidence became an insurable item in the early 1970s, however, crackssuddenly become a source of concern for house-owners.

There has, consequently, been a cultural shift from one where seasonal hairline crackswere taken for granted to one where it is viewed with dread. It also, dare I say it, provideda means of getting a makeover for no more than the excess on the policy andengendered the idea that removing the inconvenience of wall paper splitting everysummer 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 aknarled, twisted 200 year old wisteria because they were suffering seasonal movementto a 17th century cottage. They negotiated with their insurance company and agreed toremove the insurance cover from their policy and live with the seasonal movement andthe wisteria.

88)) DDiissccuussssiioonn

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

o categorized trees into six rather than 3 moisture demand categories (See table)

8/10/2019 Trees Draft 060227

24/29

OMC



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. Crownshapes are grouped as one of four potential shapes, semi-circular, ellipse, circularor 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

Robinia Laburnum Sambucus

Sorbus

(compoud-leaf)

Coniferous genera

Cupressus Chaemacyparis Sequoiadendron Cedrus Juniperus Abies

x Cupressus Taxus AraucariaTsuga Ginkco

Larix

Picea

Pinus

Figure 12 - PG Biddles tentative classification of Water Demand

Essentially this is because there are so many variables for each site. These relate to treespecies and health, ground conditions and history, climatic conditions and structuraldetail.

Thus if an apple tree is observed we do not know what graft it is. If a pine is noted wedo not know what species it is. We rarely know anything about the ground other thanperhaps reference to a site being within a geological area identified by a 1:50,000geological drift map (is it saturated, compacted etc.), do drains leak, is there a history ofmining, do foundations depths vary, what, indeed, are the foundation depths, is thesufficient 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

8/10/2019 Trees Draft 060227

25/29

OMC



elevation is a 50-year-old holly tree. It will not grow much larger, holly is a slow growingtree considered to have a low moisture abstraction capacity and the house is founded ona sandy, gravelly loam. This tree represents no threat at all to the property yet thesurveyors report advised removal.

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

There is an excessively over cautious attitude. This is understandable since we all live ina 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 offactors summarised in this paper that would be required if a realistic conclusion about thepotential threat from a tree is to be made.

At present the existing data, by default, describes situations where trees have causeddamage. 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 damagehas not occurred is indicative of how low the actual risk factor associated with actuallyappears to be. He considers this to be less than 1%.

8/10/2019 Trees Draft 060227

26/29

OMC



Photo 7Case with numerous mature high water demanding trees to property founded on a desiccated clay of PI39% but with no subsidence related damage 0314s (See photo in appendices).

Figure Plan showing layout of above

8/10/2019 Trees Draft 060227

27/29

OMC



Many of the claims or circumstances where damage has occurred (and what much of theavailable data is based on relate to poorly constructed and founded structures. Theextensions, 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 needsto be given to the implications of tree removal. At present decisions are based on apurely monetary one. A cost benefit analysis is applied whether it be to the cost offoundations, 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 climatesparticularly 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 thatpatients convalescing in hospital, recover far more quickly in rooms overlookingmature 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 promotebiodiversity,

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. Onemust try and consider whether 2000 to underpin (or re-build) a conservatory is a moresensible option than 800 to remove a large tree and the possible consequent loss ofvalue to the house (perhaps up to 20%) and, of all its 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 israrely realistically possible.

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

In an ideal world this would include a detailed assessment to gather as much of the

8/10/2019 Trees Draft 060227

28/29

OMC



requisite information to draw a reasoned conclusion. Since this is not realistic this certainminimum criteria should include foundation depths on all parts of the property, analysis ofthe founding soils and a survey from an arboriculturist, (not a tree surgeon!). Thus trialpits would be the norm.

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

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

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

Photo 8 - A pollarded tree

8/10/2019 Trees Draft 060227

29/29

OMC


By way of concluding remarks, the implementation of certain changes in the longer termcould greatly improve the likelihood of peaceful co-existence of tree and structure orremove 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 depththe bulk density increase to the extent that root growth cannot or is unlikely to occur. Ifexcessive, a raft should be constructed.

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

3) Some form of cost benefit analysis adopted that takes into consideration all thebenefits that a tree or trees provide, the cost of structural repair or root barrierinstallation 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 subsidencerelated cover.

Chris Ovebeke can be contacted on:Tel: 01223 842253Email: [email protected]

trees draft 060227

Documents