Trees and Houses: A Question of Function

Don Cameron, Experimental CS1RO Division of Building and Ivan Earl, Treemasters Pty Ltd Published by Cement and Concrete Association of Australia 1982

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CONSULTING ENGINEERS

E'5 PACIFIC HIGHWAY, PYMBLE, NSW 2073

TREES AND HOUSES : A QUESTION OF FUNCTION

Don Cameron, Experimental Officer, CSIRO Division of Building Research

and Ivan Earl, Treemasters Pty Ltd

INTRODUCTION

New houseowners usually wish to plant their own choice of shrubs and

trees in their new gardens. Frequently, replanting of an existing

established garden is required. Fast-growing trees are often planted

haphazardly to quickly change the garden's appearance. Although trees

and shrubs are desirable on a suburban block, if the garden has not been

planned carefully, much time, effort and money can be wasted. In

particular, the function of adjacent buildings may be impaired if the

interrelationship of trees and the environment is not appreciated.

TREES AND THE ENVIRONMENT

Soil Water

Trees need water. The fine hair-like roots at the extremities of a

tree's main root structure collect water stored in the soil. The type of

soil determines the amount of water it can store in two ways. Sleet

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Firstly, soil permeability controls both the amount and the extent of

penetration of rainfall. Sands and gravels are very permeable whereas

heavy clays only accept water very slowly. However, heavy clays can dry

out sufficiently to form an extensive pattern of cracks to some depth

below the ground surface. This cracking greatly increases the clay's

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permeability until sufficient water enters the cracks, then the clay

swells and the cracks close.

Secondly, the amount of water able to be held in a soil is controlled by

how strongly the water is held by the soil particles. In simple terms,

the water-holding forces are stronger for soils with smaller particles.

Of the different soil types, clays have the smallest particle size,

followed by silts, loans, sands and gravels*. So clays can contain

relatively large amounts of water.

water Uptake of Trees

Almost all water taken up by the roots is transpired through the leaves

of the tree. Water is required by the leaves to produce sugars for tree

growth. Indeed, water lost through transpiration can be related to tree

growth rate.

'Footnote: The three major soil types can be roughly identified by first

wetting the soil, kneading it (compacting it between the fingers) and

then describing the feel of the soil. Generally speaking, clove feel

either sticky or puggy, and leave stains on the hands after kneading;

silts feel smooth to silky and sometimes seem spongy when they are

kneaded; gysnds feel coarse or gritty and tend to fall apart during

kneading. If the sand is fairly coarse, the grains may be seen.

Tree species vary in their ability to produce sugars for a given amount

of water. One species may require relatively large volumes of water to

produce a growth rate similar to another species which requires

relatively little water.

A broad indicator of a tree's potential water demand is its total leaf

area, which governs its ability to transpire moisture to the atmosphere.

The amount of exposed leafage is largely determined by the size and

shape of the tree canopy and the thickness of the foliage, both of which

are species characteristics.

Water uptake is also influenced by the environmental conditions, such as

amount of sunlight, nutrient supply and availability of soil water.

Although water demand is difficult to estimate, some examples have been

cited, e.g. poplars 60 000 litres per year (Ward') and apple trees 20

000 litres per year (Bush'). A rough indication of the relative water

demand of commonly grown South Australian trees can be derived from a

large scale study of tree root intrusion into sewers, undertaken by the

South Australian Engineering and Water Supply Department. Table 1,

listing particularly 'thirsty' tree species, originated from this study.

The table shbuld be used only as a rough guideline; it is not fully

comprehensive and it may require revision as further data become

available.

Jree Root Development

The roots of most trees are able to adapt to different site conditions.

On sloping sites, the majority of roots develop up-slope to provide

stability for the tree. Soil type also has a pronounced effect on root

growth. Deep loamy soils which hold adequate supplies of moisture

encourage fine fibrous root systems. In heavy, dry clay soils, the roots

must penetrate a far greater volume of soil and so they tend to be much

thicker and longer with fewer fibrous roots. Otherwise the tree may

become stunted. Root growth is unlikely through either solid rock or

high water tables and is discouraged in poorly aerated or heavily

compacted soil. Water tables are areas below ground where the soil is

very wet. Holes dug through water tables soon fill up with water. In

Melbourne, however, water tables are generally so low that they have

little effect on root development.

Some trees have deep tap roots, which give them stability in the early

stages of their growth. However, the lateral root system which later

develops soon becomes the primary water collector. These lateral roots

may extend sideways from the trunk between 0.4 and 2.1 times the height

of the tree (Yeager'). The depth of root penetration in a clay soil is

usually between 1 and 2 m.

surrounding soil and, in extreme cases, uplifting of the adjacent

structure. Structures lighter than houses, such as fences and paths are

normally affected.

Secondly, a more common and more serious problem occurs when a tree

extracts moisture from a clay soil below a house footing and the soil

shrinks. The concrete footings bf the house will settle in much the same

pattern as the underlying soil unless they have been specially designed.

As the footings settle, brick walls crack, timber frames distort, and

doors and windows may cease to function effectively.

The five major factors which determine the risk of damage due to

shrinkage settlement are as follows:

(a) Soil type

Only clays show appreciable shrinkage on drying. Soils, however,

are normally a mixture of sand, silt and clay.

The more clayey a soil is, the more shrinkable it is.

TREES NEAR BUILDINGS

Introduction

Trees can damage houses in two ways. Firstly, a problem arises with

trees which develop large, deep boles (e.g. liquidambars, figs and

willows). The bole is that section of the tree where the trunk and roots

meet below ground level. If such trees are located against the building

perimeter, the growth in root diameter may cause compacting of

As well, clays vary in mineral composition and can therefore

respond differently to drying. The grey-brown basaltic clays of

Melbourne's western suburbs are more highly shrinkable than the

yellow-brown and grey clays derived from mudstones of the majority

of the eastern suburbs.

(b) Soil profile

The soil profile is the vertical arrangement of soil and rock

layers below the surface.

The highest risk situation is where shrinkable clays extend from

the ground surface to at least 2 m depth. If the clays are overlaid

by sands, silts or other material which does not shrink appreciably

when dried, then the risk of tree-affected settlement lessens as

the depth of this inert layer increases. If this depth exceeds 1.0

to 1.5 m, the risk could be considered to be negligible.

Where clay extends from the surface to a layer of bedrock or

permanent water table at less than 2 m depth, the thickness of the

clay layer will determine how much shrinkage settlement can be

expected. As the thickness of the clay layer decreases, so the

level of risk will decrease accordingly.

7

tree root growth patterns and the variations in trees' demand for

water. Fortunately, recent (1980) amendments to the Victorian

Uniform Building Regulations have resulted in the adoption of

stiffer footings for houses, which should lessen the risk of

damage. Where the risk of damage to a house by an expected tree

planting is deemed to be high, then the designer should consider

the higher categories of footing types offered in the Regulations.

Concrete slab-on-ground floors have performed much better in

Melbourne than conventional footings because they are generally

stiffer and less prone to rotational movements. The edge beams of

the slab are structurally tied into the floor and the internal grid

of stiffening beams. However, slabs can be affected by severe

shrinkage settlement and so some care must still be exercised.

(c) Tree species and proximity to building

The species of tree determines its potential water *uptake, the

pattern of root development for a given site and the tree's

hardiness.

The proximity of a tree to a building determines whether the soil

below the footing is within range of the roots of the tree in

question.

(d) Footing design

Footings can be designed to reduce potential movement. The footing

can be either stiffened with additional concrete and reinforcement

or based at a depth below the drying zone of the tree roots.

However, both alternatives require the designer to assess the

effect of specific tree plantings. The information he requires is

unfortunately difficult to obtain because of the complexities of

(e) Age of tree relative to age of house

Most problems occur when trees are planted after construction of a

house. Where a building is located near a mature tree without

either severing the roots of that tree during excavation works or

substantially altering the water supply to the soil, then the risk

of building damage may be considered to be negligible. If, however,

the roots are severed and root development is regenerated, some

damage may follow. Trees like poplars, ashes and willows develop

root fibres at the severed roots that eventually develop into

larger roots and so shrinkage settlements may occur 10 to 20 years

after construction. Most native trees respond poorly to damage of

their major arterial roots. They are more likely to die than

regenerate. A tree expert should be consulted in such cases to

consider the likelihood of root regeneration .

In any case, large trees within 2 to 3 m of the proposed building

line should normally be removed because of potential instability

problems. The ground adjacent to the felled tree should be

thoroughly soaked prior to construction to reduce the potential

effects of subsequent ground swell on the building.

Experiences in Melbourne

Studies of tree damage throughout Melbourne have not been comprehensive

enough to establish with any certainty whether particular species are

more dangerous than others. Present indications are that almost any

medium to large tree can cause damage if conditions are right. All trees

use water. Even if their water demand is relatively low, they can cause

damage if they are close enough.

However, a listing can be prepared, on a subjective basis only,of trees

which are commonly linked with building damage. Table 2 was prepared in

this fashion by Treemasters Pty Ltd. The reasons why some trees are. more

frequently involved than others may simply be that, in some cases,

because of their form or shape, they are commonly placed closa to

buildings (e.g. pin oak, bamboo leaved willow). Table 2 does not

represent a rigorous scientific study and should only be used as a rough

indicator of possible troublesome species.

Most cases of damage have been confined to the eastern and south-eastern

suburbs, where the clay soils are favourable for tree growth. The heavy

clays of the north and west, although highly shrinkable, appear to be

less of a risk since gardens are more difficult to establish in these

areas.

The incidence of damage is not always predictable. Situations are

encountered where one would expect damage but none has occurred.

The majority of houses throughout the eastern and south-eastern suburbs

consist of single storey brick veneer construction supported by lightly

stiffened concrete footings founded at only 450 mm. Cases of damage to

this class of structure have been reported most frequently where trees

were located closer to the building than 0.5 times their height. Where a

row of trees lines the side of a house, the wall may tilt towards the

trees so that adjoining walls separate and crack. More commonly, trees

at corners of buildings cause the corners to settle. The damage becomes

evident during dry spells. Thereafter cracks may close during winter,

only to widen the following summer. The degree of damage generally

varies from slight to moderate. Moderate damage may mean that doors and

windows have ceased to function properly.

PREVENTION OF DAMAGE

Preventative action should match the risk of damage, as outlined in

Table 3. The level of•risk is assessed by consideration of all site

conditions.

From our experiences in Melbourne, it would seem that the incidence of

tree damage may be reduced most effectively by implementing simple

planting rules for all species of trees. Table 3 gives planting rules

for low, moderate and high risk sites, specifying that trees be planted

at distances of 0.5, 0.75 and 1.0 times their mature height from the

10

building respectively'. Alternative preventative actions are given where

trees are required or have been planted closer than these rules permit.

Soaking of the ground to prevent excessive loss of soil moisture and

hence shrinkage is most economically achieved by using a drip feed

system rather than a continuous spray of water. Garden mulches can be

used in conjunction with soaking to prevent evaporative water losses.

Foliage pruning reduces leaf transpiration losses and so temporarily

reduces the water demand of the tree.

Cut-off walls are merely a physical barrier between the roots and the

building. In order to be successful, they must be impenetrable and of

adequate depth and width to permanently dissuade roots from developing

on the building side of the wall. For long term protection, lightly

reinforced concrete of 150 mm thickness should be used. Depths of 1.5 m

are commonly recommended but each site requires individual

consideration, preferably by a foundation engineer.

GETTING ADVICE

Wherever possible, horticultural or arboricultural authorities should be

consulted in the initial planning of a garden to ensure the correct soil

'Footnote: These planting rules apply for relatively light planting

densities. Heavier plantings such as rows or 'bush' gardens cause fierce

root competition and more extensive ground drying.

11

preparation, the appropriate selection of compatible plant material,

themaking of a manageable garden and the minimization of the risk of

damage to buildings. A foundation engineer may be required to help

assess the risk of damage to buildings and to suggest appropriate

preventative action if trees need to be placed close to the structure,

i.e. for most sites within 0.75 times their expected mature height on a

clayey site.

Information can be obtained from other sources such as nurseries and

current literature, but some caution is required when evaluating its

worth. Host nurserymen, if experienced, are extremely knowledgeable and

are a wealth of information on all aspects of trees. If there is some

doubt regarding the value of the information from the nursery, a

suitable botanical reference book should suffice as a cross-check.

Gardening books often quote a range of expected mature heights of trees

for Australian conditions, but don't give estimated tree growth for

specific local conditions. As briefly explained earlier, owing to the

complexities of nature it is almost impossible to predict growth

patterns for a certain area unless local experience with that species is

available. Therefore, when considering where to locate trees in a

garden, it would be best to assume the maximum potential growth will be

achieved unless there is suitable local knowledge to the contrary.

IN SUMNARY

Trees can cause damage to houses. There are many complex factors

involved when assessing the risk of damage for a particular situation.

As a general rule of thumb for most sites, if you plan to have trees

TABLE 1 PARTICULARLY 'THIRSTY' TREE SPECIES

Mature Botanical name Common name

height

H(m)

Anoophora costae Araucaris heterophvlla (and similar species) Casuarina cunninghamiang Casuarina glaucq gEicma species Cupressus species Eucalyptus bridgesianq Eucalyptus canaldulensis Eucalyptus citriodorq Eucalyptus cladocalyx Eucalyptus cornutA Eucalyptus diversicolor Eucalyptus globulus Eucalyptus leucoxylog Eucalyptus maculate Eucalyptus occidental's Eucalyptus rubidA Eucalyptus viminalis Ficus species Fraxinus oxvcarna Fraxinus "Raywood" (unless grafted or budded onto a rootstock of Fraxinus ornus (Hanna ash))

Grevillea robust Phoenix species pins species platanus species populus niora (and similar species) Quercus robur (and similar species) Robinia pseudoacacia Salix babylonicA (and similar species) $alix chilensis "Fastigiate" $chinus molls Tamaris aphYllA Ulmus procerA (and similar species)

Smooth-barked apple Norfolk Island pine River sheoak Swamp ehmoak Cedars Cypress But-but River red gum Lemon-scented Sugar gum Yate Kerr' Tasmanian blue Yellow gum Spotted gum Flat-topped yate Candlebark Manna gum Figs Desert ash

Claret ash Southern silky oak Date palms Pines Planes Black poplar English oak False acacia, Weeping willow Chilean willow Pepper tree Athel tree English elm

15 - 24 30 - 60 12 - 30 12 - 15

variable variable

24 - 30 gum to 15

15 - 30 9 - 18 to 60

gum 30 - 60 4.5 - 7.5 18 - 30

9-30 9-60 to 30 9 - 15

9 - 15 15 - 30

variable to 30

15 - 36 to 24 to 20

Black locust 9 - 15 9 - 15

6 - - 15 to 6 to 30

12 13

closer to your house than 0.75 times their expected mature height and

you know the soil is essentially clayey, then a foundation engineer or

tree expert should preferably be consulted. Once the risk for the site

is determined, then the appropriate planting rule should be adopted, if

other preventative measures are not considered. The rules are as

follows. For low, moderate and high risk sites, trees should be planted

a minimum distance of 0.5, 0.75 and 1.0 times their expected mature

height from the building respectively. Alternatively, action may be

taken which will allow closer plantings without causing damage to

neighbouring structures. Again, the preventative action should match the

level of risk: it may range from regular tree maintenance, construction

of concrete cut-off walls or re-design of footings.

REFERENCES

1. Ward, W.H. (1947). The effects of fast growing trees and shrubs on

shallow foundations. Journal of the Institute of Landscape

Architects, No. 11, pp. 7-16.

2. Bush, R. (1943). 'Tree Fruit Growing. VI'. (Penguin Books: London.)

3. Yeager, A.F. (1935). Root systems of certain trees and shrubs grown

on prairie soils. Journal of Agricultural Research, V51, pp. 1085-

1092.

The information contained in this table has been derived from:

- Baker, P.D. Tree root intrusion into sewers - Progress Report No. 2: Analysis of root chokes by species. Engineering and Water Supply Dept, SA, Sewerage Branch, Aug. 1978.

- Lord, E.E. (1970). 'Shrubs and Trees for Australian Gardens. 4th Edn. (Lothian Publishing Co.: Aust.)

TABLE 2 TREES FREQUENTLY ASSOCIATED WITH BUILDING DAMAGE IN MELBOURNE

(From Treemasters Pty Ltd)

Botanical name Genera Species

Common name

RISK OF DAMAGE'

PREVENTATIVE ACTION

Minimal Regular watering below the drip line of the tree (line below outer extremity of foliage)

Low 1. Locate trees at a distance from the building equal to 0.5 times their estimated mature height; or

2. Employ water reservoirs and/or regular foliige pruning

Moderate 1. Redesign footings to accomodate settlement, or 2. Employ cut-off walls, or 3. Locate trees at a distance from the building equal

to 0.75 times their estimated mature height; or 4. Regular deep root pruning of close trees if they

are of suitable hardy species.

High 1. Redesign footings to accomodate settlement, or 2. Employ cut-off walls, or 3. Locate trees at a distance from the building equal

to 1.0 times their estimated mature height

'Assessed from site conditions

15

14 TABLE 3 ACTION REQUIRED TO AVOID DAMAGE

Acacia dealbata decurrens

Aux =gunk Alnus iorrulensis Cuoressus torulosa Eucalyptus. botrvoidea

Citriodora gladocalvx globulua =Alta nicholi1 Oda rshmata viminalis OMEN M ravvoodii robust.% atvraciflug Nora var. Italica palustris babylonicet ghilensia pacera grocer* var. Louis van Houtte

Silver wattle Black wattle Box elder Evergreen birch Bhutan cypress Mahogany gum Lemon-scented gum Sugar gum Tasmanian blue gum Spotted gum NSW peppermint gum Swamp gum Swamp mahogany Manna gum Desert ash Claret ash Silky oak Sweet gum Lombardy poplar Pin oak Weeping willow Bamboo leaved willow English elm

Golden elm

Fraxinus

Grevillea Liouidambar Pooulus Quercus 2olix

Ulmuq

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