4.2 Building near trees

Appendix 4.2-A

Water demand and mature height of trees

Table 12

Note:

1. Where hedgerows contain trees, their effect should be assessed separately. In hedgerows, the height of the species likely to have the greatest effect should be used.
2. Within the classes of water demand, species are listed alphabetically; the order does not signify any gradation in water demand.
3. When the species is known but the sub-species is not, the greatest height listed for the species should be assumed.
4. Further information regarding trees may be obtained from the Arboricultural Association or the Arboricultural Advisory and Information service (see Appendix 4.2-F).

Appendix 4.2-B

Foundation Depth Charts

Table 13 Determination of D/H Value

Chart 1 Soils with HIGH volume change potential: Modified Plasticity Index 40% or greater

(see Design clause D5(b))

Chart 2 Soils with MEDIUM volume change potential: Modified Plasticity Index between 20% and less than 40%

(see Design clause D5(b))

Chart 3 Soils with LOW volume change potential: Modified Plasticity Index 10 to less than 20%

(see Design clause D5(b))

Appendix 4.2-C

Foundation depth tables

Table 14 - HIGH shrinkage soil and HIGH water demand tree 

Broad leafed trees

Coniferous trees

Table 15 - HIGH Shrinkage soil and MODERATE water demand tree

Broad leafed trees

Coniferous trees

Table 16 - HIGH shrinkage soil and LOW water demand tree

Broad leafed trees

Table 17 - MEDIUM shrinkage soil and HIGH water demand tree

Broad leafed trees

Coniferous trees

Table 18 - MEDIUM shrinkage soil and MODERATE water demand tree

Broad leafed trees

Coniferous trees

Table 19 - MEDIUM shrinkage soil and LOW water demand tree

Broad leafed trees

Table 20 - LOW shrinkage soil and HIGH water demand tree

Broad leafed trees

Coniferous trees

Table 21 (a)- LOW shrinkage soil and MODERATE water demand tree

Broad leafed trees

Coniferous trees

Table 22 - LOW shrinkage soil and LOW water demand tree

Broad leafed trees

Appendix 4.2-D

Climate zones

Figure 13 Reductions in foundation depth due to climate variations

The foundation depth may be reduced by the amounts shown on the map for each climatic zone (see Design clause D5(e)). Where it is unclear which zone applies, the lower reduction value should be used.

Appendix 4.2-E

Damage to trees by construction work derived from BS 5837 : 1991

In order to avoid unacceptable damage as a result of construction activities, an area around each remaining tree should be protected from disturbance by fencing. This should not be removed or breached during construction operations without prior consultation with an arboricultural specialist. The fencing should protect as large an area around the tree as possible after consideration of all construction operations in its vicinity. The minimum distance left to be undisturbed around the tree is given in the table below.

Protection of trees: minimum distances for protective fencing around trees

Note:

1. It should be emphasised that this table relates to distances from centre of tree to protective fencing. Other considerations, particularly the need to provide adequate space around the tree including allowances for future growth and working space will usually indicate that the structures should be further away.
2. With appropriate precautions, temporary site works can occur within the protected area, e.g. for access or scaffolding.
3. If it is deemed acceptable for construction works to occur closer than the minimum distance, the distance can be reduced by up to one third on one side only. If distances are reduced in this way, a corresponding increase in distances should be made in other directions.

As an alternative, the fencing may be erected below the outermost limit of the branch spread, or at a distance equal to half the height of the tree, as illustrated below. The distance by this method will usually be significantly greater than the distances in the table above.

Alternative location for protective fencing

Trenching along radii to minimise damage

If it proves essential for a service trench to be taken closer to a tree than the minimum distance in the above table, root damage can be minimised by either:

• thrust boring a hole for the service, or
• radial trenching and tunnelling as illustrated below.

If building closer than the distances recommended in the table below, precautions should be taken to allow for future growth of the tree.

Minimum distance (m) between centre of tree and structure to allow for future tree growth

*These distances assume that some movement and minor damage might occur. Guidance on distances which will generally avoid all damage is given in brackets.

Further information can be obtained by reference to BS 5837.

Appendix 4.2-F

Information sources and acknowledgements

INFORMATION SOURCES

Further recommendations and information can be obtained from:

Publications

BS 1377 'Methods of test for soils for civil engineering purposes'

BS 5837 'Guide for trees in relation to construction'

BS 5930 'Code of practice for site investigations'

BRE Digests 240, 241 and 242 'Low rise buildings on shrinkable clay soils', parts 1, 2 and 3

BRE Digest 298 'The influence of trees on house foundations in clay soils'

BRE Digest 412 'Desiccation in clay soils'

Tree Recognition - A Pocket Manual

by Ian Richardson and Rowena Gale, Richardson's Botanical Identifications,
49/51 Whiteknights Road, Reading, Berks RG6 7BB

Field Guide to the Trees of Britain and Northern Europe

by Alan Mitchell, Harper Collins, Glasgow

Geological survey maps

obtainable from British Geological Survey, Nicker Hill, Keyworth, Nottingham NG12 5GG
Tel: 0115 936 3100; www.bgs.ac.uk

Tree root damage to buildings

Vol.1 Causes, Diagnosis and Remedy
Vol. 2 Patterns of Soil Drying in Proximity to Trees on Clay Soils

by P G Biddle, Willowmead Publishing, Wantage OX12 9JA

Organisations

Arboricultural Association

Ampfield House, Ampfield, nr. Romsey, Hants SO51 9PA
Tel: 01794 368717; www.trees.org.uk

Arboricultural Advisory and Information Service

Forest Research Station, Alice Holt Lodge, Wrecclesham, Farnham, Surrey GU10 4LH
Tel: 01420 22022; www.treehelp.info
(Tree Helpline telephone no. 0906 516 1147)

Institution of Civil Engineers

1-7 Great George Street, London SW1P 3AA
Tel: 020 7222 7722; www.ice.org.uk

Institution of Structural Engineers

11 Upper Belgrave Street, London SW1X 8BH
Tel: 020 7235 4535; www.istructe.org.uk

ACKNOWLEDGEMENTS

NHBC gratefully acknowledges the help given by authoritative organisations and individuals in the preparation of this Chapter, particularly:

Building Research Establishment

Dr P G Biddle Arboricultural Consultant

Appendix 4.2-G

Worked example

How to determine foundation depths from the Charts in Appendix 4.2-B or the Tables in Appendix 4.2-C.

Step		Ref	Example
1	Determine the volume change potential of the soil. Ensure the site investigation includes representative sampling and testing.	D5(b)	Site at Oxford, building near a Lombardy Poplar (to be retained) and a Sycamore (to be removed) From laboratory tests, Plasticity Index, Ip = 36%. Test results also report that 100% of particles are smaller than 425µm. Therefore, modified Plasticity Index, From Table 1, Volume change potential = Medium (in the absence of tests assume high volume change potential) This example is typical of Oxford Clay. More than 35% of the particles are smaller than 60µm and therefore the soil is shrinkable. 100% of the particles are smaller than 425µm and therefore the l'p is the same as the lp. A typical Boulder Clay also has more than 35% of particles smaller than 60µm and is therefore also shrinkable. However, it may have only 80% of its particles smaller than 425µm in which case the l'p is 80% of the lp. A typical clayey sand may have less than 30% of its particles smaller than 60µm in which case the soil would be non shrinkable.
2	Establish the species, mature height and water demand of all trees and hedgerows within their influencing radii.	D5(c) and D5(d)	Lombardy Poplar Sycamore From Appendix 4.2-A From Appendix 4.2-A Mature height = 25m Water demand = High Mature height = 22m Water demand = Moderate
3	Plot the trees and hedgerows relative to the foundations and draw their zones of influence to determine which trees will affect the foundation design. Use a scaled plan.	D5(c)
4	Establish the appropriate tree height H to use. Always use the mature height for remaining and proposed trees and hedgerows. The appropriate height to use for removed trees and hedgerows depends on the actual height when they are removed.	D5(d)	Lombardy Poplar Sycamore Tree to remain. Therefore, H = Mature height = 25m Tree to be removed Mature height = 22m Actual height = 15m Actual height greater than 50% mature height. Therefore, H = Mature height = 22m
5	Measure the distance D from the centre of the trees or hedgerows to the face of the foundation.	D6(c)	Lombardy Poplar Sycamore Distance D = 10m from foundation Distance D = 8m from foundation
6	Select Steps 6C(a) and (b) if using Charts in Appendix 4.2-B to derive depths or select Step 6T if using Tables in Appendix 4.2-C to derive depths. Alternatively the NHBC foundation depth calculator may be used (see Sitework clause S2).
6C (a)	Calculate D/H i.e. distance D from face of foundation (Step 5) divided by the appropriate tree height H (Step 4). Alternatively D/H can be obtained from Table 13 in Appendix 4.2-B.		Lombardy Poplar Sycamore
6C(b)	Determine foundation depth using the Charts in Appendix 4.2-B as follows: Volume change potential Chart number High 1 Medium 2 Low 3		Lombardy Poplar Sycamore In this example the volume change potential is Medium, then from Chart 2 for broadleafed high water demand trees at Foundation depth = 2.33m In this example the volume change potential is Medium, then from Chart 2 for broadleafed moderate water demand trees at Foundation depth = 1.50m The Lombardy Poplar is the tree requiring the greater depth (2.33m)
6T	Determine foundation depth using the Tables in Appendix 4.2-C as follows: Volume change potential Tree water demand Table number High High Moderate Low 14 15 16 Medium High Moderate Low 17 18 19 Low High Moderate Low 20 21 22		Lombardy Poplar Sycamore In this example the volume change potential is Medium and the water demand is High, then from Table 17, for broad leafed high water demand trees at D = 10m and H = 25m, Foundation depth = 2.33m (by interpolation) In this example the volume change potential is Medium and the water demand is Moderate, then from Table 18, for broad leafed moderate water demand trees at D = 8m and H = 22m, Foundation depth = 1.50m The Lombardy Poplar is the tree requiring the greater depth (2.33m)
7	Adjust the depth according to the climatic zone. A reduction may be made for distance north and west of London but the final depth should not be less than the minimum given in each Chart and Table.	D5(e)	Oxford is between 50 and 100 miles NW of London. From Appendix 4.2-D, a reduction of 0.05m is permitted. Final foundation depth = 2.33 - 0.05 = 2.28m
8	Check that the recommendations of this Chapter have been met for: Acceptable foundation types New planting (including shrubs) Non shrinkable soil overlying shrinkable soil Variations in foundation depths Foundations on sloping ground Precautions against heave (including suspended floors) Measurement of foundation depths Foundation trench bottoms Precautions for drainage	D6(a) D6(d), D6(e) D6(f) D6(g), S3(b) D6(h) D8,S4 S3(a) S3(c) S5

 Note:

The above process may be repeated to allow the foundation to be stepped as its distance from the tree increases.