Building near trees

4.2

Also See:

4.2.7Foundations in shrinkable soils

Foundations shall be capable of accommodating the effects of trees, shrubs and hedgerows on shrinkable soils without excessive movement. Items to be taken into account include:

foundation type
distance between tree and foundation
method of assessment of foundation depths
foundation depths related to the zone of influence of new tree planting
foundation depths related to new shrub planting.

Landscape and foundation designs should be compatible, and planting schedules produced by a qualified landscape architect or other suitably qualified person and agreed with the local planning authority before work commences on site.

Foundation type

Foundations to all permanent structures, including garages, porches and conservatories, should take account of the effects of soil desiccation. Foundation types that are acceptable in shrinkable soils include strip, trench fill, pier and beam, pile and beam, and raft, providing they:

are capable of supporting the applied loads without undue settlement

include suitable heave precautions.

Variations to the foundation depths derived from this chapter may be permitted where:

it is necessary to take account of local ground conditions

other foundation depths are traditionally acceptable

designed in accordance with Technical Requirement R5.

Root barriers are not an acceptable alternative to the guidance given.

Distance between tree and foundation

The distance (D) between the centre of the trunk and the nearest face of the foundation should be used to derive the foundation depths.

D = 2m where trees which have been, or are to be, removed from within 2m of the face of the proposed foundation are less than 50% of the mature height as given in Table 3. This is to avoid a situation where, for example, a ‘sapling’ removed from the foundation line would otherwise require an unnecessarily deep foundation since the D/H value would always be zero, regardless of the height H of the tree.

Method of assessment of foundation depths

Foundation depths should be determined according to the guidance provided in this document. If in doubt, assume the worst conditions or consult an engineer. Foundations deeper than 2.5m should be designed by an engineer in accordance with Technical Requirement R5.

One of the following methods may be used to assess the foundation depth where foundations are in the zone of influence of existing or proposed trees.

Method	Taking account of	Comments
Design in accordance with this chapter to a depth derived from the charts in Clause 4.2.12, tables in Clause 4.2.13 or the Foundation Depth Calculator App	■ site investigation ■ soil volume change potential ■ water demand of the tree ■ appropriate tree height (H) ■ distance (D) of the tree(s) from the foundations ■ geographical location of the site north and west of London ■ appropriate heave precautions.	The most onerous conditions should be assumed in the absence of derived information.
Design by an engineer in accordance with Technical Requirement R5	■ the recommendations of this chapter ■ site investigation ■ advice, when necessary, from a registered arboriculturalist or other competent person whose qualifications are acceptable to NHBC.	When this method is used and it results in foundation depths or other details less onerous than those derived from this chapter, the design should be submitted to NHBC for approval prior to work commencing on site.

Foundation depths related to the zone of influence of new tree planting

Foundation depths relating to the zone of influence of proposed tree planting should be in accordance with any of the following:

Foundation depth charts in Clause 4.2.12.

Tables in Clause 4.2.13.

The Foundation Depth Calculator App

Minimum foundation depths outside of the zone of influence of trees can be determined from Tables 4 and 5.

Table 4: Minimum foundation depths

Volume change potential	A) Minimum foundation depth (m) (allowing for restricted new planting)	B) Minimum foundation depth (m) (where planting is outside the zone of influence of trees)
High	1.50	1.0
Medium	1.25	0.9
Low	1.0	0.75

Table 5: Where foundation depths are in accordance with column A or column B in Table 4, tree planting should be restricted to:

Water demand	No tree planting zone for column A in Table 4	No tree planting zone / zone of influence for column B in Table 4
High	1.0 x mature height	1.25 x mature height
Moderate	0.5 x mature height	0.75 x mature height
Low	0.2 x mature height	0.50 x mature height

Foundation depths related to new shrub planting

Shrubs have considerable potential to cause changes in soil moisture content. The foundation design should consider shrub planting in accordance with Table 6.

Table 6: Shrub planting

Volume change potential	A) Minimum foundation depth (m)	B) Minimum foundation depth (m)
High	1.50	1.0
Medium	1.25	0.9
Low	1.0	0.75

The foundation design should consider shrub planting as follows:

Shrubs that have a maximum mature height of 1.8m	Use foundation depth from column B.
Climbing shrubs which require wall support and have a maximum mature height of 5.0m	Use foundation depth from column B.
Pyracantha and cotoneaster whose mature height exceeds 1.8m	Use foundation depth from column B and plant at least 1.0 x mature height from foundation, or use foundation depth from column A and plant at least 0.5 x mature height from foundation.
All others	Use foundation depth from column B and plant at least 0.75 x mature height from foundation, or use foundation depth from column A with no restriction on minimum distance from foundation.