The NHBC Standard 2008 came into force in September 2008. Chapter 4.2 of this document is the industry standard for calculation foundation design in relation to existing and proposed trees. The revised standard has some significant consequences for Engineers, Architects, Arboriculturists and Landscape Architects.

  1. There is a section included, on how to calculate soil heave following the removal of a tree. A pre-strip survey of all vegetation is required, otherwise an engineer has calculate the worse conditions – i.e. all trees are high water demand and therefore bigger foundations are required.
  2. Foundations of garages, porches and free standing walls must be taken into account – previously these tended to be ignored in relation to trees. Now an engineer will have to build bigger foundations.
  3. Root barriers – ‘are not a reliable means of reducing the effects of trees on foundations’. This weakens the argument for placing larger trees close to buildings and compensating with root barriers.
  4. Foundations now have to take into account shrubs and climbers:
    • shrubs/climbers under 1.8m mature height – no affect
    • Pyracantha and Cotoneaster with mature height over 1.8m, plant 1X mature height away from foundation to avoid any affect – closer bigger foundation
    • All other shrubs/climbers over 1.8m – plant 0.75x mature height away from foundation to avoid affect.

Conclusions –

Early site survey work could reduce the depth of foundations by identifying tree species prior to their removal.

Sufficient space within a development will need to be provided by layout designers.

Conflicts for structural landscaping and the cost of additional foundations will be common as developers strive to achieve higher densities and site viability.