| [College of ACES] | [University of Illinois] | [Illinois CES] |
Developers are often faced with a number of criteria prior to new build situations. One of these includes subscribing to NHBC guidelines in development situations. Problems arise when the technical data from the current National House Building Council (NHBC) Chapter 4.2 is employed as the template for building near trees. These guidelines became operative from October 1992, although our current focus is towards a more common sense approach, rather than the questionable data on trees cited in Chapter 4.2.
It is accepted that guidelines are needed as a benchmark for building near trees, however, the tree data of Chapter 4.2 raises certain questions. For many years doubts as to tree moisture usage by specific species has been raised with little or no resolve. It was not until the late 1970s that scientific effort was directed towards the role of trees as soil drying agents. Work by prominent arborists led to the production of the updated NHBC guidelines which contained a "water demand" table which categorised trees as High, Medium or Low in their demand for water.
The term water demand has become accepted as a definitive scientific descriptive term. Although it has recently become acknowledged that the term "water demand is not accurate in biological terms", the concept of "water demand" should be taken to mean "the lateral extent, depth and intensity of soil drying which is achieved by different species."
Problems arise because there is little historical statistical data on specific species moisture uptake. NHBC tables indicate trees such as Oak, Poplar, Elm, Willow, Eucalyptus and coniferous Cypress as having a high "water demand." There is no real statistical data to support this theory. It seems bizarre that developers are forced to increase foundation depth close to these trees on the basis of theory and not scientific fact. This somewhat hit and miss situation creates an unnecessary financial burden on the developer.
We must insist that the current Chapter 4.2 guidelines for building near trees are updated yet again. We have learned a great deal since 1992 and this valuable experience should be taken into account when devising new and more reasonable and common sense guidelines based on the tree as a dynamic living organism and not some static object that abstracts soil moisture like some monstrous suction pump.
There is no data on Wellingtonia - Assume worst case scenario eg. High water demand and high shrinkability/low permiable clay soils with remaining innumerable variables.
Why on earth would anyone want to build so close to a 33m high tree. In all events, it seems unlikely that you will get past Local Authority outline planning for development.
Paul H.