Tree root development is entirely opportunistic and spreads horizontally to a distance and depth entirely dependent upon the ground conditions encountered. Very few trees have a 'tap root' after the first few years. Roots require oxygen and water to function and therefore most will remain close to the surface, research has shown that 90% of tree roots are to be found in the top 600mm of soil. Roots may extend horizontally for considerable distances and where conditions are suitable this distance may be equivalent to two or even three times the tree height.
The majority of roots are the easily overlooked fine, fibrous roots that absorb water, oxygen and nutrients from the soil; these are easily damaged by crushing and removal during soil stripping operations. The main structural support roots are usually found within a few metres of the tree stem and these are linked to the fibrous roots by a network of cable like roots that also provide additional anchorage.
All tree roots are important.
Root protection to prevent long-term damage
The guidance contained within BS 5837 Trees in relation to design, demolition and construction – Recommendations identifies a Root Protection Area (RPA) based on the stem diameter but protective measures may need to be increased, for example, to the extent of the branch spread to avoid damage to the above ground parts of the tree.
Tree protective measures are detailed within the British Standard, the default specification for a protective barrier is shown in the diagram below.