When a tree canopy is too low then the canopy will need lifting or raising. The circumstances vary but often it can be due to being too low over a footpath e.g. below 2.5m clearance. Too low over a road (5.2m clearance). Low over a lawn preventing light or causing risk of eye injury to whoever mows the site in which case 2.5m clearance is generally used. When trees are close to properties then raising the canopy can allow light to come underneath the foliage, this is particularly useful when large trees are concerned and reductions will not remedy a light issue. Sometimes when trees are in direct contact the canopy can be raised on the building side to create clearance between the tree and building. Care must be taken not to unbalance the tree to a degree where it becomes unstable.
When specifying the raising or lifting there are two common ways employed to determine the clearance. Firstly is to give a measurement from the lowest point of the canopy. This is more often employed when the canopy is being raised to allow light etc. However, BS3998 Tree Work Recommendations suggest the use of measuring from ground level and giving the clearance height before the lowest branches / foliage are present. So with the latter specification “raising the canopy to 5.2m” would indicate that there would be no foliage present on the tree upto a height of 5.2m from ground level.
When raising the canopy consideration should be given to the clearance when in full leaf or out of leaf. Pruning cuts need to be in the correct position so that reshooting does not immediately occur, thus exacerbating the situation.
It is also very important not to over raise the tree as branches act as shock absorbers reducing the twist and flexing of the stem.