This will depend on what you are trying to protect or achieve. As a general rule, don’t direct roots deeper than you need to. For example, don t go 1m deep to protect a pedestrian kerb structure where a 300mm deep barrier would be sufficient.
Always give the tree as much space as possible. For smaller species we can use small root directors to manage roots downwards. If we are installing a barrier deeper than a RootDirector all around the tree then we need to take into consideration the trees need for anchorage and access to soil nutrients.
We would normally suggest that a barrier extends to a depth of 2-300mm below the invert level of the service or utility.
The recommended distance is again about 300mm away from the utility. This will ensure that any pressure against the root barrier is not transmitted directly to the service utility. The 300mm is effectively a buffer zone.
Generally we find that if you can show that you have planned the tree planting design to incorporate protection for their utility, they have no difficulty. What they do not like is indiscriminate planting without regard to their investment. We do have on file some copies of letter from utilities approving the use of ReRoot 2000 for protection of their installations.
Any root barrier used must finish at least 10mm above any planting medium (i.e. topsoil) on the tree side otherwise roots could grow over the top. This top edge can be incorporated in a pedestrian kerb detail or disguised by groundcover plants or suitable edging material. It does need protecting from traffic or mowers which could damage it.