The Critical Root Zone (CRZ) is so important not to disturb to keep your trees healthy and strong. Ideally, tree roots should not be cut if there are other options. If you must disturb the CRZ, that is where root pruning comes in. Roots are typically pruned when they interfere with infrastructure to limit vascular disease or to correct defects that affect tree health or stability. Much root cutting occurs on construction sites during excavation for foundations, installing underground utilities, and changing the grade.
The consequences of root pruning have not been well studied. Small-diameter root cuts and damage limited to root sapwood are thought to be compartmentalized well and don't usually lead to extensive decay. When larger roots are cut, and heartwood is exposed, decay is more likely to occur.
There are two general methods used in cutting roots. Selective root pruning is one; this involves locating and exposing roots, then making targeted cuts. The second way is nonselective root cutting, which involves mechanical excavation trenching the solid. This is indiscriminate root cutting.
To minimize tree health impacts when roots must be pruned, they should be cut as far from the trunk as practical.