Root Zone Protection Calculator

Critical Root Zone (CRZ) Radius starts from the common arborist rule of thumb of 1 foot of radius per inch of Trunk Diameter (DBH), then applies a conservative widening for older trees, capped at double the base radius by age 100, and for soil type, where sandy soil widens it further. That age adjustment is this calculator's own safety margin, not part of the ISA rule — the published standard is a function of trunk diameter alone, since DBH already encodes most of what age would tell you. The 1-foot-per-inch figure is a floor, not a midpoint: the International Society of Arboriculture treats it as the minimum and municipal codes that depart from it use 1.25 or 1.5 feet per inch, so the clay-soil setting reduces the widening applied on top of the base rather than shrinking the zone below it. Trunk Diameter sets the entire base radius before either the age or soil adjustment is applied, so raising it always raises CRZ Radius directly — though switching Soil Type between its sandy and clay extremes (a 1.5x spread) can shift the result by more than a modest change to Trunk Diameter alone would.

Construction Type does not change CRZ Radius itself — CRZ Radius represents the tree's full root zone regardless of what work is planned nearby — instead it scales CRZ Radius down to a smaller Min Protection Radius, reflecting this calculator's own ordering of how immediately destructive each activity is to roots: trenching severs them outright, grading removes soil over them, and paving covers them. That ordering is about immediate root severance, not long-term health — permanent paving over a root zone eliminates gas and water exchange and can be the more damaging of the three over a decade, so treat the paving setting as the least conservative option rather than a safe one. Protection Fence Length is simply the circumference of that minimum protection circle, and Penalty Zone Depth is the gap between the full CRZ and the minimum protection radius — the band where root damage becomes likely if construction activity encroaches. This is a planning-level rule-of-thumb estimate, not a substitute for an on-site root system assessment by a certified arborist, particularly for high-value or already-stressed trees.