Tree Protection / Critical Root Zone
The ANSI A300 Part 5 tree protection zone (critical root zone) to fence off during construction.
Example
You enter
- DBH (in) 20
- Radius factor 1
You get
- Protection radius 20
- Fenced area 1257 ft^2
Checked against published worked examples from: ANSI A300 Part 5 / ISA critical root zone.
Details, formula, and sources
Scoperadius = radius_factor x DBH (1.0 ft/in standard, 1.5 ft/in mature), area = pi x radius^2. The zone is set by the trunk diameter, NOT the canopy dripline -- a narrow-crowned tree still needs the full radius, so fencing to the visible canopy under-protects the roots.
Formula or tableradius = radius_factor x DBH; area = pi x radius^2 (radius_factor 1.0 ft/in standard, 1.5 ft/in conservative/mature).
Source and editionTree protection zone / critical root zone (ANSI A300 Part 5; ISA critical root zone), by name.
Free accessThe one-foot-per-inch protection-radius rule is published arboricultural practice; a qualified arborist and the local ordinance govern.
What governsEstimate. AHJ and licensed professional govern.
API field namesdbh_in, radius_factor, radius_ft, area_ft2
- Trunk-based radius radius = radius_factor x DBH, set by the trunk not the canopy; 1.0 ft/in standard, 1.5 ft/in matureANSI A300 Part 5
- Cumulative protection fencing plus no grade change, compaction, or trenching to the protected soil depthISA critical root zone