# Root-cut estimator

> Enter a tree's trunk diameter and how close a trench, fence or driveway will run. See its root zones, the share of the zone the cut takes and a safer distance.

Blue Whale Trees, a native tree nursery in the United States. Web version: https://bluewhaletrees.com/tools/root-cut/

Before a driveway, a fence or a utility trench goes in near a tree, the estimator shows how much of the tree's root zone the digging takes. Enter the trunk's diameter, the distance from the trunk to the digging, the shape of the cut and its depth. It gives both root zones, the share of the critical zone beyond the cut, how that share compares with the limits, and the closest distance within them. The estimator needs only the trunk's diameter, so it covers every tree and shrub we grow, and any other with a trunk. Every rule is shown with its source, and the answer is an estimate for planning, not an arborist's verdict.

The estimator needs JavaScript. These are its rules. The critical root zone is a circle around the trunk whose radius in feet equals the trunk's diameter in inches, by the City of Austin's Environmental Criteria Manual. Austin measures the diameter 4.5 feet above the ground. Arlington County and Fairfax County use the same zone, and Purdue Extension gives 1.5 feet per inch for sensitive trees. The structural root zone, which Arlington County calls the area essential for the tree's stability, reaches six trunk diameters. The share a cut takes is the part of the critical root zone's area beyond it. A straight line leaves a segment of the circle beyond it, two lines on opposite sides twice that, and a cut all the way round the whole ring outside it. Up to a quarter of the zone, Purdue Extension and University of Maryland Extension expect a tree to recover. Past 40 percent, Fairfax County warns that the tree may not survive, and Mississippi State Extension says to move the trench. The City of Austin counts a tree as removed when less than half its zone stays at natural grade. Austin allows no cut deeper than 4 inches within half the zone's radius, and none at all within a quarter. Bartlett Tree Research Laboratories prefers a cut on one side no closer than three trunk diameters, and cuts on more than one side no closer than five to six. Closer than 1.5 trunk diameters, Bartlett finds stability very likely affected and says removal should be considered. Any cut inside the structural root zone of a tree that could hit a house, a road or people is a question for a certified arborist first. Bartlett and the University of Florida find saturated soil or a leaning tree makes the same cut worse, and Bartlett keeps grade cuts on a slope outside the dripline.

## How to dig near a tree

- **Measure to the edge of the digging.** The City of Austin draws the line where the excavation runs, not where the finished edge sits, since a curb is dug out behind its back.
- **Go under instead.** Bartlett Tree Research Laboratories and the City of Austin both name boring or tunneling under the roots as the alternative to a trench.
- **Expose the roots, then cut them clean.** Bartlett's laboratory calls a trencher or an excavator one of the worst ways to cut roots. Dig by hand, with air or with water, and cut with a sharp saw.
- **Spare the big roots.** Bartlett's laboratory avoids cutting a root over 3 inches across wherever it can, and on a mature tree any root over 2 inches.
- **Move the walk, or bridge it.** Bartlett's laboratory curves a sidewalk around a tree or raises it over the roots, which spares the large roots that hold the tree up.
- **Backfill and water the same day.** Purdue Extension asks for the trench to be backfilled within an hour of cutting roots and the tree watered within 24 hours.
- **Look up at the crown.** Bartlett's laboratory finds that root cuts outside a normal dripline rarely cause permanent damage, and Purdue Extension keeps all work out from under it.
- **Keep machines off the rest of the zone.** Purdue Extension warns that compacting the soil around a tree destroys its fine absorbing roots, so keep vehicles and stacked materials off it.
- **Watch for years.** Purdue Extension finds that a tree often takes several years to show construction damage, so a tree that looks well the next summer may still decline.

The critical root zone follows the City of Austin's [Environmental Criteria Manual](https://library.municode.com/tx/austin/codes/environmental_criteria_manual?nodeId=S3TRNAARPR_3.5.0DECR_3.5.2PR), section 3.5.2, where its radius is "a number of feet equal to the diameter in inches of the tree". Austin measures the trunk "at 4-½ feet above grade", and a tree with several trunks as "the diameter of the largest trunk plus ½ the diameter of each additional trunk". [Arlington County's standard detail 311300.3](https://www.arlingtonva.us/files/sharedassets/public/v/3/forestry/311300.3-crz-determination.pdf) and [Fairfax County](https://www.fairfaxcounty.gov/landdevelopment/protecting-your-trees-during-neighboring-construction) give the zone the same foot per inch. The sources disagree on its size. [Purdue Extension's Construction and Trees](https://www.extension.purdue.edu/extmedia/fnr/fnr-463-w.pdf) multiplies the diameter "by 1.5 to get the distance in feet" for trees grown in woods and young trees. [Mississippi State University Extension](https://extension.msstate.edu/publications/preserving-trees-construction-sites) gives "1.25 feet of protection radius for every 1 inch of stem diameter", and 1.5 for sensitive trees. The estimator follows the foot per inch, since Austin's and Fairfax County's limits are measured on that zone, and shows Purdue's wider zone beside it. Arlington County takes a tree with several stems as the square root of the sum of their squares, a smaller figure, and the hint gives Austin's.

The structural root zone is Arlington County's: the area essential for the tree's stability, generally six trunk diameters from the base of the trunk. The stability distances are those of Bartlett Tree Research Laboratories. Its report on [root pruning](https://www.bartlett.com/dynamic/pdf/technical-reports/Root%20Pruning.pdf) gives "5 to 6 times the DBH" as the preferred minimum, and "three times the DBH" for cuts on one side only. Its pull tests on young red maples and willow oaks are reported in [Getting to the Root of Tree Stability and Construction](https://www.bartlett.com/dynamic/pdf/technical-reports/TreeStabilityandConstruction.pdf). They found "a measurable change in tree stability when cuts are made closer to the trunk than three times the trunk diameter". Bartlett's report on [sidewalk repair near trees](https://www.bartlett.com/dynamic/pdf/technical-reports/Sidewalk-Repair-Near-Trees.pdf) adds that inside 1.5 trunk diameters "it is very likely that tree stability will be affected". Mississippi State caps the root plate at about 10 feet. The estimator keeps six diameters for every trunk, since Bartlett's distances grow with the trunk and its researchers urge more caution on larger trees. The [University of Florida](https://web.archive.org/web/20260121185037/https://hort.ifas.ufl.edu/woody/root-prune-guidelines.shtml) reports experts who keep cuts on mature trees "6 to 8 inches from the trunk for each inch in trunk diameter", and others three to five trunk diameters. That page has since left the university's site, and it is read from the Internet Archive's copy of January 21, 2026.

The limits on the share run from a quarter to a half. Purdue Extension writes "Do not allow more than 25 percent of the roots in the PRZ to be damaged", its protected root zone. [University of Maryland Extension](https://extension.umd.edu/resource/damaged-tree-roots) finds many recover "if the damage does not exceed 1/4 of the total root zone". Fairfax County warns that past 40 percent of the zone "the tree may not survive", and Mississippi State writes "Limit root loss to 40 percent or less." The City of Austin keeps "At least fifty percent of the total area" of the zone at natural grade, and counts a tree as removed when that fails. The estimator calls a quarter or less within the limits, since two sources set it and it is the lowest. Past 40 percent it calls decline likely, the line of Fairfax County and Mississippi State, and Austin's half is shown as the permit limit it is. Each source sets its figure on its own zone, so applying them all to the foot-per-inch zone's area is the estimator's own reading.

The share is the area of the zone beyond the cut, by the geometry of a circle cut by a straight line. Mississippi State's trench guide, after Coder, counts more: by it, a trench at 60 percent of the radius loses "30 percent of the critical root area", where the area beyond is 14 percent. The estimator follows the area, the measure Austin's rule uses, and shows the other count with each answer. Austin allows "cut or fill of four inches or less" within half the zone's radius and none within a quarter. A cut of any depth is counted the same, since Purdue's [Why Is My Tree Dying?](https://www.extension.purdue.edu/extmedia/FNR/FNR-FAQ-11-W.pdf) finds that "a large portion of most tree roots grow in the upper 12 inches of the soil profile".

Bartlett's pull tests found that "if the soil is saturated with water, the tree is much less stable when pulled toward the cut roots". Its root pruning report calls three trunk diameters "too close if there is pavement over the other side of the root system", or if the tree leans or has root decay. The University of Florida finds root cuts more injurious "on shallow, compacted soils or on soils that drain poorly". On a slope, Bartlett's report on [grade changes](https://www.bartlett.com/dynamic/pdf/technical-reports/Preventing-Damage-to-Trees-from-Grade-Changes.pdf) puts every cut in the natural grade outside the dripline. [Preventing Construction Damage to Trees](https://www.bartlett.com/dynamic/pdf/technical-reports/Preventing%20Construction%20Damage%20to%20Trees.pdf) warns that cut roots can lead to "structural decay of support roots" and failure years later. [UF/IFAS Extension](https://ask.ifas.ufl.edu/publication/UW323) fences a tree to its drip line and warns that "Even with this method, 50% or more of the roots will be impacted by construction."

It is an estimate for planning, not an arborist's verdict. How far roots spread, and what trenches and fill do to them, is in [Tree roots run shallow and wide, not deep](https://bluewhaletrees.com/articles/tree-roots-run-shallow-and-wide/index.html.md). Keeping weight and fill off the rest of the zone is in [The habitat around a tree is part of the tree](https://bluewhaletrees.com/articles/habitat-around-a-tree/index.html.md). [Fungus on a tree is not a death sentence](https://bluewhaletrees.com/articles/fungus-is-not-a-death-sentence/index.html.md) tells when decay in a cut root matters. Questions the page leaves open can go to us through the [support page](https://bluewhaletrees.com/support/index.html.md#contact).