Root-cut estimator

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.
• Any tree. The estimator needs only the trunk's diameter, so it covers every tree and shrub we grow, and any other with a trunk.
• Mark a sensitive or young tree. Purdue Extension and Mississippi State Extension give a tree grown in woods with a narrow form, a young tree or one in poor health a wider zone, 1.5 feet per inch. Choose Sensitive or young for such a tree, and the estimator reads the share and every limit on that zone.
• Read the first line. The result opens with the gravest verdict that applies. Past 40 percent of the zone, Fairfax County warns that the tree may not survive, and Mississippi State Extension says to move the trench. Inside 1.5 trunk diameters, Bartlett Tree Research Laboratories finds stability very likely affected. Past a quarter, the cut is beyond the limit Purdue Extension and University of Maryland Extension give. Inside Arlington County's structural root zone, a certified arborist comes first, as Bartlett's laboratory asks for any root cutting. Otherwise the cut is within the limits the sources give. Past the tested 60 inches, a trunk's figures follow the same rules, under a warning above the verdict that sends the cut to a certified arborist on site.
• 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 address carries the inputs. A link copied from the estimator opens with the same trunk, distance, cut, depth and tree. The page draws the estimate and its plan view before any script runs, so a shared link and the page's Markdown copy show the figures themselves.

The critical root zone follows the City of Austin's Environmental Criteria Manual, 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 and Fairfax County give the zone the same foot per inch. The sources disagree on its size. Purdue Extension's Construction and Trees multiplies the diameter "by 1.5 to get the distance in feet" for trees grown in woods and young trees. Mississippi State University Extension 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. With Sensitive or young chosen, it reads the share and every limit on Purdue's wider zone instead. Purdue gives that zone to "sensitive trees that have been growing naturally in a wooded area with a more narrow form, or juvenile trees", and Mississippi State to "species sensitive to damage". Purdue also writes that "newly planted or younger trees typically are more resilient in construction zones", and names juvenile trees for the wider zone all the same. Mississippi State takes "1.5 feet per inch of stem diameter, or the dripline, whichever is greater" for such a tree; the estimator asks nothing of the crown, so the dripline is not applied. 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 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. 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 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 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 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. Fairfax County's and Austin's are set on the foot-per-inch zone the estimator follows. Purdue's quarter is set on its 1.5-foot zone, so the estimate tests it there as well and says whether the cut passes it. Applying Maryland's and Mississippi State's 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? 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 puts every cut in the natural grade outside the dripline. Preventing Construction Damage to Trees warns that cut roots can lead to "structural decay of support roots" and failure years later. UF/IFAS Extension 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. Keeping weight and fill off the rest of the zone is in The habitat around a tree is part of the tree. Fungus on a tree is not a death sentence tells when decay in a cut root matters. Questions the page leaves open can go to us through the support page.

The address carries the root-cut estimator's inputs, so a link opens it filled in, and Copy link on a result writes one: dbh (the trunk diameter in inches), distance (the feet from the trunk to the digging), cut (one for a straight cut past one side, two for two sides, around for a cut all the way round), depth (shallow for under 12 inches, deep for 12 or more), tree (typical, or sensitive for a sensitive or young tree). For example, /tools/root-cut/?dbh=16&distance=6&cut=one&depth=shallow&tree=typical.