Free tool · AS 4970-2025
Encroachment Calculator
Put the works next to the tree and read off the encroachment: the area, the share of the Notional Root Zone, whether the Structural Root Zone is touched, and which of AS 4970-2025's three categories that puts you in. Draw as many works areas as the job has, shape each one corner by corner, and every figure follows as you go. Nothing leaves the browser.
The tree
Metres, filling from the right: type 55 for 0.55 m.
NRZ radius — m · SRZ radius — m
DSH at 1.4 m drives the NRZ; DAB above the buttress drives the SRZ. How these are derived
The works
As many separate areas as the job has — a slab, a driveway, a service trench. They are measured together, and where two of them overlap the shared ground is counted once.
Setback is the gap from the trunk to the nearest point of the works, wherever that falls. Bearing is the direction it lies in, degrees clockwise from the top of the plan. Width and length are the shape’s own extents, so they keep meaning something after you have edited it.
NRZ encroachment
— %
Area severed
— m²
SRZ encroachment
— %
NRZ area
— m²
Two ways to count the same works
Ground taken: —% (— m²)
The area of the zone the works physically sits on. Straightforward, and what most encroachment calculators report.
Roots severed: —% (— m²)
The works, plus everything behind it. A root cut at the near edge of a trench is lost for its whole length, not just the 300 mm you dug. This is the figure above.
The three categories
Minor — up to 10% of the NRZ area, entirely clear of the SRZ. Standard tree protection measures are usually enough, and the lost area can generally be compensated elsewhere on the site.
Moderate — more than 10% and up to 20% of the NRZ area, still clear of the SRZ. The project arborist has to justify it: root investigation, site-specific controls, supervision of the works.
Major — more than 20% of the NRZ area, or any encroachment into the SRZ at all. The second half of that sentence is the one that catches people. A works area that takes 4% of the NRZ is still a major encroachment if it clips the SRZ, because the SRZ is what holds the tree up.
Percentages are of the zone's area, not of its radius and not of the directions blocked. On a 6 m NRZ that is 113 m² to work against.
How this measures it
Three hundred and sixty rays leave the trunk. Each one is followed until it first meets the works; from that point out to the boundary, every root in that direction is severed from the tree, so all of it counts. Summing those wedges gives the area, and the area over the zone's area gives the percentage.
Where the works presents a single straight edge across the zone this reduces to the circular segment behind that edge, which is checkable by hand: a works edge 6 m from a 7 m zone gives 4.88 m², or 3.1%. The rays here return the same figure. Where the works is a trench or an odd shape there is no closed form, and the rays are what is left.
AS 4970-2025 Protection of trees on development sites is an advisory standard, not a regulatory instrument. This page is arithmetic on the numbers you typed, not a site assessment, and it cannot see the soil, the root plate or the condition of the tree. Have a qualified project arborist confirm anything that goes into a development application.