Tree pit soil volume: the figures a Rutgers guide and a UK street tree guide give
Street trees are often planted into a small pit of loose soil surrounded by compacted ground. This page reports the soil volume figures printed in a Rutgers stormwater tree bed guide and in the Trees and Design Action Group guide to trees in hard landscapes, and how each source describes the ways of providing that volume.
Checked against the sources at the bottom of this page on October 9, 2026. Rules, fees and pay change: the source has the last word.
This page reports what two documents say about soil volume. It is not a design manual. How new street trees fail, and the city standards written to prevent it, are on why new street trees die and planting specs compared by city.
The Rutgers rule of thumb
The Rutgers Cooperative Extension Water Resources Program publishes a document titled Stormwater Tree Bed Guidance. It says tree roots need enough uncompacted soil volume to support the tree, and that surrounding soils in cities are typically very compacted to carry paved surfaces. Its general recommendation is two cubic feet of uncompacted soil per one square foot of mature tree canopy, a figure it attributes to Lindsey and Bassuk, 1992. It adds that the figure does not apply to all species, and that narrow growing trees would need more soil relative to their canopy.
| Mature canopy diameter | Soil volume stated | Surface area at 3 feet deep |
|---|---|---|
| 10 feet | About 157 cubic feet | 49 square feet |
| 20 feet | 628 cubic feet | 210 square feet |
| 30 feet | 1,413 cubic feet | 471 square feet |
The guide says it is not always easy to provide this volume given limits on space and utilities, and that every effort should be taken to come as close to the criterion as possible. It also gives a tree trench example: a trench 5 feet by 25 feet at 3 feet deep gives 375 cubic feet, for a canopy diameter of about 15.5 feet, and extending the soil under 5 feet of adjoining sidewalk gives 750 cubic feet, for a canopy of about 22 feet.
Three ways of supplying the volume
The Rutgers guide names structural cells, suspended sidewalks and structural soils as strategies that provide soil volume while carrying the pavement above.
| Type | How the Rutgers guide describes it | What the UK guide adds |
|---|---|---|
| Structural cells | Modular manufactured cells placed and filled with soil in the void space; more complicated to install than structural soils. | Called crate systems: plastic, usually glass-reinforced polymer, or concrete, with load capacity depending on the cell material. |
| Suspended sidewalks | Pillar structures support the pavement in place of cells; any soil media can be used. | Not published here. |
| Structural soils | A gravel and soil mix in which the gravel carries the pavement and the voids hold soil; lower nutrient capacity, and porosity of 25% against 35% for a typical sandy loam. | Described with aggregate mixes and installation in layers; a case study uses a medium aggregate mix on a residential street. |
The Rutgers guide also says depth of soil amendments is recommended at a maximum of 3 feet, because deeper soil gives no significant benefit to the tree, with a minimum of 2 feet recommended. For bioretention style tree beds it describes a soil of 85% to 95% sand amended with 3% to 7% organics by weight.
What the Trees and Design Action Group guide reports
Trees in Hard Landscapes: A Guide for Delivery, released in September 2014 by the Trees and Design Action Group in the United Kingdom, was produced with several professional institutions and is aimed at highway engineers, public realm professionals and tree specialists. It does not print one universal volume. It says published soil volume estimates for trees vary widely because of differences in soils, climate and assumptions, with some based on observation of existing urban trees and others on the water demand of the tree.
Two of its case studies state volumes in metric units. In a Leeds square, a London plane tree was given 28 cubic metres of non-compacted rooting environment supported by a crate system. In a New Milton car park, a district council tree officer asked for 20 cubic metres of soil volume per tree, a load-bearing crate system, and a connection to downpipes collecting runoff from the 800 square metre store roof. The guide also discusses linking street tree roots to nearby soil areas through a break-out or root channel, and says the width of a planting trench needs to exceed the size of the root balls planted.
Where the figures differ
The two documents use different units, different tree types and different purposes, so their numbers are not interchangeable. Rutgers ties volume to canopy size and stormwater storage in New Jersey. The UK guide reports volumes chosen on particular projects. Neither document, as read for this page, gives a single legal minimum.
Sources
- Rutgers Cooperative Extension Water Resources Program, Stormwater Tree Bed Guidance, PDF, read October 9, 2026
- Trees and Design Action Group, Trees in Hard Landscapes, guide page, read October 9, 2026
- Trees and Design Action Group, Trees in Hard Landscapes: A Guide for Delivery, PDF, read October 9, 2026