FIELD NOTE · SUPPLEMENTAL SET
The sidewalk makes the root: Los Angeles and a thirty-year repair obligation
ISSUEDJULY 11, 2026DRAWN BY THE NIGHTLY SWEEP
The sidewalk makes the root that lifts it. That is not how anybody tells it. It is what the research says.
The Forest Service put it plainly. Contrary to common viewpoint, sidewalks apparently promote rather than deter development of shallow tree roots. The mechanism is water and it is entirely mechanical.
A concrete slab stops evaporation from the soil under it. It stops transpiration. It blocks rainfall from percolating into the ground it covers. Then it works as a thermal plate. Concrete warms faster than soil and radiates that heat down. It cools faster too. So soil moisture condenses on the underside of the slab and sits there, evaporating back into the ground whenever the slab heats up ahead of the soil again.
The best soil on the parcel ends up a few inches under the pavement. Wet, aerated, temperature buffered, never disturbed.
Then it grows in girth. That is the part that does the damage. The lift comes from radial growth, not from the root pushing forward. A root that develops at depth makes the same force and spends it through a much larger volume of soil first. A root two inches under a four-inch slab has nothing between it and the concrete.
The panel lifts at the joint and stops being flat. Nothing above grade caused it.
A circling root is one that hits the barrier and turns instead of diving. It stays shallow. It also wraps the tree it belongs to. The barrier moved the problem and the tree picked up a second one.
That is the trouble with all the usual fixes. Grinding a lifted panel takes off the trip hazard and leaves the root. Replacing the panel hands the root a fresh slab. Neither one touches the reason.
Los Angeles ended up owing something much larger. A class action was filed August 4, 2010 in the Central District of California, for all persons with mobility disabilities denied access to the city's pedestrian rights of way. It settled. The City agreed to expend a total of $1,367,142,684 during a thirty-year compliance period on program access improvement. Final approval came August 25, 2016. Class counsel took $10,269,745.07 in fees and $1,533,751.77 in costs the same day.
The money was scheduled, not budgeted year to year. The term sheet sets an annual commitment in five-year steps and lifts it by 15.3 percent at each step to hold the present value: $31,000,000 a year in years 1 to 5, $35,743,000 in years 6 to 10, $41,211,679 in years 11 to 15, $47,517,066 in years 16 to 20, $54,787,177 in years 21 to 25, and $63,169,615 a year in years 26 to 30. There is a floor under it too. In no fiscal year after the first may the City spend less than $25 million. The Bureau of Engineering stood the program up in the last week of 2016.
THE SCHEDULE THE CITY AGREED TO, USD PER YEAR YEARS 1 TO 5 31,000,000 YEARS 6 TO 10 35,743,000 YEARS 11 TO 15 41,211,679 YEARS 16 TO 20 47,517,066 YEARS 21 TO 25 54,787,177 YEARS 26 TO 30 63,169,615
The City's own figure for the network has moved, which matters before anybody does arithmetic on it. At the 2016 launch the number in circulation was approximately 11,000 miles over 472 square miles. The 2025 presentation says about 9,000. Nobody has reconciled them.
The rate is the rate either way. A 2021 audit by the City Controller, Repairing L.A.'s Broken Sidewalk Strategy, found Willits repairs completed at 2,100 sites against roughly 50,000 reported sidewalk problems still open. At the pace it was then running, the audit calculated nearly 500 years to fix all of the city's buckled and cracked sidewalks. The average basic sidewalk fix took 41 days in fiscal 2021. A pothole took 3.
Those numbers are five years old. The newer ones are not better.
Species and spacing get decided long before any of this reaches a concrete contractor. Beverly Hills wrote a master plan governing what grows on which street. That document, not a repair spec, is where a sidewalk's next forty years get set.
For the contractor there are two markets here. One is a rolling public obligation with a court order behind it. The other is every owner fronting a street, who owes the repair under Section 5610 whether they know it or not. Filings scored nightly for paving contractors across the Los Angeles record show where the flatwork and the curb ramps are landing.
The panel gets replaced and the root keeps growing under the new one. The new slab blocks the same evaporation and condenses the same water. Thirty years buys a lot of concrete. It does not change what is underneath it.
The Forest Service put it plainly. Contrary to common viewpoint, sidewalks apparently promote rather than deter development of shallow tree roots. The mechanism is water and it is entirely mechanical.
A concrete slab stops evaporation from the soil under it. It stops transpiration. It blocks rainfall from percolating into the ground it covers. Then it works as a thermal plate. Concrete warms faster than soil and radiates that heat down. It cools faster too. So soil moisture condenses on the underside of the slab and sits there, evaporating back into the ground whenever the slab heats up ahead of the soil again.
The best soil on the parcel ends up a few inches under the pavement. Wet, aerated, temperature buffered, never disturbed.
Where the root goes
The root goes there because nothing else on the site is better. The base course under a sidewalk is the least compacted layer in the section. Open, granular, full of voids. A root that outgrows its planting hole takes the path with the least resistance in it, and under the slab it finds water and room in the same place.Then it grows in girth. That is the part that does the damage. The lift comes from radial growth, not from the root pushing forward. A root that develops at depth makes the same force and spends it through a much larger volume of soil first. A root two inches under a four-inch slab has nothing between it and the concrete.
The panel lifts at the joint and stops being flat. Nothing above grade caused it.
A barrier makes a different root
Root barriers are the standard answer. They work in one direction only. Field experiments at the same Forest Service center tested three barrier types at 30 centimetres deep. Barrier-induced circling roots were a common problem across all three, and they were particularly abundant inside plastic barriers with no internal vertical ribs.A circling root is one that hits the barrier and turns instead of diving. It stays shallow. It also wraps the tree it belongs to. The barrier moved the problem and the tree picked up a second one.
That is the trouble with all the usual fixes. Grinding a lifted panel takes off the trip hazard and leaves the root. Replacing the panel hands the root a fresh slab. Neither one touches the reason.
Who owes the repair
California puts the duty on the owner. Streets and Highways Code Section 5610 requires the owners of lots fronting a public street to keep the sidewalk in a condition that will not endanger persons or property and will not interfere with the public's use of it, and then carves out an exception for a condition created or maintained by somebody other than the owner under a permit or a right granted by law or by the city authorities. The duty is the owner's. The exception is where the arguing happens.Los Angeles ended up owing something much larger. A class action was filed August 4, 2010 in the Central District of California, for all persons with mobility disabilities denied access to the city's pedestrian rights of way. It settled. The City agreed to expend a total of $1,367,142,684 during a thirty-year compliance period on program access improvement. Final approval came August 25, 2016. Class counsel took $10,269,745.07 in fees and $1,533,751.77 in costs the same day.
The money was scheduled, not budgeted year to year. The term sheet sets an annual commitment in five-year steps and lifts it by 15.3 percent at each step to hold the present value: $31,000,000 a year in years 1 to 5, $35,743,000 in years 6 to 10, $41,211,679 in years 11 to 15, $47,517,066 in years 16 to 20, $54,787,177 in years 21 to 25, and $63,169,615 a year in years 26 to 30. There is a floor under it too. In no fiscal year after the first may the City spend less than $25 million. The Bureau of Engineering stood the program up in the last week of 2016.
THE SCHEDULE THE CITY AGREED TO, USD PER YEAR YEARS 1 TO 5 31,000,000 YEARS 6 TO 10 35,743,000 YEARS 11 TO 15 41,211,679 YEARS 16 TO 20 47,517,066 YEARS 21 TO 25 54,787,177 YEARS 26 TO 30 63,169,615
Fifteen miles a year
Nine years in, the program's own numbers are the thing to read. In a 2025 presentation the City's ADA coordinator described it repairing roughly 15 miles of sidewalk a year and adding 50 curb ramps. The same presentation put the network at about 9,000 miles. Around 1,000 access requests come in a year and about 300 get completed. More than eight years after the settlement took effect, the dashboard of major street projects it required was still being built.The City's own figure for the network has moved, which matters before anybody does arithmetic on it. At the 2016 launch the number in circulation was approximately 11,000 miles over 472 square miles. The 2025 presentation says about 9,000. Nobody has reconciled them.
The rate is the rate either way. A 2021 audit by the City Controller, Repairing L.A.'s Broken Sidewalk Strategy, found Willits repairs completed at 2,100 sites against roughly 50,000 reported sidewalk problems still open. At the pace it was then running, the audit calculated nearly 500 years to fix all of the city's buckled and cracked sidewalks. The average basic sidewalk fix took 41 days in fiscal 2021. A pothole took 3.
Those numbers are five years old. The newer ones are not better.
What the tree is worth
None of this argues against street trees. It argues that a tree in a parkway strip is a structure sharing ground with a pavement. Somebody planted it thinking about the canopy, the street and how the block would look in thirty years. That is a drawing question. Drawings do not settle in the rain.Species and spacing get decided long before any of this reaches a concrete contractor. Beverly Hills wrote a master plan governing what grows on which street. That document, not a repair spec, is where a sidewalk's next forty years get set.
For the contractor there are two markets here. One is a rolling public obligation with a court order behind it. The other is every owner fronting a street, who owes the repair under Section 5610 whether they know it or not. Filings scored nightly for paving contractors across the Los Angeles record show where the flatwork and the curb ramps are landing.
The panel gets replaced and the root keeps growing under the new one. The new slab blocks the same evaporation and condenses the same water. Thirty years buys a lot of concrete. It does not change what is underneath it.