Why do tree roots grow under sidewalks?
Because the soil just under a slab is often damp and airy. A UC Davis study found soil under sidewalks was moister and warmer than soil under nearby lawn. Water vapor condenses on the underside of the slab as it cools at night.
The soil below that is often packed hard to hold up the paving. So roots stay in the thin layer right under the slab.
Most tree roots are shallow anyway. University of Illinois Extension puts over 90% of a tree's root mass in the top 18 in of soil. The roots can spread up to 3 times as wide as the canopy.
How does a root lift a slab?
A root grows thicker every year, not just longer. As a root under a slab widens, it presses up and lifts one edge of the panel into a trip hazard. Curbs, driveways and patios lift the same way.
Which trees and sites are most at risk?
A research review by Randrup, McPherson and Costello found conflicts most likely with:
- Tree species that grow large or fast
- Trees older than about 15 to 20 years
- Small or packed soil spaces, such as a narrow strip between the curb and the walk
- Paving within about 7 to 10 ft (2 to 3 m) of the tree
Iowa State University Extension says a rooting area under 4 ft wide is too small for any tree. Large shade trees need at least 10 ft.
How does a root barrier help?
A root barrier is a wall in the soil between the tree and the paving. Sidewalk Shield is black HDPE, 80 or 100 mil thick, designed to turn tree roots down and away from paving. A root that reaches it is guided down its face and grows on under the bottom edge, deeper in the soil.
Research supports this, with limits. In a 10-year trial at the Bartlett Tree Research Laboratory, walks beside an 18 in vertical barrier had little lifting. The roots under them were fewer and deeper. In a University of Florida trial with 12 in barriers, many deflected roots came back up by the far side of the walk.
So a barrier buys depth and time. It steers roots; it doesn't stop them.
Where does the barrier go, and which way does it lean?
- Line: right along the edge of the sidewalk, curb or driveway you want to protect, on the tree side, centered on the trunk. See where a tree root barrier goes.
- Length: about the mature canopy width plus 2 ft in all, so it reaches about 1 ft past the mature drip line on each side. If your city detail gives a length, use that.
- Lean: none by default. Set it vertical, the way city details draw it. Where there is room, you can lean it about 10°, top edge toward the tree, so roots that reach it are guided down.
- Top edge: ½ to 1 in above finished grade. Dig the trench to the barrier depth minus that: 17 to 17½ in for the 18 in barrier, 29 to 29½ in for the 30 in and 35 to 35½ in for the 36 in.
- Joints: overlap the ends 12 in and clamp them with the stainless connecting kit, one kit per joint. Add 12 in to your order for each joint. See how cut-to-length ordering works.
- Backfill: 6 in lifts, firmed one at a time, so the barrier stays where you set it.
- Established tree: talk to an arborist first, because the trench cuts the roots on that line.
- Utilities: Call 811 before you dig.
If your project plan or city detail differs, follow the plan. Each step is in the Sidewalk Shield install manual.
Example: a tree that will spread 40 ft takes a run of about 40 + 2 = 42 ft, centered on the trunk. Order 45 ft, the next 5 ft step: 22½ ft each way.
Which depth do I need?
Use the depth your plan or spec calls for. Sidewalk Shield comes 18 or 30 in deep at 80 mil, or 36 in deep at 100 mil. Iowa State University Extension says at least 18 in for sidewalks.
See which depth to choose for puncture numbers, the longest piece and a 24 in spec. With no plan, send us the tree species and the site. Prices and kits are on the Sidewalk Shield page.
Barrier, root pruning or structural soil: which one?
They do different jobs, and they are often used together.
| Option | What it does | Best for | Limits |
|---|---|---|---|
| Root barrier | Steers roots down and away from the paving | New plantings and slab replacements | Won't move roots already under the slab. Roots pass under it, and many come back up past the walk. |
| Root pruning | Cuts the roots that are lifting the slab | Replacing a slab next to an established tree | Roots grow back. Cuts near the trunk can make the tree less stable. |
| Structural soil | A stone and soil mix that holds up paving and leaves room for roots | New streets and plazas built around trees | Only goes in with new paving, usually about 24 in deep. |
What the research found. Cut roots grow back: in a study of mature live oaks, only 38 of 557 cut roots had not regrown five years later.
In the Bartlett trial, a 4 in layer of structural soil under the walk had the most lifting. That is much thinner than the usual 24 in. A 4 in layer of gravel or foam under the walk lifted about as little as the 18 in barrier.
A 6 in gravel layer under the walk also did well in the Florida trial, especially on well-drained soil.
How to choose:
- New tree by an existing walk: put the barrier in at planting time.
- Established tree, walk still flat: talk to an arborist first, because the trench cuts the roots on that line. Then set the barrier along the edge of the walk, or where your plan shows it.
- Lifted slab beside an established tree: replace the slab, and have an arborist mark which roots can be cut. Set the barrier along the slab edge before the pour, or where your plan shows it. In one study, barriers set against cut roots let far fewer regrowing roots through.
- New street or plaza: ask the designer about structural soil or a gravel layer under the walk, plus barriers.
- Large tree in a strip under 4 ft wide: no option fixes a tree too big for its space. Iowa State says that strip is too small for any tree. Ask your city forester where a new tree could go.
What can't a root barrier do?
- It won't flatten a slab that has already lifted. The panel needs repair or replacement, and the barrier goes in during that work.
- It won't make room the site doesn't have. A large tree in a narrow strip will still outgrow its space. Ask an arborist or your city forester.
When should you call an arborist?
Before any digging that cuts roots of an established tree, because a barrier trench cuts every root it crosses. Call one when:
- The work will cut roots thicker than 2 in, which Iowa State Extension advises against.
- Any cut would come within 3 trunk diameters of the trunk.
- It's a street tree. Some cities require a permit: Portland, Oregon, does for street-tree roots 1/4 in thick or more.
For the trunk-distance math, see do root barriers hurt trees? Find an ISA Certified Arborist near you.
Common mistakes
- Leaning the top edge away from the tree. Set Sidewalk Shield vertical, or lean the top toward the tree where there is room.
- Butting or taping the ends instead of a 12 in clamped overlap.
- Ordering the bare run length. Add 12 in for each joint.
- Burying the top edge. Leave ½ to 1 in above finished grade. Iowa State says the top must stay above the soil and mulch, or roots can grow over it.
- Filling the trench in one go instead of 6 in lifts.
Who is Sidewalk Shield not for?
- A walk that has already lifted, unless it is being repaired at the same time.
- Running bamboo. Use Bamboo Shield, with its top tilted away from the bamboo and a 2 in lip, so rhizomes turn up where you can cut them.
FAQ
Will a root barrier fix my lifted sidewalk?
No: the root under the slab stays where it is, so the raised panel stays raised until it is repaired or replaced. Put the barrier in during that work, before the new slab is poured, so new roots are guided down and away. Have an arborist decide which roots can be cut.
Can I grind down the lifted edge instead?
For a while. Grinding shaves the high edge into a gentle slope. One borough's repair sheet says to grind back 12 in for each 1 in of rise. It warns the fix may only be temporary, because the root keeps growing. Iowa State says a slab can be ground to about half its thickness.
Who pays to fix a sidewalk a street tree lifted?
It depends on the city. In Portland, Oregon, the owner of the property next to the walk must repair damage from street trees. The work needs a sidewalk permit, and sometimes a root-pruning permit. Los Angeles fixes many walks through a city-funded program. Ask your city's transportation or forestry office before you hire anyone.
Is root pruning enough on its own?
Not for long: cut roots grow back. In one live oak study, all but 38 of 557 cut roots regrew within five years. Cuts near the trunk can make a tree less stable. Pair pruning with a barrier set along the slab edge before the new pour, or where your plan shows it.