· by Michael Blair, Nashville's Concrete
One panel of the front walk has tipped up at a joint and become a trip edge, and there is a maple fifteen feet away. Or the driveway has a hump running across it that was not there five years ago. Most slab movement in Middle Tennessee is the clay settling or swelling, but a raised edge close to a tree is a different problem with different repairs. The fix has to deal with the root and keep the tree alive, and on a sidewalk by the street it may not be yours to fix at all.
Why roots end up under concrete
Tree roots grow toward water and air, and most of them live in the top foot or two of soil. A slab changes the ground beneath it: it stops evaporation, so moisture collects on its underside, and the thin gap between concrete and base is loose enough for a root to follow. Once a root is there, it thickens every year, and the slab above it takes the load.
Fast-growing trees with shallow roots are the usual suspects on Nashville lots and streets, silver maples and sweetgums especially, and many older neighborhoods have mature ones planted close to walks and drives. Clay makes it worse, because roots in heavy soil stay closer to the surface than they would in loose ground.
Telling root lift from clay movement
Root lift is local and one-sided. The raised edge is on the side of the panel nearest the tree, it is often at a joint or a crack, and the panel tilts up toward the trunk rather than sinking. Sometimes the root is visible at the edge of the slab or in the lawn beside it. Clay movement tends to affect a wider area and follows wet and dry seasons, rising in spring and dropping in late summer.
The difference matters because the repairs are opposite. A slab that settled is lifted or replaced on a better base. A slab lifted by a root cannot be lowered until the root is dealt with, and slab lifting foam injected under it would only raise the low side to meet the high one.
Repairs that do not kill the tree
Cutting a large root close to the trunk can destabilize a tree or starve one side of it, and a mature tree is worth more to the property than a section of walk. An arborist can say which roots can be pruned safely and which cannot, and is worth calling before anyone takes a saw to a root thicker than a wrist. Root work is generally gentler on a tree in the dormant season, which starts as the leaves come down.
The concrete side of the repair then has several options, and the right one depends on how big the root is and how close the tree stands.
- Grind the trip edge down where the lift is small and the tree must stay untouched
- Remove the panel, prune the root on an arborist’s advice, and pour a new panel with a root barrier
- Bridge over a root that cannot be cut with a reinforced, thickened slab
- Curve the walk or drive away from the tree, or switch that section to pavers that can be reset
Who owns the sidewalk out front
In much of Nashville the sidewalk along the street and the tree beside it sit in the public right-of-way, not on your lot, even though you mow around them. Work on a public sidewalk, a curb or a driveway apron that meets the street usually needs a permit from the city’s transportation department, and cutting the roots of a street tree may need approval of its own. Call Metro before any work starts; in Franklin, Brentwood, Murfreesboro or another city, ask that city’s public works office.
On your own drive and walk, the choice is yours, and the timing is good now. Fall brings mild temperatures for pouring, and the dormant season that follows suits the root work. When we replace a lifted panel, the base gets compacted and the root barrier set before the forms go in, because a new slab poured over the same root starts rising again within a few years.