The expansion of tree roots beneath driveways, sidewalks, and foundations is a common cause of costly structural damage. This problem arises because roots naturally seek out water, nutrients, and oxygen, which are often trapped or concentrated beneath impervious concrete surfaces. As the roots grow, their increasing girth exerts immense pressure, lifting and cracking the concrete slabs above them. Successfully mitigating this issue requires a multi-pronged strategy that begins with informed planning and extends to physical barriers, controlled root management, and specialized repair techniques.
Selecting and Locating Trees Safely
The most effective strategy for preventing concrete damage is to avoid planting problematic trees near hardscapes, as not all tree species have the same root growth habits. Some develop deep taproots while others spread aggressive, shallow lateral roots close to the surface. Trees with less invasive root systems, such as Japanese Maple, Dogwood, Hedge Maple, and Ginkgo Biloba, are safer choices for proximity to structures. Conversely, species like Red Maple, Ash, Sweetgum, Willow, and Poplar are notorious for their shallow, vigorous root growth and should be planted much further away.
Proper placement is just as important as species selection, and distance should be calculated based on the tree’s mature size. As a guideline, trees that grow up to 30 feet tall should be planted at least 8 to 10 feet away from any structure or pavement. Larger trees, exceeding 50 feet in height at maturity, require a buffer zone of 20 feet or more to prevent future root conflicts. A conservative approach suggests planting the tree at a distance equal to at least half of its maximum expected crown width.
Installing Root Barriers
When planting near concrete is unavoidable, or for protecting existing structures from nearby trees, physical root barriers provide a practical solution. These barriers are typically rigid panels made of materials like high-density polyethylene (HDPE) or a durable, non-woven fabric impregnated with a root growth inhibitor. The barrier’s function is not to stop root growth completely, but to deflect the roots downward and away from the protected structure.
Correct installation depth is paramount for the barrier’s effectiveness, as roots will grow beneath a panel that is too shallow. For protecting sidewalks and driveways, a minimum depth of 30 inches is recommended, with some aggressive species requiring barriers up to 48 inches deep. The barrier must be placed vertically in a trench parallel to the structure being protected, ensuring the top edge is slightly above the soil line or sealed into the concrete edge. This top-edge placement prevents roots from growing over the barrier and then turning down toward the pavement.
Many commercial root barriers feature vertical ribs or fins that must face the tree, actively guiding roots downward along the barrier’s surface until they reach the bottom edge. The barrier should extend horizontally a few feet beyond the problematic area on either side to ensure roots do not grow around the ends. It is important to install the barrier only on the side facing the structure to avoid girdling the tree’s root system, which can compromise its stability and health.
Techniques for Controlling Established Roots
When an established tree’s roots are already threatening or causing damage, intervention is necessary, often involving a combination of physical and chemical methods. Structural root pruning is the process of selectively removing the problem roots to make room for repairs or barrier installation. This procedure carries risks and must be executed with precision using sharp tools to make clean cuts, avoiding the tearing that can lead to disease or decay.
The amount of root material removed must be limited to prevent tree decline or a loss of structural stability. Arborists advise against cutting any root larger than three inches in diameter, as these are often structural support roots. Root cutting should occur no closer to the trunk than three to five times the trunk’s diameter, measured at breast height. Removing more than 25% of the total root zone on one side can severely stress the tree, making consultation with a certified arborist advisable before proceeding.
Chemical root growth inhibitors offer a non-physical method of control, particularly useful for roots infiltrating sewer lines or growing directly under a slab. Copper sulfate is a common non-systemic agent that kills contacted roots by dehydrating their cells. Dichlobenil acts as a root growth inhibitor by preventing cell division at the root tips. These chemicals are applied locally, often through small holes or directly into sewer cleanouts, and are designed to affect only the target roots without harming the parent tree.
Repairing Damaged Concrete
Addressing concrete that has already been lifted or cracked by root growth involves repair methods that also incorporate future root mitigation. For slabs that are merely lifted or uneven, but not severely fractured, polyurethane foam injection is a repair technique. This process involves drilling small holes and injecting an expanding foam beneath the slab, which lifts and levels the concrete back to its original position by filling voids created by root movement.
For severely cracked or broken sections, full concrete replacement may be necessary, but only after the underlying root issue has been addressed. Simply removing the old concrete and trimming the roots risks the new slab resettling as the disturbed soil compacts. Before pouring new concrete, a root barrier should be installed in the trench created by the root removal to protect the new slab. Incorporating flexible joints into the new concrete design can also help accommodate minor future movement, minimizing the chance of cracking.