Trees benefit urban and suburban landscapes, providing shade, beauty, and environmental benefits. However, their roots can sometimes cause significant damage to sidewalks, driveways, and roads, leading to safety hazards and costly repairs. Understanding how tree roots affect sidewalks and exploring potential solutions can help mitigate these issues.
How Trees Cause Damage to Sidewalks
As tree roots seek water and nutrients, they spread horizontally near the soil surface. And when they encounter a sidewalk, they simple grow beneath it. Roots start small, but as they mature, they increase in diameter (referred to as secondary growth). This outward expansion of roots will exert pressure on nearby structures, including sidewalks.
This pressure can cause several types of damage. As roots expand, they can push up sections of the sidewalk, resulting in buckling and lifting. This creates uneven surfaces and trip hazards. Roots can infiltrate small cracks in the sidewalk, widening them over time. The resulting cracking can lead to extensive damage and potentially uneven surfaces. The growth of large roots can also displace entire sections of the sidewalk, making it difficult to walk or use mobility aids.
Preventing Damage to Sidewalks from Trees
An ounce of prevention is worth a pound of cure. When trees are planted near sidewalks, chances are good that tree roots will eventually cause damage to those sidewalks and other infrastructure. Several strategies can be used to prevent root-related sidewalk damage.
Select an Appropriate Tree Species
Large trees require large rooting volumes. If sidewalks will be in close proximity to large-maturing trees, choosing smaller trees becomes the preferred strategy. Rooting areas less than four feet wide should not be considered for any tree species. Small-statured trees (those with a mature height under 30 feet), like crabapple, redbud, or Japanese tree lilac, can be good options when the rooting area is greater than four feet wide but less than ten. Large shade trees should have a rooting area at least ten feet wide (and wider would be better). By giving the tree an appropriate amount of rooting area, damage to sidewalks can be avoided.
Additionally, avoid planting shallow-rooted tree species near sidewalks. While all trees have roots near the surface, some species are more likely to have an abundance of shallow roots that are more likely to cause lifting, cracking, or displacement of sidewalks. Trees that tend to have abundant surface roots include:
- Norway maple (Acer platanoides)
- red maple (Acer rubrum)
- sugar maple (Acer saccharum)
- ash (Fraxinus spp.)
- sweetgum (Liquidambar styraciflua)
- tuliptree (Liriodendron tulipifera)
- pin oak (Quercus palustris)
- poplars and cottonwoods (Populus spp.)
- willows (Salix spp.)
- American elm (Ulmus americana)
Use Flexible Sidewalk Materials
Using materials like concrete pavers, bricks, rubber pavers, or flexible asphalt can accommodate root growth better than rigid concrete, reducing the risk of cracking and buckling. While these materials will not prevent the potential upward movement of the paving material, if will provide a smoother transition to displaced areas and reduce potential tripping hazards.
Reroute or Reduce the Sidewalk
If possible, adjust the location of the sidewalk to place more distance between the tree and the pavement. Provide a minimum distance of 4-6 feet for smaller trees and 10+ feet for larger trees. Sometimes, the sidewalk can simply be narrowed to provide this distance. As long as the sidewalk is at least 39 inches across (as required by the ADA), this can be a viable option to provide more rooting space.
Root Barriers
Installing root barriers can direct roots downward and away from sidewalks, reducing the likelihood of surface damage. These root barriers must be at least 18 inches deep, and the top must project above the soil and mulch surface to be effective. Even with barriers, roots may still find a way to grow beneath sidewalks. However, these devices can reduce the likelihood of buckling, lifting, or cracking.
Root-excluding base layers may encourage roots to grow deeper under sidewalks, reducing the probability that secondary growth will lift or buckle the sidewalk above. Provide a base layer under the concrete at least four inches thick. In well-drained soils, at least four inches of gravel can be used. When possible, base layers deeper than four inches would be even more effective. Foam boards can also be used. These boards are commonly available and come in a variety of thicknesses. Glue together two 2-inch foam boards to provide a four-inch base that will protect the concrete from potential damage.
Repairing Damage to Sidewalks from Trees
Grinding, Shaving, and Patching the Sidewalk
Grinding down raised sections and patching cracks can provide a temporary fix, but it won’t stop further root growth. By shaving or grinding the upper surface to the same level as the lower surface, the tripping hazard is significantly reduced. Sidewalks can be reduced to about half their original thickness.
Alternatively, asphalt or concrete wedges or patches can be placed over the lower surface to make it level with the upper surface, eliminating an edge to potentially trip on.
Bridging the Roots
Sidewalks do not have to be level to be safe to traverse. The buckled sidewalk can be replaced with one that slopes up and over the roots. The maximum allowed slope to abide by ADA regulations is 1:20.
The new sidewalk can be supported by a thicker base layer or with concrete piers strategically spaced around the existing root system. Be sure to install a root excluding base layer under this new raised section of sidewalk.
Avoid Root Pruning
Simply cutting the offending roots and installing the sidewalk is often tempting. This can lead to some significant challenges for the tree. When major or buttress roots are removed, a large part of the root system is severed from the rest of the tree. This could lead to branch and crown dieback or death. Additionally, the tree might become structurally compromised and more likely to fail in high wind events.
The further away from the tree you can get, the less the sidewalk installation will affect the tree's health and longevity. As you get further from the base, the roots become smaller in diameter. Try to avoid cutting roots over 2" in diameter. If smaller roots must be severed, make clean cuts with a saw or hand pruners (instead of ripping them off with a backhoe). If the root system of a tree is exposed during construction, do not allow the roots to dry out. If possible, install a root excluding base layer under a newly raised section of sidewalk to avoid the reoccurrence of the problem.
When root pruning is unavoidable, it is recommended that buttress roots not be cut closer to the trunk than a distance equal to three times the trunk's diameter measured three feet up from the ground. When roots must be cut closer than this distance, damage to the tree (crown dieback, prone to failure in wind storms, etc.) is much more likely. When roots must be cut at a distance at or less than 1.5 times the trunk's diameter, tree removal is a more prudent option.
Tree Removal
Depending on the condition, size, and value of the tree it may be better to simply remove the tree than to damage it with sidewalk replacement only to have it fail or die 3 to 5 years later. Replace the tree with a smaller species that will fit in the available rooting space, or plant a larger tree in a new location at least 10 feet away from the sidewalk.
More Information
- Sidewalk Repair Near Trees from Bartlett Tree Experts (pdf)
- How to Diagnose Problems with Trees in the Landscape
- What effects will construction activities have on nearby trees?
- Preventing Grade Change Damage to Trees
- All About Stem Girdling Roots
- Identifying Tree Problems (pdf)
- Causes of Branch Dieback on Trees
- Shedding, Peeling, and Splitting Bark on Shade Trees