Tree Roots & Sidewalk Heaving Expert Article

Along sidewalks, the root systems of trees can displace or lift up the sidewalk over time in a phenomenon referred to as “sidewalk heaving.” Sidewalk heaving can present a trip and fall hazard for pedestrians, as well as accessibility challenges for wheelchairs and other mobility aids.

In this article, board-certified master arborist Christopher Larson discusses sidewalk heaving from the perspective of his forensic investigations.

Tree Roots & Sidewalk Damage

One cause of sidewalk heaving may be the increased growth of tree roots that have come into contact with sidewalks and displaced them. Some species of trees have growth characteristics that are known to interfere with sidewalks. When in close proximity to sidewalks, utilizing tree species that develop large buttress roots or species that grow shallow surface roots increases the potential for sidewalk damage. 

The following species are some of the more common surface rooters that cause sidewalk and infrastructure damage, and are well known to cause sidewalk heaving. They include but are not limited to:

• Ashes
• Cottonwood and Poplars
• Elms
• Locusts
• Maples
• Southern Magnolia
• Sweetgums
• Willows

Sidewalk Heaving & Walkway Safety

Sidewalk heaving can affect the safety of walkway surfaces. Sidewalk heaving is a common cause of trips and falls.

The damage to the sidewalk can be expensive to fix. There are a variety of strategies for mitigating trip and fall hazards and reducing tree sidewalk conflicts. Such strategies can be preventative, remedial, or both. The strategies are tree based, infrastructure based, or root zone based.

Tree- and root zone-based strategies:

• Proper tree species selection: Selecting tree species based on knowledge of tree size at maturity, tree trunk flare traits, planting site space, and species performance in similar sites.
• Root Shaving: Uses a root cutting machine to reduce the thickness of an offending root without severing the root entirely; this is used to retain part of the root for tree structural strength and for general tree health; very infrequently used because of the potential for root decay and the callus growth itself can damage the sidewalk.
• Root Pruning: Pruning off roots, causing damage without destabilizing the tree or compromising the tree’s longevity; this is considered a last resort.
• Root Bridging: Used to avoid the removal of tree roots by bridging over them; the heaved sidewalk is replaced with an elevated sidewalk over the roots; it is important to consider degree of slope for code compliance.
• Root Barriers: Block and direct roots away from sidewalks.
• Root Channels: Create favorable rooting zones to channel roots, under pavement and compacted soils, to adjacent areas with favorable soil.

Infrastructure-based strategies:

• Sidewalk design: Many different sidewalk layouts, materials used for the sidewalk itself (concrete alternatives), and specific materials for underlayment and base (sometimes structural soils).
• Sidewalk grinding: A temporary measure in which a lifted sidewalk is ground down to remove the lifted edge and establish a continuous surface between the two concrete slabs. This is usually done if the lift is not more than 1 inch and is located at an expansion joint. Eventually, the sidewalk will continue to heave and it must be replaced or ground again. The slab strength will be reduced if grinding removes more than half the slab thickness.
• Slab jacking (Mudjacking): When a sidewalk slab is heaved or sunken, slab jacking can be used to reset the concrete slab back to grade or made level. The process pumps cement grout under the lowered section and raises the slab to grade. Transitions may still be needed and this is only a temporary solution because the offending root stays in place and radial growth can heave the slab again. This strategy increases the cost when the slab eventually has to be removed. This usually isn’t done if more than one slab is lifted because other repairs are more economical in the long run.
• Reinforced slab: Concrete sidewalks can be reinforced with rebar which reduces the potential for damage from tree roots but does not eliminate sidewalk heaving.
• Thicker Slabs: Reduces the potential for sidewalk heaving by increasing sidewalk thickness to greater than 4 inches (>6” often used to reduce potential damage); resistance to cracking is a function of the concrete thickness as well as the base coarse thickness.
• Foam underlayment: Foam material used as a backing material under new concrete and over existing roots; is a remedial treatment that absorbs the radial growth and compresses the foam without heaving the concrete; sometimes used after root pruning and in conjunction with herbicide impregnated landscape fabric.

*(It is important to note that this is not an exhaustive list, but a list of some common strategies used to prevent and remediate tree and sidewalk conflicts.)

Who is Responsible for the Sidewalk and/or the Tree?

When a sidewalk has a conflict with a tree there are several potential parties responsible.

Potential parties responsible for the maintenance and repair of a sidewalk can be the private property owner, commercial business owner, homeowner associations, contractor, the municipality, or government entity. Depending on the specific circumstances and the locality/municipality where the incident occurred, there can be multiple parties responsible.

Trip and fall cases may be lacking a time frame for how long the sidewalk was dangerous. Analysis of the incident tree and the sidewalk heave can often resolve the time frame issue and establish how long those responsible for the maintenance had to address the hazard. Involving a tree expert to analyze tree sidewalk heaving can be the key to completing the case.

Tree Sidewalk Heaving Investigations

The experts at Robson Forensic are frequently retained to investigate injuries involving tree-related sidewalk trips and falls. Within the context of a tree sidewalk heaving investigation, we are positioned to provide a thorough and comprehensive investigation by addressing every aspect of the case, from structural engineering and materials failures to specialty aspects of tree forensics. Experts involved in these matters may include architects, engineers, materials scientists, or board-certified master arborists. 

For more information, submit an inquiry or call us at 800.813.6736.

Featured Expert

Christopher Larson, BCMA, TRAQ

Board Certified Master Arborist & Tree Expert

Chris Larson is a Board Certified Master Arborist with over 20 years of experience in urban forestry, arboriculture, and landscape design/maintenance. He applies his expertise to a broad range of… read more.