I was retained by a defendant attorney to determine whether her client’s tree was responsible for lifting the adjacent civil sidewalk that resulted in a plaintiff trip and fall lawsuit. Based on the photographs and tree species, White mulberry (Morus alba), I was fairly certain her client’s tree was responsible for the sidewalk damage.
The attorney retained me and requested a site inspection and tree root assessment. I trenched adjacent to the sidewalk and within a short time, encountered a three-inch diameter root growing under the sidewalk. Further excavation revealed a network of two to four-inch diameter roots that had grown under the sidewalk, lifting a sidewalk panel, creating a hazardous condition resulting in a trip and fall accident.
I informed the attorney that root encroachment from her client’s tree resulted in the sidewalk damage, she said her civil engineer expert claimed it impossible for a tree root to lift a concrete sidewalk. I questioned the engineers knowledge and experience with trees, was the civil engineer a certified arborist or horticulturist?
Although the answer was no, she felt his qualification as a civil engineer was superior to my qualifications as a certified arborist, tree risk assessor qualified, registered consulting arborist and a college educated horticulturist. Her dilemma was having two experts who disagreed and she requested I alter my opinion.
I told the attorney altering my opinion to suit her needs was unethical, and I removed myself from the case.
The common myth is trees have tap roots that grow straight down into the soil. While this may occur in very deep, loam soils, the reality is most containerized nursery grown trees lose their tap root in the container. Once planted in our poor Southern California soils, when the tap root encounters physical soil obstructions, such as rock, clay or hardpan, the tap root divides and grows multiple roots around the obstacle, forming a fibrous root system. Tap root grows into fibrous root system.
Trees require a spreading root system to maintain structural stability. Trees dissipate energy generated during a storm or wind event by transmitting leaf, limb and trunk movement down to the roots. A spreading root system anchors the tree movement and dissipates the energy far more effectively than a single tap root system.
Most tree roots grow within the top 24-inches of the soil horizon. Roots in top 24″ of soil. Over time, structural tree roots (2-inch diameter and greater) growing within a shallow soil adjacent to sidewalks, footings, foundations, walls or othersub-surface infrastructure may cause damage.
Just as a twig grows into a branch, and then a limb, roots increase in length and circumference. Irrigation water, sewer or water service leaks increase subsoil moisture beneath sidewalks or garage slabs, creating a perfect environment for root growth.
As the root circumference increased, it exerts pressure on the concrete slab or footing above the root. Depending on the species, root diameter might increase 1/8-1/4” annually.
Within five to eight years, a small feeder root may grow to one-inch in diameter or greater.
The root growth may be compared to the action of a hydraulic jack, as the circumference increases, the upward pressure on the sidewalk or slab may crack and or eventually lift. Concrete lifting may often occur at an expansion joint between concrete panels
Roots seek out soil moisture, they can and will grow under walls, footings and garage slabs. Roots are opportunistic, leaky plumbing, old cast iron sewer lateral or water services contribute to the soil moisture needed for roots to flourish.
Roots may infiltrate pvc, abs and cast-iron pipe through even the smallest of cracks or holes.
Once inside, the roots expand in size and quantity, eventually completely clogging the utility.
If the tree crown has grown over a sidewalk or adjacent structure, it is a reasonable assumption structural roots (two-inch diameter and greater) have grown under the sidewalk, slab or footing.
Planter areas confined by concrete pose one of the greatest risks for root damage. City sidewalks often incorporate small, square planters within the sidewalk easement.
Confined planters quickly fill with structural roots, as well as damaging girdling root. As the tree crown grows, so to do water absorbing feeder roots. Over time, root mass and size increases, structural roots may begin to grow beneath concrete improvement while seeking out moisture.
The tree pictured above and to the right had a leaky water meter adjacent to the small sidewalk planter. The leaky service provided idea soil moisture conditions for the roots to lift the water meter box, adjacent sidewalks and crack the curb and gutter.
Preventative measures to minimize root encroachment include a variety of root barrier methods. All root barrier systems work best when the tree is installed. Once roots have enlarged and matured, barrier mitigation is not successful.
In summary, our Southern California poor, shallow soils do not support deep tree tap root systems. Most trees grow fibrous spreading root systems. Structural roots emanating from the root collar extend to the edge of the tree crown, (drip line). Most structural roots growing beneath sidewalks range from two to four inches in diameter, lifting sidewalk panels on average one-two inches. Trees growing in confined planters or adjacent to concrete, utilities or foundations may develop structural roots capable of lifting, cracking or damaging adjacent improvements.
Attorneys should select an expert based on the case criteria, not simply a title, license or certificate. Choose an expert most appropriate to address the cause of the problem and develop opinions based on sound, industry practices.