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Tree Roots versus Sidewalks

Edited by Len Phillips

Problems with roots from municipal trees begin when shallow-rooted trees are forced to grow in the tree lawn or parkway, or whatever you call that ribbon of grass between the street and the sidewalk. In search of better growing conditions, roots travel under the sidewalk to utilize the moisture trapped by the sidewalk as well as the better soil on nearby residential lawns. Over time, the roots grow in diameter and lift the sidewalk, creating a trip hazard. New innovations in repairing or replacing sidewalks can reduce the damage to trees by accommodating rather than ignoring tree roots. However, sidewalk damage is more often caused by the soil in its expansion and contraction and not by tree roots.

Sidewalk Construction

Sidewalk construction requires compacted soil that will generally deflect tree roots downward. However, in time a small gap develops between the compacted soil and the pavement where water collects from the condensation of soil moisture rising to the surface. Finding water and oxygen in this gap, opportunistic roots will grow and increase in diameter, raising the sidewalk and curb just as they raise the soil in areas where there is no sidewalk.

When the municipality decides it is time to correct the sidewalk problems, a whole new series of problems with the trees can result. The horizontal roots that are causing the problems are the same roots that make a tree stable against storms and wind. Although these buttress roots supporting the tree cause the worst damage to sidewalks, it is not advisable to prune any of these roots in the interest of saving the walk.

Solutions

The following alternatives are listed from the most to least desirable, in this editor's opinion.

• Solving the problem of heaving walks is best done when the trees are planted. The community should have solutions built into construction specifications and subdivision regulations.

There should at least one foot (30 cm) of soil between the curb and the tree trunk at maturity. If this space will not be available, create better conditions, move the tree to a better site, or select a smaller stature tree with roots that are not as invasive. In other words, plant the right tree in the right place.

Proper tree selection near a sidewalk is critical to long-term success. The amount of space available should determine the size of the tree being planted. Here are general guidelines:

Tree Lawn Width Mature Tree Height

less than 3 feet (less than 0.9 m) No trees

3-5 feet (0.9-1.5 m) 10-30 feet (3 – 9 m)

5-7 feet (1.5-2.1 m) 30-40 feet (9 – 12 m)

7-9 feet (2.1-2.9 m) 40-50 feet (12 – 15 m)

9 feet or wider (2.9 m+) 50 feet or taller (15 m+)

• Another solution is the use of rubber sidewalks. This product uses recycled rubber tire crumbs to form a molded sidewalk. The modular design minimizes aggressive root growth and it is durable enough to last for many years longer than concrete. Tests show that after five years, there is no displacement of the rubber sidewalk by roots. The segments are affixed to each other with epoxy or dowels and locked on the sides with anchors. If a segment has to be removed for root pruning or regrading, it can be released and lifted, then anchored back into place. The rubber's porosity allows water and air to pass through the product. This feature creates an environment where the tree roots develop as fibrous roots instead of large diameter roots that would lift the sidewalk. The pavers do not expand or shrink in weather and are flame resistant. They can be hosed off when they get dirty and are resistant to most compounds except sulfuric acid or long-time exposure to oil based products. The advantage of rubber panels is the thin cross section. They are less than two inches thick and can be placed on top of a root if necessary to delay removal or root pruning.

Rubber pavers have other advantages.

• They can be recycled.

• They require little maintenance.

• There is less trip-hazard liability.

• They reduce lawsuits caused by pedestrians injuring themselves on concrete.

• They are safer.

• They are more economical and aesthetically pleasing.  

• They can be easily installed.

• Using structural soil under a sidewalk also works well. The large pores encourage root growth deep under the walk while the rock provides a stable base for the street, curb, and walk. Structural soils require the removal of all roots in the subgrade area and are only recommended for new installations.

• A favorite of many communities is reversing the sidewalk and grass strip locations when practical, to reduce the need to root prune and to provide space for new trees. With the trees near private property, the roots have plenty of space to develop. In those situations where the sidewalk that is next to the curb, the tree can be planted next to the lawn or on private property with an easement.

• New trees should not be centered in the planting strip but should be located a minimum distance from the curb. This solution also considers more sidewalk material options such as thinner cross section products like asphalt that can be placed directly over the roots and not require the same base preparation as concrete. Asphalt can be used with the new colored and stamped products. This solution uses easements to run the sidewalk on private property and increase the tree lawn width. This solution also allows curved sidewalks or curbs to go around trees.

The list below indicates some less desirable ways of dealing with sidewalk problems:

• If the tree lawn is less than 3 feet (1 m) wide, use easements to plant trees on private property a minimal distance of 3 feet (1 m) beyond the sidewalk.

• Use physical root barriers to deflect the roots downward.

• Chemically treat the asphalt and concrete walks to retard root invasion.

• Install a root barrier before the roots become a problem.

• Improve soil conditions to help tree roots grow more deeply and away from the curbs and walks.

• Remove and replace the tree with a deep root species.

• Remove and replace the sidewalks, making them higher than before.

• Landscape pavers can be used to replace the damaged sidewalk. As roots begin to expand, individual pavers could easily be displaced without much damage to the entire sidewalk.

• Use expansion joints in concrete walks so if a heave occurs, that section is easily replaced after the problem is corrected.

• Drill a hole in the lower slab and pump in concrete grout to raise the slab level with the raised slab.

• Use asphalt or concrete ramps to bridge over the problem roots. Such repairs rarely correct the problem, but they do reduce the trip hazard to pedestrians.

• Grinding the raised slab is generally limited to slabs with less than 1 inch (2.54 cm) of displacement.

• Install asphalt over the top of exposed tree roots. Since asphalt is flexible, root expansion would occur only in the area of the root itself.

• Root pruning is the least desirable solution because the roots are cut, removed, and a sidewalk is reinstalled. However, since pruned roots will quickly regrow, the problem will come back unless a barrier is used to deflect roots down and away from the sidewalk. Root pruning can also result in root decay that is difficult to monitor and inspect. It is also a very expensive alternative.

• If root pruning is done, the crown of the tree could be reduced to cut back on the wind sail to make the tree safer if the branches are very thick. In this case, the pruning should be thinning cuts, and not necessarily on the side of the tree that has had the root severance. Some trees appear to respond with limb shedding on the same side as the root cutting and some on the opposite side of the root cutting. Since it may be difficult to predict which branches the tree will shed in response to root cutting, it is better to avoid pruning unless necessary and return later to remove any dieback. Ideally, trees should have their crown pruned one or two years prior to the root pruning if this pruning is going to become necessary.

The new ANSI root zone management standards will require 3 to 5 times the trunk diameter distance from the trunk for root pruning, not the three foot (3’) (0.9 m) minimum which is the current standard. Soil excavation work is permitted closer than this distance if all the excavation of soil is done by hand and no roots greater than 2" (5 cm) in diameter are cut.

If time is not a factor, root pruning should not be done one month prior to or after bud break in the spring. All root-pruned trees should receive annual inspections to check their stability and recovery. A trained arborist should do all root pruning.

Sources

• D'Amato, Nicholas E. T., Davis Sydnor, Robin Hunt, and Bert Bishop, "Root Growth Beneath Sidewalks", Journal of Arboriculture, Vol. 28, No. 6, November 2002.

• D'Amato, Nicholas E. T., Davis Sydnor, Robin Hunt, and Bert Bishop, "The Root Vs The Crack", Journal of Arboriculture, Vol. 28, No. 6, November 2002.

• Dodge, Linda and Jim Geiger, "Tree Roots and Sidewalk Damage", City Trees, The Journal of The Society of Municipal Arborists Vol 37, No. 4, July/August, 2001.

• Kane, Brian & H. Dennis P. Ryan, "Tree Roots and Hazard Tree Evaluation", TCI Magazine, December 2002.

• McPherson, E Gregory, "Conflicts With Street Tree Roots", Journal of Arboriculture 26(6): 289-297.

• Nicoll, B. C. & A. Armstrong, "Tree Roots & Pavement Damage", Arboricultural Research & Information Note 138/9 71SII N.

• “Rubbersidewalks”, Online Seminars, March/April 2008.

• “Structural Soils”, Online Seminars, March/April 2008.

• Sydnor, T. Davis et. al, "Trees Are Not The Source Of Sidewalk Problems", Journal of Arboriculture 26(1):20 – 29.

• Urban and Community Forestry Assistance Program, "Protect Roots When Replacing Walks", City Trees, The Journal of The Society of Municipal Arborists Vol 36, No. 6, November/December 2000.

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