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Methods for remediating tree and sidewalk conflicts
Jill Odom | May 16, 2017
pavement broken up by tree roots

One of the main reasons for tree-sidewalk conflicts is not giving trees the proper growing space.
Photo: mrhayata/Flickr

Majority of people have probably tripped over a buckled sidewalk at least once in their life and often the culprits are industrious tree roots that have set out to invade new territory as they search for nutrients.

The number one way to prevent trees from warping sidewalks and driveways is to follow the wise practice of planting the right tree for the right place. According to the Georgia Forestry Commission, tree root systems extend about one to one-and-a-half feet out from the trunk for every inch of trunk diameter measured about four feet above the ground.

This means that a 12-inch diameter tree will have roots spreading 12 to 15 feet out in every direction. Roots grow in search of water and one good way to keep roots out of the way is to encourage deep growth by watering longer and less frequently, soaking several feet of soil instead of just the surface.

If sidewalk replacement or other work will be regular and root cutting is expected, the Arbor Day Foundation says that Norway maple, ginkgo, hackberry, hawthorns, cherries and river birch are more tolerant of root damage. Also, the City of Portland has a list of trees that are suitable for streets based on their height and presence of power lines.

Tree and sidewalk conflicts are far more common than they should be and they can be easily avoided with the proper care and planning beforehand, but sometimes you are called in after a mature tree has done its work.

Because removing the tree often defeats the purpose of it being planted there in the first place and is sometimes prohibited by local ordinances, here are some options on how to repair, accommodate and adapt to the situation.

Bridging

This option has two definitions and is for roots that are far too large to be cut. One definition is pouring concrete over the damaged portion and creating a hump in the sidewalk. As it wears down it can be removed and replaced over time. Another version of bridging is when the tree roots have expanded above the natural grade so an actual bridge sidewalk has to be built over them.

Curve the sidewalk

If there is space, instead of just repairing the sidewalk in the exact same spot and hoping that the roots will lose their desire to grow, move the path away from the tree. The distance should be three times that of tree’s diameter. Meandering sidewalks can break up the monotony of a trek and make a walk more enjoyable.

Place gravel beneath the sidewalk

Recent studies have found that when gravel is placed beneath a sidewalk slab the roots will grow under the gravel instead of directly under the slab. It prevents the likelihood of the sidewalk being pushed up by roots while providing air spaces for roots to grow in the gravel layer.

Pour thicker concrete

If moving the sidewalk further away isn’t an option, increasing the concrete thickness from 4 to 6 inches will make it harder for roots to lift or break the slabs. Another way to reinforce the concrete is to use rebar or wire mesh when re-pouring a slab. It is important that the rebar is in more than one slab so the root is pushing against two or three slabs instead of one.

Cutting roots

Cutting tree roots only serves as a temporary solution and should only be done as a last resort if other methods are not feasible. Pruning tree roots will create a ritual of trimming every few years to keep the roots in check.

Cutting roots also increases the risk of the tree falling or dying. The removal of large lateral roots can destabilize a tree and make it more likely to be blown over by the wind.

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