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Trees - Oak Tree
Expert: Jim Hyland - 9/30/2008
Question
My son and his wife are about to become fist time homeowners. The house sits on a very small city street with a huge Oak tree on the front sidewalk. My guess is this tree is approx. 40-50 yrs. old. The side walk is lifted and the porch steps are leaning forward. My guess is that this is root damage. they want to replace the sidewalk and steps. I'm afraid this mighty oak will only damage it again. What should they do?
Answer
I would try replacing the sidewalk. Try not cut any of the roots just add to the surface over the roots. Do not add more than 3 inches deep.
Consider installing expansion joints in the sidewalks near trees. This will limit possible sidewalk replacements to just a few sections rather than large areas. Curving and narrowing sidewalk sections near trees might also reduce heaving. Building sidewalks on beds of coarse gravel is also effective at slowing or stopping the heaving. Tree roots will not grow through porous gravel; instead, they grow deeper. The installation of removable or adjustable semi-permanent pavers near trees can serve as alternatives to poured sidewalks. The pavers can be altered when required to compensate for tree root growth.
Install root barriers along the tree-side edge of the sidewalk. The barrier will force tree roots to grow deep below the sidewalk, thus preventing heaving. Barriers can be made of plastic or geotextile fabric. It should extend one foot deep and at least 5 or 6 feet in both directions from the point on the sidewalk edge closest to the tree.
Whenever trees are root pruned, there is always some risk of tree failure. Many factors are involved. Tree species, age, size, site conditions, existing problems, vigor and extent of pruning are just some of the factors. Mature trees are less tolerant of root pruning than young trees, trees on sites exposed to high winds are less tolerant than sheltered trees, and trees with defects or poor general health are not good candidates for root pruning. The closer to the trunk the roots are pruned, the greater the effect on the tree. A rule-of-thumb is to make all cuts at least a distance of three times the trunk diameter from the outside of the trunk. Thus, root pruning of a tree with a trunk diameter of two feet should be done no closer than six feet from the trunk. Make all cuts even farther from the trunk for trees which are judged intolerant of root pruning.
When root pruning mature and intolerant trees use a stump grinder to level the offending roots. Grinding produces less damage than indiscriminate root pruning. After grinding the offending roots, add coarse gravel as a base for a new sidewalk or pavers. Root pruning machines and vibrating plows cause less damage than do trenchers and backhoes. Prune only one quadrant of a tree's root system in a given year; wait at least two years before pruning another quadrant.
I can not visualize the step problem but if the roots are lifting the front of the steps, I would cut these roots and replace the steps.
It took many many years for the roots to grow to the point of lifting the walkway so it will take many for this to be a problem again. In the meantime the tree will give shade for the house and this can not be replaced in a decent time frame so I would not take the tree down.
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