Question
How much, if any, of a root system can we remove from a Red Oak that is
tearing up our sidewalks.  Trees are very large, 80-100ft tall, and seem to be
very healthy.  We have them pruned every 2 years or so.  The city is making us
repair the sidewalk and I'm afraid to hurt the trees by taking out the roots that
are causing the damage.  Thanks.

Answer
It depends on how close the tree is to the sidewalk. Generally the tree will be ok if not more than 25% of the roots are cut. If you htink of the roots system as a pie and the walk disects the pie --the closer to the center of the pie will cut more of the system.

Here are a few things to consider when the siealk is replaced.

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.

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.

Trees tolerant of root pruning include elm, ginkgo (Ginkgo biloba), honeylocust (Gleditsia triacanthos var. inermis), red maple, silver maple (Acer saccharinum), and sycamore (Platanus occidentalis). Intermediate trees include ash, linden (Tilia spp.), Norway maple, oak, and willow.