Questions I here many times in the field are “will cutting roots kill my tree or how close to the tree can I cut the roots without harming the tree or what percentage of tree roots can be cut without harming the tree heath?”
Of course cutting all the tree roots will in fact kill the tree.
A general rule for cutting tree roots is don’t cut more than 25% of the root system.
Measure the trunk diameter at about 4 feet from the ground. Take that measurement and multiply it by 6. Mark off that distance from the trunk and that is about where 25% of the roots can be cut. See image below.
For example if you have a 12″ diameter trunk 4 feet measured from the ground, multiply that by 6 and draw the line 6 feet from the trunk. Another example would be to look at the drip line of the tree as a pie and just cut 1/4 of the pie. It is really important to stay as far away from the trunk as possible.
Roots Are Lifting Walkways
In some cases an alternative to cutting through the entire tree root is to shave the top of the root. This can be done when roots are lifting walkways or new walkways are being installed. The amount shaved should be kept to a minimum. No more than 1/3rd of the top of the root should be shaved.
Make clean cuts when cutting roots. Use sharp loppers or a sharp hand saw. Don’t leave ragged edges on the root ends. Try to cut them off where there are root junctions – just like pruning a tree branch back to the branch collar. Clean cuts aid in the healing process and help form a seal to ward off disease.
Generally the small fine roots in the top 18 inches of the soil are responsible for nutrient uptake. Try not to harm any of these roots by digging or compaction with vehicles or equipment. Try and keep a layer of mulch or leaf litter over the fine roots within the drip line. This helps keep moisture in the soil and the breakdown of organic material creates much needed soil nutrients.
Try not to cut any of the real large roots. The larger roots give the tree structural stability. Cutting the large roots on just one side of the tree poses potential danger of the tree falling in that direction in a violent wind storm. Large roots on the back side also add much needed support.