If you need a root cut, and do not want to or can't afford a "tree expert", here is a way to do it yourself and be safe in doing so, to yourself and your tree.
If, after your research, you feel that you can cut the root safely, then first, make your cut as far from the trunk as possible. To see how close to the tree base you can cut the root, measure the diameter of the trunk at a point 4.5 foot above the ground. Take this number and multiply it by 6. That result will tell you how far from the tree base that you will need to make the cut.