The CRZ of a tree, which is also called the “tree protection zone”, is often defined as an imaginary circle on the ground that corresponds with the “dripline” of the tree. The dripline of a tree is where the greatest extent of a tree’s branches end. For narrow-crowned trees the dripline distance should be expanded to insure that the critical tree roots will be protected.
A more effective way to define your tree's critical root zone is to calculate a circle on the ground below the tree that has a radius equivalent to 1.5 feet for every inch in trunk diameter. A tree with a trunk diameter of 12 inches has a critical root zone radius of 18 feet.
The trunk diameter of a tree should be measured at 4.5 feet above the ground. You can estimate this diameter, or calculate it by first measuring the circumference of the tree with a tape, then dividing by 3.14 (a constant known as pi). For example, a circumference of 36 inches is roughly equivalent to a diameter of 12 inches.