Question
what is the root span of a 100 foot lime tree

Answer
The root area is not based on the age but the size of the tree. Tree roots are generally located in the top eighteen inches of soil. Roots do not extend deep in the soil because oxygen amounts below this level are usually too low to accommodate growth. Requiring moisture and oxygen equally, root growth ceases as either of these two elements become minimal. Typically a tree's root system extends as much as two to three times the distance from the trunk to the dripline. The dripline is the outer most edge of the crown (foliage). If you start her and go sround the tree this would be the dripline. So the roots will extend way past this line.