Question
We have an englemann spruce tree whose trunk is 32" in diameter and the tree is about 80' tall. What is the size of the root plate? We are considering an addition to our home and the foundation will come to within three diameters of the tree trunk. The foundation encroachment would represent 3% of a root zone established six diameters away from the trunk. ...The tree in question is in on neighbor's property. What do you think?
Thanks,
Joseph

Answer
The most critical root zone encompasses the main root plate, the area adjacent to the base of the tree containing the largest roots, This is generally the area starting directly below the outer most limbs (drip line) back toward the trunk. The entire root system will extend about 1 1/2 -2 times this width.
The general rule is not to cut more than 20-25 % of the roots within the  dripline. If you are not damaging but 3% of the root system you will not see any effect on the health of the tree. Sounds like you have done your homework and are ready to begin the addition.