Question
We are building a new house and trying to respect/maintain all large trees.  We need to lay pipes to hook up sewer from house to street.  Along the way will be two large trees: a red maple (approx 10years old) and a mountain ash (approx 25 years old.)  The trench needs to be 6-8ft deep.  What can you you recommend to minimize tree root damage?

Answer
If you damage more than 30% of the trees roots the trees foliage  will show major die back or maybe part of the tree will die. The trees root system is in the first 12-24 inches of soil and a trench dug near the tree will cut the root system. To estimate the damage draw a circle around the tree out where the branches reach (width of the branch spread) and then make aline through the circle where the trench will be. This cut of the "pie" will be the amount of roots cut. If this is more than 25-30% of the circle you will get major damage to the tree.

Another method would be to tunnel under the tree and the roots this would cause no damage to the tree. Here is a web link to this type operation. http://www.tlcfortrees.info/what_will_damage_trees.htm

Another serious mistake is to allow machinery to drive under the trees and over the roots. This will cause compaction of the soil and kill the roots and greatly damage the tree. Maybe as damaging as cutting the roots. Place barriers around the trees and do not allow machinery or piling soil etc under the trees during the construction. Here are a couple of web link to information on this.

http://www.ext.colostate.edu/pubs/garden/07420.html

http://www.extension.umn.edu/distribution/housingandclothing/dk6135.html