Question
We have a row of trees lining our driveway, a mix of cedar (Thuja plicata) and Douglas Fir (pseudotsuga menziesii).  All are aproximately 8"-10" in diameter and aprox a foot to a foot and a half apart from each other.  There are a dozen of them (crazy, I know).  They are quite close to the house and driveway - 5 feet to driveway and 6 to 12 feet from house.  Immdeiately to the otherside of the trees the land drops off at a 45 degree angle, which then meets the property line 2 feet away for some trees and 6 feet away for others.
Our problem:  we have to dig a trench 4-5 feet deep to connect to the public sewar right past all these trees.  If we go the route on the slope, we will definately have to takeout two trees and our privacy with it!
We're worried that cutting in to the side near the house will cause trees to fall, especially since the grade is so steep on the otherside.
We're waiting for an expert to come see them, but i thought I'd see what info I could find online - and found this site.
Any thoughts?

Answer
I am having some trouble getting a good picture of the set up but I can give you information on the root system. Root of tree will extend out about 1 1/2 to 2 times the width of the branches. The roots are in the first couple of feet of soil (the fir may have a tap root that extends down under the trunk area). IF you cut a trench through the root system you will kill the roots that are cut off. You can damage up to about 1/3 of the roots without seriously damaging the tree. Sounds like this would be what would happen if the trench was dug near the trees. Cutting the roots should have little effect on the stability of the tree but can have a great effect on the overall health of the trees.

YES too many trees in this very small space. Thuja plicata can grow at maturity about 25 feet wide. It can be pruned to keep it narrower. Pseudotsuga menziesii grows to a 20 foot width and also is tall 60-80 feet. IT does not do as well as far as pruning as does the thuja. Not sure what effect you are trying to achieve with the trees but it sounds like you may have the wrong species for the space. Without a picture or being there I am handicapped. You may want to think about installing the sewer line and then put in a hedge type plant for your privacy. There several evergreen hedges that would fit these conditions.

Without seeing the area that is about all I can recommend.