Question
I purchased a home recently and began having sewer issues due to one of the previous owners planting a pin oak directly over the sewer line.  The plumber thinks the shear weight of the tree is slowly crushing the sewer pipe.  I am going to have the plumber go around the tree as much as possible.  How far from the tree does he need to stay from doing major damage to the roots and possibly killing the tree?  Let me add some specifics.
* 50-60% of the roots will not be touched in any way.
* We can stay 20-21 feet away from the trunk for another 25% of the roots.  This will put us inside the outer edge of the canopy but by how far I can't tell.  A guess would say a good 6 ft.
* The last 25% of the roots will be cut eventually getting to within 6-7ft of the trunk due to the proximity of gas and power lines buried nearby.  I'm sure the plumber will want to tie back in before he gets to those utilities.

Can this tree survive that kind of root damage or is this going to be a futile attempt to save it and I should just cut it down now and get it over with?  Thank you in advance for your answer.

Answer
Hard sewer pipes will not be crushed by the tree. If the sewer pipes are the old felt type this could happen. If there is a crack in the pipe the roots could clog the pipe but chemical treatments can control the roots.
If you cut more than 25-30 % of the roots system then the tree will start to show damage--dieback of limbs and branches. The roots extend out about 1 1/2 times the width of the branches and if you draw a circle around the tree at the ends of the branches and if the trench for the pipe cuts more than 25-30 % of this area you can expect some dieback. This can be off set by fertilizing the tree with 10-10-10 fertilizer at the rate of 1 lb of fertilizer per inch of trunk diameter scattered around the tree and watered in good. Do this in the spring and again in the late Fall after leaf drop.

You can install the pipes by tunneling under the tree instead of trenching. her is a web link to this information.

http://www.tlcfortrees.info/what_will_damage_trees.htm