Great article by Roger Cook in the June 2014 issue. But another hint is to NOT plant a tree in line with any water or sewer lines. The roots of the tree can grow into the lines and cause problems, such as backups and cracked pipes.
It's true when you have clay sewer line.
If you have ABS, no roots can get inside, unless the line is broken.
No matter what the pipe is made of, I would never plant a tree directly over any water/sewer line. Who knows about the power of roots to displace and break pipe in the future.
If the pipe needs to be replaced, any tree wound have to come down possibly adding thousands to the job.
Generally, I would agree with ed21 - it's not a good idea to plant trees on top of a major pipe. But some trees have long reach with their root systems, making it almost impossible to find a spot far away from pipes, unless of course you live on a very large lot.
A rule of thumb: the size of the root system is about the size of the branches and leaves.
I've seen 4" PVC broken from root intrusion where it grew underneath it and lifted/crushed the pipe. And if you have a septic system then being that close to a solid line is often too close to the drain field. Most trees root systems match their 'canopy' but some have deep 'tap roots' (such as pines) while others have roots that will seek water sources for long distances such as a slight leak at a pipe joint (such as willows). Those can actually grow around the pipe and the fix is to remove that root all or most of the way back to the tree, a rather large job, plus fixing the pipe which may be displaced for some distance to both sides.
Best to just not mix the space pipes and trees need to avoid the problem :cool: