I have a maple tree in my back yard planted some time in the 1980's. I need to know how deep the spreading roots are because I want to keep the tree if possible, but I have a septic system back there.If they aren't to deep I was going to run a ditch witch along the outside edge of the septic system to keep the roots from growing into the lines.

Exposed Tree Roots: What To Do With A Tree With Roots Showing (http://www.gardeningknowhow.com/ornamental/trees/tgen/exposed-tree-roots.htm)

Although stroud's information is good, I don't think you can determine how deep the roots are. Unless of course you call an arborist who may have a way to determine this. Sounds expensive !

Your question is backwards. You need to tell us (or a local tree expert) how far the tree is from the septic, and what kind of soil you have. There is no general rule for tree roots. But maples usually get huge and of course will be a problem no matter what you do, and if too close to the septic, I'd take it down now.