Tree roots - cutting big roots vs. sucker roots/root shoots

 https://gardening.stackexchange.com/questions/1294/can-i-cut-off-exposed-tree-roots-without-harming-the-tree

Basically, the closer you cut a big root near the trunk, the more damage/stress a tree will have. An 8inch diameter trunk will have a 12 foot radius safe zone around it by this sort of thinking. Granted, these are tree people who are fairly serious about not harming the tree. 

TL; DR;

• Measure the trunk diameter,
• multiply by 18
• don't touch anything within that distance from the tree.

Sucker roots went here: https://en.wikipedia.org/wiki/Basal_shoot