Question
can I cut roots that r raising in my yard from a ash tree that's 20 years old.... they just started showing up acouple of years ago.... whats the rule of thumb

Answer
Cutting the roots will cause damage to the tree. Not a good idea, covering them would be best.  

Most people are surprised to learn that 90 percent of the fine roots that absorb water and minerals are in the upper 6 to 12 inches of soil. Roots require space, air, and water. Roots grow best where these requirements are met, which is usually near the soil surface. Piling soil over the root system or increasing the grade smothers the roots. It takes only a few inches of added soil to kill a sensitive mature tree.

You can cover the exposed  roots of trees as long as you do not add more than 2 inches over the top of the roots. Sand is the best since it is less heavier than clay or loam. Do not pack the sand down. Since the water table is not far beneath the surface over the years the roots may again become exposed from the growth. When this happens just add more sand. There is not really  good  data to say how long before they will reach the surface OR if they will. I would say guessing over 5 years.