Question
We have a mature soft maple tree that is growing on a slight slope and the roots are on top of the ground, can we cover the roots with soil without damaging the tree?

Answer
90 percent of a tree's roots occupy the top 12 to 18 inches of soil. These fine feeder roots spread out, pancake-like, away from the trunk. Roots can extend from the trunk to a distance of 1 to 2 times the width of the tree. Roots take up water, nutrients and oxygen, and release carbon dioxide. Because roots need oxygen, any activity that compacts the soil or raises/lowers the existing soil level can damage tree roots and diminish tree health.

Changing the grade drastically is not something that trees can tolerate very well. At the most, a 2 inch layer of sand at a time is the most one should apply.
The very shallow roots are responsible for absorbing all of the water for the plant, for the uptake of dissolved minerals, and for holding the plant in place. Those top few inches of top soil provide all that the roots will seek, including plenty of oxygen without which they (roots) will die. By adding too much more soil on top, you will be smothering those roots. 2 or so inches during a season and the roots can slowly grow UP, before adding another 2 inches. Use sand. You can then add grass sod over the area.

Also keep the soil away from the tree trunk about a foot. the tree needs the swell of the trunk as it enters the ground-covering it will effect the health of the tree.