If you are going to sod over some roots I would add a couple inches of soil at least so you get good soil contact. In a recent university study it was found that tree roots actually put out a mild poison to discourage grass and other plant growth which competes with the tree roots. Also in a university study it was found that trees grow twice as fast when an area of mulch is maintained and it doesn't have to compete with roots of other plants. Just things to consider.
We plant lots of Fescue Sod at http://www.nashvillesod.com