In normal circumstances, the root ball of a tree more or less mirrors the crown of the tree. That's kind of the general rule of thumb, but it really depends on local growing conditions. If the soil is somehow in certain areas not as permeable as in other areas, the roots will find the best place for them to go, so you can have very idiosyncratic patterns of root growth.
On April 2nd, 2019. See this statement in context.