A recent blog published on Deeproot Urban Landscape looked at the depth of roots for several different types of trees. They compared several different research papers to debunk a common myth about tree roots. This group was interested in learning the answer to this question because they are focused on the improving green infrastructure through street trees. As a home owner or landscape designer the study does have several relevant implications.
The deeper the roots the more drought resistant a tree is. So be careful when planting a drought resistant tree because we know that they will grow deep roots.
Read entire blog here and find out how deep roots grow.