Question
Does an oak tree have a fibrous or tap root system?  How deep do the roots usually grow?

Answer
It is generally a rather shallow root system that is composed of many smaller fibrous roots. The root system is usually in the first two feet or so of the ground surface and extend out about 1 1/2 times the width of the foliage. The roots right under the trunk may extend down to 4-6 feet deep.