That 90% of a tree root system can be found in the top 18 inches of soil. That's why doing any construction, even light construction, near a tree can be extremely hazardous to its health.

When a tree grows, it grows from the top and the sides. A branch emerging from the trunk at a point three feet from the ground when the tree is five feet tall will still be three feet from the ground when the tree is 40 feet tall.