Question
The neighbors 30+ old oak tree is growing over the boundry line into my yard and over my house.  It is probably only 15 - 20 ft from my house, and the water and gas lines run through the space between my home and their tree.  It has already taken out an old fence that "I replaced".  How far do the roots go out.  I am concerned about their mangle my water and gas pipes.

Answer
A tree's root system will extend about 1 1/2 the distance that the branches extend. The majority of the roots and all of the larger roots are within the area from the  branch ends to the trunk. The roots are in the top 24 inches of soil. Roots can get into sewer pipes since they seek water and if the pipe is cracked they can get into these. Water and gas pipes are not a problem. Normally these are deeper than 2 feet and are below the roots. if they were above the roots than there could be a problem with the roots pushing up the pipes (similar to roots under a sidewalk). I do not think you will have a problem with the pipes.
If the limbs are over your house make sure they do not rub the house this could damage the roof. These overhanging limbs can be prune back.