Question
Our young friend bought property that is loaded with oak trees.  One (trunk) is approximately 11 feet in diameter.  Another is even larger.  The one above is located about twenty feet from his house.  He wants to cut it down and cut down another that is ten feet from his garage. (It's trunk is about 8 feet in diameter.  They are both healthy and gorgeous.  
His argument is that they could fall on his house of his garage.
I told him that the root system is very deep.  He questioned that fact.  After reading a lot of your answers, I learned about the root system and diameter of the roots.  But "how deep?" would the root system be on this huge oak tree.  Would it, in fact, only be about two feet deep as I've read or would it be deeper the taller the tree grows.

Nancy

Answer
The majority of the root system is within the first 2-3 feet of the soil BUT there are roots that extend much deeper AND the roots system is massive. The roots will extend out about 1 1/2 times the distance of the foliage over hang. This is what gives the support. A tree this large has experienced all types of weather events and the wind action over the years has strengthened the tree and the root system making it almost impossible to push over. IF the tree is healthy there is NO reason to cut it based on "it might fall on the house". What you will lose if the tree is cut is the shade and the power bill will increase during the summer due to the sun on the roof heating up the house. Not accounting for the very large cost of removal of the trees.
I see NO reason to cut the trees. They will be there long after the owner has moved and/or died.