Thanks, Ghost,
That's very helpful. The real problem I am having is how do I determine what number to set for the maximum height if I want to make a 5th floor (one above the roof)? If I can find out that, I can make 4-5 more floors above the roof. I saw a 18x18 on Atl that has 6 floors above the roof. Used ladders to get to each of the levels, plus marked the holes with torches to remember how to get up and down.