I have an old fireplace, and the chimney is an old rock chimney with "mud" holding it together. It's solid, but I must run a liner if I want to use an insert in the fireplace again. Problem is that the chimney top has been covered over by a new roof. Why? Because last year we figured gas heat was the way to go. Besides, the previous owner had removed old chimney cap/bricks (or whatever it's called), and covered it with a tin roof. We changed our minds about the gas, and want to put in an insert, but have to somehow rebuild the chimney extension beyond the roof line.

Question is: what do I build it out of, brick, hollow cynder block squares, other? What are my guidelines?

Also, what is the formula for figuring out where the center of the chimney is from atop the roof?

Please assist if you could (or if you're up to doing the job for me, I live in north central Tennessee).