Hey all, I just found this site about a week or so ago and just want to say that I've all ready learned a lot! I am about to start building a concrete block RF smoker( which which will have a decorative stone face surrounding it). Needless to say the FB and CC will be square in design. I've used the calculator and all seems pretty clear except for the stack. For my build it suggested for a 6" stack that it raises 17 inches. Here's what's getting me. The inside of the CC is going to be lined with 2.5 inch thick refractory brick(including the ceiling). After the brick at the ceiling there will be a 1.5 inch air gap followed by a 2.5 inch decorative concrete slab. At this point I have 6.5 inches between the inside of the CC to the exterior of the smoker. Do I include this in my 17 inches that was suggested or do I not and just make my stack from the start of the exterior of the smoker going 17 inches up? One other question for you: as far as the stack goes, I'm thinking of not using a 6" pipe as I believe it will take away from my useable CC space. Anything wrong with me using thin steel, welding in a rectangle (about 3"x 18") and putting it in instead? The calculator wants a chimney volume 485.97 cu/in. Does the cubic inches come from that 17 inches sticking out of the smoker? Thanks for any help you can throw my way!