You could go with square tubing instead of round pipe. Or make your own rectangular stack to your own dimensions to make it the exact height you want by making it wider or narrower to get the right volume while making it high enough for your roof but not a flagpole sticking out. 4" square tubing will be close to the same volume as 5" pipe, little less but close. Also, dual stacks will work as far as I know. Hope this helps.
The dual stack option will work fine but they will need to be on the same end of the CC for the Rf to work right. Some folks weld them right together. Just use the size pipe you have for the calc and cut it in half to get the right length..
jm2cw
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........
What RC said! I asked a similar question on my build. Since you are new and I'm barely ahead of you by on a few months maybe this will help.
If you have 4" pipe and the calculator says 40" tall. Make two exhaust 20" tall out of the 4" pipe. Or make your own custom exhaust like motor said. Some of these pros it's like adding 1+1 with us newbies questions. Sometimes we aren't for sure where the 1 came from. As you get in it. Keep asking questions. But we do like pics. Look at me 3-4 month member using "we".
I used Feldons calculator also until I found this site. Look for the site's calculator on the main forum page.
If you have a Napa Auto close to you, they sell 5" exhaust pipe for like $10/ft. That's what I bought for my smoker. But yes, that is very tall. I'd use 6" pipe and bring it down to somewhere close to 30" tall.
I found a shop that worked with underground water lines, and got a section om EMT tubing from them in painted steel. Took me 5 minutes with a grinder to take the old paint off ,and I was left with a NICE steel pipe that wasn't so thick and heavy I had to worry about bracing it.
How tall is your roof above your smoker? Figure the height of that and get it through the roof. The length of that stack may change your diameter of pipe needed.
Thanks guys I did think about 5" exhaust pipe which is probably what I'm gonna go with just to stick with the whole keep it simple thing lol. But I am kinda basing my diameter off of how high my roof will be and the total finished height of the rig but I really appreciate all the feedback and I will try to get some pictures up as I go just to get some input in case there is something I'm overlooking that y'all's experienced eyes might catch. Thanks again guys!!!