Re: concession hood instalation
If you have poked around here enough, you should know that whoever enforces all the rules in your particular area is where you should go first.
Some areas require that your hood and fan be installed by a professional only.
But in any case, most jurisdictions will require that it be done to code. NFPA96 code. It's not a fun read.
Unless you have a 100% metal roof with nothing between the hood and roof, I think there still needs to be an air gap.
The duct from the hood to the fan is supposed to be welded and welds ground smooth. NFPA96 is pretty specific.
Please check with your local authorities first.