DIY: Installing a kitchen exhaust fan

DIYer here, researching the feasibility of installing my own kitchen exhaust fan over a gas island cooktop.
I'm curious about connecting duct to a roof mounted fan on a pitched roof. The sleeve on the fan is going to be entering the attic at an angle. What's the best strategy to route the duct to meet the angle of the sleeve? I can't put the fan directly over the opening. I have to go over a couple of joists and higher up on the roof. Can I just come out perpendicular from each opening with two short pieces of duct and an adjustable elbow at the end, point the 2 elbows at each other and do a straight run between them?
I'm going for the Fantech RE model. Any opinions there?
Where's the best place to put the backdraft damper? Seems to me near the end of the run, close to the roof. This way there's less external air in your duct.
Am I dumb thinking I can do this myself? A dedicated circuit is already in place so it's just duct work and installing & wiring the fan. I'm not seasoned handyman but have a good track record so far with construction and minor electrical. Thanks.
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My city wants a $138 plan check fee, $98 mechanical permit and $58 fee for inspection of a ducted kitchen or bathroom exhaust fan.
Shhh don't tell anyone you are doing it.
Everything you need should be at your local home center like HomLowes-Depot. Ask the sales person in the isle to help (ha, good luck with that suggestion)

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Actually, this is the last correction in our remodel so we can get finalled. Yeah, I learned my lesson 'bout being too honest. I'm anxious to get finalled so we move forward on some other, er, "minor" projects.
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That's about it. I assume that you don't have to go through any joists or rafters, just above or below them. Elbows add to the effective length of the duct so you need to take that into account when sizing the fan.

Fantech makes a very good product.

I've always seen backdraft dampers on the intake side. Check out http://rewci.com/416splobada.html for a spring loaded one.

Definitely has DIY potential but I'm always leery when it comes to cutting holes in my roof. You need to make sure that it is flashed and sealed properly or else...
You also might break a shingle or two so it is helpful to have some spares around just in case.
-al sung Rapid Realm Technology, Inc. Hopkinton, MA
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Thanks Al. And, yup, that roof work is the crux of the job for me. But after watching some of these hack subs work on my house... well, it may take me 4 times longer but it'll 8 times better in quality.
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