How can I install a hood exaust vent with a joist in the way?

Question

I was preparing to install a hood vent this weekend and found myself with a bit of an issue...
Here's what it looks like:

The larger hole in the middle is the center point over my cooktop and the red line is a 2x4 ceiling joist going over the top. The 4 smaller holes are where the mounting plate for the hood attaches (they look off but that's the angle of the picture).
I need to put a 6" circle in the ceiling to run the ducting through and that's pretty impossible here. I'm assuming I can't cut the joist, or maybe I can if I hire a carpenter to somehow shore it up. I think I could remove some metal from the mounting bracket and fit it though. The bracket looks like this:

If I basically cut out one side I can probably get a flexible 6" duct through the offset hole. However I'm worried that this could impact the strength of the bracket, and also that flexible ducting might collect grease in the ridges of the tube.
Any thoughts or other suggestions on how I could tackle this and what the best option may be?

P2000 · Answer

I would cut your "Top Support Plate" (as it's called in the picture you linked in one of your comments), as you suggest in the picture of your question.
Attach a metal 6in 90 to drop down (as I suppose you were going to), and then 6in ducting down and attached to the vent unit, all inside the shrouded box.
There seems to be enough distance down to allow a minor slant to this portion of the vent, form the off centre exit point at the top plate, down to the fan unit.
And attach the plate to the joist for extra strength.

handyman · Answer

I wouldn't be too concerned cutting the mounting plate since the fasteners are in the corner. But there is another option if the parts are available where you are.
And that is to buy a couple of round to square connectors and some flat square pipe (well, it's oblong or rectangular really).
Flat oblong piping is often used on top of cabinets for it's lower profile, so it's not seen from the floor.
Cut the round ally duct under the ceiling, convert to square pipe and then once you're past the joist, convert back again.

How can I install a hood exaust vent with a joist in the way?

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