Kitchen hood not level

When we remodeled the kitchen about 8 years ago we installed a Vent-a-Hood over the range top. The hood has a chimney-like spacer with a 12"x12" cross-section that attaches at the ceiling.
It is slightly out of level. Over the 42" length it drops about 3/4". My theory is that the cause is a slight unevenness in the plaster ceiling, either due to sag in the joists or thickness variation in the plaster. Note that a difference of only 0.2 " over 12" will cause 3/4" difference over the 42" length.
Several years ago, I talked the the contractor who installed it. He proposed to carefully jack the ceiling up from below, then go into the attic and nail in a 2x4 vertical brace from the joists between which the hood is mounted and a cross-piece between two rafters above. For reasons I can't quite remember I decided against that.
Now, I'm thinking about essentially the same idea, but implemented differently. I'm thinking about attaching a length of fairly heavy chain by means of lag screws to the joists, and another length to a cross brace between the rafters above. Between the two chains I will put a turnbuckle. Cinching up on the turnbuckle at the low end should lift the joist just enough to level the hood.
Any opinions on the idea?
TIA
Ed
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does it vent poorly because it is out of level? what is your reason for fixing it?
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Charles,
It works fine. It's just that when there is some problem in a room and I know about it my eye seems to be drawn to it. OTOH, I've lived with it for 8 years now, so...
Ed
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I think you are heading for a cracked ceiling. Are you sure the unit doesn't have any leveling feature?

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Actually, that's what got me thinking seriously about it again. There is already a crack in the ceiling and I was thinking about having the plaster repair man fix it when he's here to do some work on a bathroom remodel. I realized that it would be foolish to patch the ceiling and then start jockying it around.
THere's no leveling feature per se, but I suppose it could/should have been shimmed at the ceiling.
Ed

unit
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Have you looked at the installation manual. Seems to me that you basically have a duct with an end cap and it would have been easy for the manufacturer to make it possible to slide the bottom portion in and out a bit before fastening for leveling purposes.

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Art,
No, the hood is attached to a 12x12 spacer piece, which is first attached to 2x4 cross-pieces between two ceiling joists. The duct is inside the spacer.
As I write this I'm wondering now what the installer did to accomodate the 1" gyp-board lath/plaster ceiling thickness. I'll have to get up in the attic and check. Could be that he just cinched it up till the spacer was snug to the plaster all the way around. If so, a little difference in plaster thickness would account for at least some of the tilt.
Thanks.
Ed

basically
manufacturer
before
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I'd be very concerned if the rafters or joists are sagging. You may have some serious structural problems.
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why level the house around the hood, when all you need to do is level the hood....
randy

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Randy,
thanks to this discussion, I'm coming to that conclusions. All I have to do is figure out how.
Ed

level the

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shim one side with washers?
randy

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