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Rim joist violated?

edee_em's Avatar

Join Date: Dec 2011
Posts: 99

11-14-16, 03:51 PM   #1  
Rim joist violated?

I am in the process of replacing a kitchen exhaust fan. It requires 6" ducting and 5" was installed in the house 30+ years ago. No biggie to swap it out, or so I thought.

Having everything pulled out I noticed the rim joist was cut to allow the 3 1/2x10" vent to the outside. So, there is a roughly 4x10" hole taken out of a 2x8" rim joist. Joists are 16"oc and there is a bedroom above the kitchen.

It looks like it is severely compromised but after 30+ years of being this way I haven't noticed anything untoward going on structurally. Also, at the vent opening there doesn't seem to be any stress cracks or such things as that.

I guess my questions are: can a rim joist for one joist bay be cut out like this? And, if I wanted to cut it more, say to install a 6" round duct instead of the 3 1/2x10, can I do that?


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pugsl's Avatar
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11-14-16, 03:57 PM   #2  
I would reduce the vent down to the 5 inch. The amount of loss you will never notice. Not sure about joist.

XSleeper's Avatar
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11-14-16, 04:05 PM   #3  
Rim joists sit solidly on the sill plate and foundation. Unlike a joist, which has to span from point to point, there are no restrictions on holes in a rim joist.

CarbideTipped's Avatar

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11-14-16, 04:08 PM   #4  
As Pugsl said, you can run 6" for most of the run and then reduce to go out the rim.

That said, most of the load to the rim joist is transferred from the wall studs above, and they are usually aligned with the floor joists. So cutting out the rim in between two studs and/or two joists is usually not a big deal. The bottom plate of the wall above also helps spread the load.

And....having said all that...the area of a 6" round pipe is about 28 sq inches. Your 3.5 x 10 is 35 sq inches. So all you really have to do is swap out the 5 to 3.5x10 for a 6x 3.5x10 and you will be good to go with no further cutting.

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11-14-16, 04:40 PM   #5  
the rim joist was cut to allow the 3 1/2x10" vent to the outside.
You don't need a new 6" round wall cap, just install a new 3.25 x 10 wall cap.
Where your new 6" duct meets the rectangular cap, use a 3.25 x 10 x 6" round adapter.
There's no problem with adapting 6" round to rectangular.
Microwave hoods and some high quality vent hoods even require and specify that 7" round be adapted to 3.25 x 10 at some point.
You'll have plenty of air flow.

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