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Old 11-17-2010, 08:53 AM   #1
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Duct cutouts in joists


Hello. I'm adding an island vent hood as part of a kitchen remodel and need to run some new duct. In order to vent to the outside wall, I need to go perpendicular through 3 joists in the kitchen ceiling. Joist construction is 2x10 @ 16" O.C. with a 14' span. Space above the kitchen is a bedroom.

The duct coming off the vent hood is a 6"round, but of course I will need to transition to something else so the openings are within 1/3 of the joist height. I plan to use 3.25x10 rectangular duct to pass through the joists. So, I plan to cut 3.25x10 out of three adjacent joists. I know that slightly more than 1/3 but very close.

Two questions...

1. Where is the best place along the span to make this opening? I've been told that doing it at 1/3 the span is best? Does that sound right?

2. How should I reinforce the joists after making the openings? I plan to place 1/4" steel plates on each side of the joist and bolt them through the joist. Plates would be 9x30 with the rectangular cutout in the middle and 9 bolts on each side. Does this sound right? Does the steel need to be glued to the joists? What size bolts should I use? Is 30" long enough?

Any other suggestions? Thanks!!

:-peter

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Old 11-17-2010, 09:38 AM   #2
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Duct cutouts in joists


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Originally Posted by independentpete View Post
Hello. I'm adding an island vent hood as part of a kitchen remodel and need to run some new duct. In order to vent to the outside wall, I need to go perpendicular through 3 joists in the kitchen ceiling. Joist construction is 2x10 @ 16" O.C. with a 14' span. Space above the kitchen is a bedroom.

The duct coming off the vent hood is a 6"round, but of course I will need to transition to something else so the openings are within 1/3 of the joist height. I plan to use 3.25x10 rectangular duct to pass through the joists. So, I plan to cut 3.25x10 out of three adjacent joists. I know that slightly more than 1/3 but very close.

Two questions...

1. Where is the best place along the span to make this opening? I've been told that doing it at 1/3 the span is best? Does that sound right?

2. How should I reinforce the joists after making the openings? I plan to place 1/4" steel plates on each side of the joist and bolt them through the joist. Plates would be 9x30 with the rectangular cutout in the middle and 9 bolts on each side. Does this sound right? Does the steel need to be glued to the joists? What size bolts should I use? Is 30" long enough?

Any other suggestions? Thanks!!

:-peter
None of what you want to do would be correct. Not the hole size or the plate size. Call in a professional for guidance.
Ron

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Old 11-17-2010, 11:49 AM   #3
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Duct cutouts in joists


100% agree.
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Old 11-17-2010, 01:25 PM   #4
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Duct cutouts in joists


Thanks, I'll do that. In the meantime, would you mind elaborating on why the hole size is wrong? I realize its 3/16" taller than 1/3 the joist height. I might be able to squeeze it down 3/16" to keep within the 1/3 rule. Is that it, or is there something else wrong with the hole size?
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Old 11-17-2010, 01:39 PM   #5
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Duct cutouts in joists


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Originally Posted by independentpete View Post
Thanks, I'll do that. In the meantime, would you mind elaborating on why the hole size is wrong? I realize its 3/16" taller than 1/3 the joist height. I might be able to squeeze it down 3/16" to keep within the 1/3 rule. Is that it, or is there something else wrong with the hole size?
Your understanding of the hole size is incorrect. The largest size hole I would put in a 2x10 would be a 2" in diameter hole.
Did the hole width of 10" ever concern you? You have a very vague idea of the hole size and location and spouting it like gospel.
I would call the local code office and get the precise information you need before ramping up the power tools.
You might be able to box out the area to provide a path for the ductwork, but I can't see the structure you're dealing with, so I don't know.
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Old 11-17-2010, 01:55 PM   #6
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The largest size hole I would put in a 2x10 would be a 2" in diameter hole.
My understanding per IRC and other codes is that you can put an opening that is up to 1/3 the height of the joist and no closer than 2" to the top or bottom of the joist. That would equate to about a maximum 3 1/16" hole. Your 2" max seems conservative based on this. Yes, I am not sure about the width of the opening because the code doesn't address that. That's why I'm asking before I cut.
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Old 11-17-2010, 02:05 PM   #7
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Duct cutouts in joists


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My understanding per IRC and other codes is that you can put an opening that is up to 1/3 the height of the joist and no closer than 2" to the top or bottom of the joist. That would equate to about a maximum 3 1/16" hole. Your 2" max seems conservative based on this. Yes, I am not sure about the width of the opening because the code doesn't address that. That's why I'm asking before I cut.
Again...,ask a professional. One who has actually seen the situation and the loads involved.
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Old 11-17-2010, 03:18 PM   #8
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http://arch.umd.edu/Tech/Structural_..._Guide_A11.pdf

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Old 11-17-2010, 03:30 PM   #9
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Thanks for the link Gary. I've previously read that and many others like it. Unfortunately, they only show round bores and make no reference to any "rules" for a rectangular "bore". The notching rules don't apply, since I'm not notching.
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Old 11-17-2010, 05:29 PM   #10
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Duct cutouts in joists


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Originally Posted by independentpete View Post
Unfortunately, they only show round bores and make no reference to any "rules" for a rectangular "bore". .

Quote:
Holes: Do not bore holes closer
than 2" from joist edges, nor
make them larger than 1/3 the
depth of the joist.
that says you cannot make the hole any larger than 1/3 the depth of the board. Since that calculated dimension is 3.17", then you cannot make a hole larger than 3.17" in diameter. Your oblong would be greater than 3.17" limitation.

definition of diameter for objects other than circles:

Quote:
For a convex shape in the plane, the diameter is defined to be the largest distance that can be formed between two opposite parallel lines tangent to its boundary, and the width is defined to be the smallest such distance.
So, for your cut, the diameter would be the larger measurement across the center point from end to end of your shape. That would be larger than 3.17" so therefore, not acceptable without specific engineering approval.
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Old 11-17-2010, 05:41 PM   #11
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Duct cutouts in joists


Could you save all this debate and build a soffit below the ceiling to carry the vent duct?

I'm nowhere near a pro, but cutting joists for ducting is something I've never seen.
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Old 11-23-2010, 05:24 PM   #12
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Could you save all this debate and build a soffit below the ceiling to carry the vent duct?

I'm nowhere near a pro, but cutting joists for ducting is something I've never seen.
I've seen this done on several projects. There are joist reinforcements specifically made for this kind of situation. It took me a few days to find the info on the manufacturer I was looking for. Hopefully this will do the trick.
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Old 11-23-2010, 07:03 PM   #13
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Duct cutouts in joists


Quote:
Originally Posted by independentpete View Post
Unfortunately, they only show round bores and make no reference to any "rules" for a rectangular "bore".
They're not there because they're not allowed with dimensional lumber and that's why they're not there. You can't do it. Fiind anoother way. It will never pass inspection your joists will fail guaranteed.
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Old 11-23-2010, 07:35 PM   #14
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Originally Posted by is5kevin View Post
I've seen this done on several projects. There are joist reinforcements specifically made for this kind of situation. It took me a few days to find the info on the manufacturer I was looking for. Hopefully this will do the trick.
If you're an architect, you would have spec'ed them. So it took you "a few days" to find them?
Sounds fishy(spam fish) to me.
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Old 11-23-2010, 07:59 PM   #15
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Duct cutouts in joists


yes, it does sounds like spam.

but beyond that, it would appear to use one of those, you still need a professional. I'm not big on building codes in general but in the documents associated with the device, it appears to suggest that this item must be approved for a particular installation. It provided direction to apply for a permit for that permission and along with other items required, give direction to have the specific intended use to be approved by a licensed professional (engineer) from the state involved.

OP would be better off just calling an engineer and asking them to design a method and if it included this device, so be it.

The fact remains: OP cannot do what he wants without an engineers approval of the design of any suggested installation method.

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