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I'm working on my kitchen to add a range hood that goes outside the house. I'm putting the 6" exhaust pipe in my ceiling, between my 1st and 2nd floor, going in the direction of the ceiling joists, as represented by the red arrow in the next picture:

While going this direction however I found 2 boards put together of at least 12 inch height, probably more, in perpendicular direction of the other joists, which marks the end of the inside of the house (Siding would be attached to the other side of the board).

As you can see in the following picture, there are 2 sides in the ceiling:

Side A is inside the house, the blue lines are the studs in the ceiling holding the second floor. Side B is a part that extends a bit out of the house (backyard), there is nothing on top of it, just a small roof on the backyard.

I want to put the exit of my exhaust on this small roof. My question is: Is this, is this a loading bearing joist? Is it safe to put a 6" hole in it (you can see I started it before realizing what I was doing)? What can I do to safely put the exhaust through it?

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    All I read was the title of the post, but I'm pretty confident in saying NO, don't cut a 6 inch hole in a joist. I think the rule is 1/3 of the width of the beam, max (and if you do that, there are rules to follow about where in the beam you can put the hole). – Z4-tier Oct 22 at 5:01
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    If you're looking for a very quick way to bring all your upstairs stuff down into your kitchen then this would work quite well. In all seriousness, proceed no further with your hole drilling. The joist is doubled up for structural reasons, not because the builder had excess lumber lying around. If you get through that then that joist effectively becomes only as strong as the thinnest part which is really thin!! – MonkeyZeus Oct 22 at 13:13
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    How deep have you drilled already? – Caius Jard Oct 22 at 15:59
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    I love that you got halfway through before pausing to wonder. Were there strange creaking sounds, I wonder? – Strawberry Oct 22 at 16:19
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    If I might add, and it should have been obvious earlier...That doubled joist is actually a beam, not a joist. Those follow a more stringent rule for drilling. I do not know what the largest allowable hole is, short of a 3/4" hole for electrical wiring, but that is all I do know. The mention by the OP about the outside roof and inside the house, that beam is carrying the outside wall above it with possibly the roof portion as well. I will add this as an edit to my answer. There is no amount of reinforcement you can add that will allow you to get a 6" hole in there. @MonkeyZeus had it right – Jack Oct 24 at 5:41
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That joist is carrying something, even if it the weight of the material that make up that part of the house. I would say it is more than that since the new 2X with the framing anchor is attached to it, is telling me it is passing the load from the new 2X onto the one you wish to drill.

Now onto drilling dimensional lumber. Code only allows you to drill a hole that is no larger than 1/3 the total height of the material you plan to drill. 12" material, 4" hole maximum. There are metal brackets available online that will allow larger holes to be drilled.

10-23 edit

If I might add, and it should have been obvious earlier...That doubled joist is actually a beam, not a joist. Those follow a more stringent rule for drilling. I do not know what the largest allowable hole is, short of a 3/4" hole for electrical wiring, but that is all I do know. The mention by the OP about the outside roof and inside the house, that beam is carrying the outside wall above it with possibly the roof portion as well

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    I think some codes also specify how close to the edge of the lumber the hole can be. – Steve Wellens Oct 22 at 4:37
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    @SteveWellensYep, that is right.The 1/3 rule applies, but you can't, for example, make a notch that is that deep. Also, greetings from St. Louis Park. – Z4-tier Oct 22 at 5:03
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    In dimensional lumber it is to be centered, in engineered lumber, you can make a type of hole to the top and bottom cords. Need to follow the makers specs for engineered lumber. This is not the case here.... – Jack Oct 22 at 5:36
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As J comments, this is not a joist, it is a beam on which the joists are hung. Likely there used to be a load-bearing wall there that a prior owner had removed to enlarge the space. They could have opted to have the beam under the joists, which would have allowed space above it but would have reduced ceiling height. Instead they opted (surely at higher cost) to cut it into the joists, allowing a nice smooth ceiling, but blocking crosswise space. I have several such beams in my house, following renos, and I have some wiring runs that are crazy roundabout to avoid going through them.

This kind of work may have been custom-engineered based on load estimates. It's carrying a lot of weight. I'd be loath to drill 3/4" through it for wiring, never mind 6" for an exhaust duct, especially if (and it's hard to tell from the photo) it's engineered lumber and not just two-by spruce/pine/fir.

At either end of this beam, possibly buried within the finished walls, will be posts taking the load down to the house's foundation. If one is in the middle of the house, it will be on a load-bearing wall or on a post going down to solid ground. Find those and make a note that those are important, too.

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Clues that it is likely load bearing are the sheer size (two side by side planks, you don't do that without a reason) and the fact that it looks like it's supporting the studs. What is the span between walls if you follow its length? I don't think you can safely put a hole in that without reinforcing it, and you'd need an engineer to tell you how much reinforcing is necessary.

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    Well, the other clue that it's load bearing is that it's the ceiling of the 1st floor and the floor of the 2nd floor.... every joist hangs on that beam. You don't need any clues here. – J... Oct 22 at 13:43
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You not allowed to make that type off hole there is double LVL beam supporting the second floor exterior wall

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ABSOLUTELY NOT!!! That’s a double LVL, which is much more expensive than dimensional lumber, and only used in load-bearing situations. Even a small hole of less than 1” would require an engineer to sign off on it first. Since you’ve already begun drilling, I highly recommend having an engineer look at the damage you’ve already caused to make sure you don’t need to repair it. This is why framing is not DIY.

Looks like you’ll need to figure out another way to vent the exhaust. Sorry for the bad news, but better to find out this way than to have seriously compromised the structure of your home.

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