floor joist hole allowance

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  #1  
Old 02-10-19, 12:26 PM
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floor joist hole allowance

I installed a bathroom in my basement and need to run a 3 1/2 inch vent through one floor joist, the joist is 2x8 (1 1/2 X 7 3/8) with an 8 ft span across a cinder block wall. From what I have read per code you are not supposed to cut a hole larger than 1/3 the depth of the joist. So my thoughts are to add a 2ft long section of 3/4 ply on both sides of the joist before drilling my 3 1/2 hole, is this okay? thanks
 
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  #2  
Old 02-10-19, 12:37 PM
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I don't believe adding the plywood to either or both sides will meet code. Will it help your situation. Most likely, But I believe you must sister the full floor joist to be able meet code. You may need to consult you own engineer.
 
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Old 02-11-19, 09:06 AM
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so if I were to sister the joist with another joist over the expansion then it will be okay to cut the 3 1/2 hole through the both of them?
 
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Old 02-11-19, 05:11 PM
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I misspelled in my last post. I meant to say "talk to your TOWN engineer" for clarification.

I really can't answer that. I'm not a structural engineer. But my gut feeling is, yes it will do just fine if that is the only hole and it's not close to the end.
 
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Old 02-11-19, 06:36 PM
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you are not supposed to cut a hole larger than 1/3 the depth of the joist.
How does making it thicker make it okay?
 
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Old 02-11-19, 06:59 PM
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it is harder to snap a 4x4 in half or a 2x4?
 
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Old 02-11-19, 07:18 PM
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LOL! If you drilled a 3 1/2" hole ALL THE WAY THROUGH a 7 1/2" board it wouldn't matter how wide it is, it would still deflect and/or snap if you put enough weight directly over the oversized hole. Holes larger than 1/3 are not allowed period, because the deflection would be beyond what is safe.

That's why the codes on notching exist. The code does not change no matter how many members you stack or sister together.

http://www.scpdc.org/wp-content/uplo...od-members.pdf
 
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Old 02-11-19, 08:17 PM
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"if you put enough weight" over a solid joist it will also snap. What im saying is a 2x4 with a perpendicular hole through it is 50% more likely to give under a load then a 4x4. or in my case a 4x8 vs a 2x8, more surface area is less surface tension. Not asking if its code, I was asking an opinion on the strength. Which Ive received. This will be my last response on the matter.

Thanks
 
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Old 02-11-19, 08:27 PM
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Well, you have one opinion either way. Guess you can take whichever opinion you want. I got the impression you were "here to learn".

it is 50% more likely to give under a load
Not sure that's entirely correct. There is 50% more wood, but I don't think that increases the strength by 100%.

Keep in mind that by drilling a hole larger than what code allows you open yourself up to the failure of future home inspections. One of the benefits of this forum is that good advice that is based on structural codes (which are usually the bare minimum, quality wise) "should" stop people from doing things that are blatantly wrong.
 
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Old 02-11-19, 09:37 PM
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(I meant 100% more wood as far as thickness is concerned.... not 50%.)
 
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Old 02-12-19, 03:25 AM
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Would a sufficiently sized steel plate bolted to the joist allow for a larger hole than normally allowed to be used?
 
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Old 02-12-19, 04:11 AM
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That would be for a structural engineer to determine and provide written documentation of.
 
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Old 02-12-19, 04:13 AM
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Thats not a really wide vent, I know it's not your question, but I would just run it under the joist by the wall and build a small bulkhead. I don't imagine it would be that distracting when all done.
 
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Old 02-12-19, 04:41 AM
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That would be for a structural engineer to determine and provide written documentation of.
I have to agree. I wish I hadn't said that my gut feeling would be OK.
 
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