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It looks so neat! (thelemmy.club)
submitted 10 months ago by to c/wtf@lemmy.wtf
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[–] 11 points 10 months ago (7 children)

Assuming it's centered vertically and not too close to the ends, then the joists would still have essentially full strength. Because the top and bottom are seeing compression and tension, there's an area in the middle that's not so critical.

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  • [–] 6 points 10 months ago (6 children)

    No, no they wouldn't. In the UK there are very strict rules about notching or putting holes in wooden joists. Precisely because it weakens them.

    Full joist the load is spread through all the fibres of the timber. Drill a big hole in the middle and the load only goes through the top and the bottom fibres. So essentially it's as strong as those two blocks of fibers. So why not make the joist just that thin the the first place then? The answer is because it's nowhere near as strong.

    There are various steel, composite and wooden joists with a special frame construction that can have hollow sections. A standard piece of timber with a big hole drilled in the middle of it is not one of them

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  • [–] 29 points 10 months ago* (5 children)

    I was trying to keep it simple for the sake of answering the question, but I'm literally a carpenter and drilling solid wood joists is totally fine (at least in Canada) as long as you roughly follow what I said. Maybe I could've said "design strength" instead of full strength but again, trying to keep it short and simple.

    I've worked mostly commercial though so you've inspired me to look back in my books to make sure I wasn't way off base in my recollection.

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  • [–] 3 points 10 months ago (2 children)

    Thanks that exactly proves my point. As your diagram says. You're allowed to drill a hole at a max of 1/4 the width of the joist. So even if that's an 8 inch wide joist the biggest hole you can put in it is 2 inches wide.

    That looks like 4" waste pipe. So to drill a hole through a joist for that the joist would need to be 16" inches wide.

    So you can't just drill holes in joists in the "middle that isn't critical" As I said there are specific rules about where the size of holes you can put in joists because the size and location matters very much to the strength of the joist.

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  • [–] 6 points 10 months ago (1 child)

    Well, the joist loses perhaps 25 % of its strength to the big hole, as opposed to some 75 % to the bottom notch. It's still fair to call it "much better".

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  • [–] 1 point 10 months ago

    It also “won’t pass inspection” and it has the additional effect of “making your property worthless without repairs”.

    Additionally it might still “kill you” when the fridge slides over and tips on top of you when your floor is suddenly 28 degrees sloped downward because it cracked in half.

    Imagine dying pinned under your own couch because you did this thinking it made your house stronger

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