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I just bought a new house and I'm looking to run some cables. One of the areas I would like to run it to would require drilling through a 2x10 sistered joist. Realizing that going through the joist should be a last resort, if the hole is 1/4" and in the middle of one of the joists will this weaken the integrity?

I have seen lots of articles about drilling through the depth of a joist, but very few regarding drilling vertically.

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Don't do it.

The reason that you can drill through the face of a beam in the center is because the bending stress is essentially zero there.

enter image description here

If you drill from the top to the bottom, you're removing the portions that carry the brunt of the loading (the top and bottom of the beam). If a joist that big is sistered, there's a good bet that it's a major load bearing beam section that you don't want to mess with.

Find a different route for your wire. Ask yourself this question: Is it worth risking major structural problems in order to hide one unsightly wire?

Image Source

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  • Really? Even with a hole as small as 1/4"? – user13655 Jun 25 '13 at 16:17
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    A 1/4" hole through a 1.5" wide beam is 1/6 of the cross-sectional area. So you're reducing your load-bearing material by 17%, while also introducing a stress concentration to an anisotropic material. So yes, even with a hole as small as a 1/4", you still significantly weaken the beam. – Doresoom Jun 25 '13 at 16:25
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    Avoid stress risers in the tension face of timber. Just that simple... – Fiasco Labs Jun 25 '13 at 16:30
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You should angle the drill to exit the face of the joist. OR

Consider running the cable just around the nearest corner (tucked behind the baseboard (I'm assuming its a low voltage, not a power cable)) and just avoid the paired joists.

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  • I don't believe that this is an option unfortunately because it's quite awkward. Is there a way to drill it and re-enforce it as to not weaken the integrity? If the joist has been drilled through can it be repaired? – user13655 Jun 25 '13 at 15:34
  • No on the repair. Why can't you move the down spot? – HerrBag Jun 25 '13 at 15:43
  • The wall that the cable would go into is a small wall adjacent to the stairwell. The joist runs alongside the entire stairwell, and beside that (the other wall that's about 8" wide) has vertical wood support beams running through the whole thing. – user13655 Jun 25 '13 at 15:51
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In most cases, maximum stress in a beam occurs at the midspan of the beam. Since this hole was apparently drilled well off mid span, it is likely that the reduction in strength will not be a problem. HOWEVER, in some cases a beam is loaded with one or more point loads, in which case maximum stress DOES NOT occur at midspan.

A vertical hole is completely different, since you are removing material from the top and bottom of the beam, which carry the majority of stress. This results in the remaining beam width being forced to pick up the load, which results in a significant stress increase.

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    Hi @AvDrilling, welcome to DIY.SE. We're not a fan of overt promotion here: this is a site for answering questions, not advertising your company. If you need a link to your website in order to answer a question, that's ok (as long as you note your affiliation, which you did). But in this answer your website link isn't required, and is just tagged on. If you edited to remove the link, your answer would be much improved. – AndyT Feb 16 '16 at 9:25

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