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.

2 Answers 2


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.

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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

  • Really? Even with a hole as small as 1/4"?
    – user13655
    Commented Jun 25, 2013 at 16:17
  • 13
    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
    Commented Jun 25, 2013 at 16:25
  • 2
    Avoid stress risers in the tension face of timber. Just that simple... Commented Jun 25, 2013 at 16:30

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.

  • 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
    Commented Jun 25, 2013 at 15:34
  • No on the repair. Why can't you move the down spot?
    – HerrBag
    Commented Jun 25, 2013 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
    Commented Jun 25, 2013 at 15:51

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