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To be able to handle live loads, we recently reinforced the 2x6 joists in our attic by sistering them with 2x8s. To achieve level, the 2x8s are not flush with the 2x6s on the bottom; they range anywhere from 1/2 inch to 1 1/2 inch higher.

The question is then how to apply the guidelines for drilling electrical holes. When they say to stay "2 inches away from the top and bottom" does that apply to both boards individually or just 2 inches away from the bottom of the 2x6 and 2 inches away from the top of the 2x8?

Update

Here's a cross-sectional view of the worst case scenario:

enter image description here

Holes are for wiring only; so 3/4" - 1" in size is all that's needed.

So the question is: How far down from the top of the 2x8 should I drill a 1" hole?

If I drill the hole so it starts at 4 1/4" down, this leaves 2" of 2x8 below the hole, but only 1" above the hole on the 2x6.

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  • Now the off-set is 2”? – Lee Sam May 16 '18 at 22:32
  • Had a contractor do the joist work, so I haven’t looked at each one, just eyeballing it at various places. About 2” looks like the worst spots where the old joists were sagging quite a bit. Most places are 1/2 to 1 1/2”. – Aeryk May 17 '18 at 1:56
  • Well, 1” hole is better than 2”. I’d still stay 2” clear from bottom of 2x8. Do you know the span and spacing? – Lee Sam May 17 '18 at 4:43
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The rules for notching and boring joists allow you to

  • bore holes of diameter up to 1/3 the height of the joist, a minimum of 2" from the top or bottom of the joist

  • make notches in the outer thirds of the joist, width of notch up to 1/3 height of joist, height of notch up to 1/6 height of joist

notching and boring rules

The question and comments seem to say there is as little as 4" of overlap between the sistered joists, making it impossible to make bored holes that are 2" away from the top and bottom edges of both joists.

However if a bored hole is less than the 1/6 max height for a notch, it stands to reason the bored hole can be in that 2" zone; certainly a bored hole removes less wood than a notch.

Note that the height of a 2x6 is 5-1/2" and a 7/8" bored hole is less than 1/6th the height of a 2x6.

Therefore, as long as you

  • make holes no larger than 7/8" (5/8" or 3/4" should be adequate)
  • keep them close to the walls / beams, in the outer thirds of the joists

you satisfy the rules and run your wires without significantly weakening the joists. I'd aim for the approximate center of the overlap if possible, that should be the strongest spot to drill through.

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If the two joists are secured (nailed) together and the tops and bottoms are not aligned, then you’ve moved the neutral axis from the center of the beams.

Normally an equal amount of the beam is in compression (top) and tension (bottom)...thus the neutral axis would be in the middle of the beam. Now, it sounds like you’ve moved the neutral axis down (lower) relative to the 2x8.

Yes, I’d stay 2” away from the bottom of the 2x8 and avoid the top of the 2x8.

Also, I’d avoid the middle third of the span.

I’d look along the line of drilling and make sure none of the holes will be located next to a large knothole. If so, I’d move the hole a few inches.

BTW, if the joists are not secured together, none of my answer matters.

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  • "bottom of the 2x8 and avoid the top of the 2x8" -- So I can basically ignore the 2x6s? If I go through the center of the 2x8s, then I'm close to the top of the sistered 2x6. – Aeryk May 15 '18 at 16:51
  • @Aeryk No, No, No. I would not go through the “center” of the 2x8’s. Depending on the size of the hole, (2” and smaller,) I’d stay in the lower portion of the 2x8’s...and yes, you’ll be in the top of the 2x6. In the worst case, you’ll have 2” of the 2x8 plus 3 1/2” of the 2x6 on the bottom AND 3 1/2” of the 2x8 on top. In the best case, you’ll have 2” of the 2x8 plus 2 1/2” Of the 2x6 on the bottom AND 3 1/2” of the 2x8 plus 1” of the 2x6 on the top. What is the span and what is the percentage of each condition? – Lee Sam May 15 '18 at 18:44
  • I think I get what you're saying. I updated the post with a picture. Drilling a 1" hole centered at 4 3/4" leaves 4 1/4" of the 2X8 above the hole and 4" of the 2x6 below the hole. And the 1" in the 2x6 above the hole is good enough? – Aeryk May 16 '18 at 17:36
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putting holes near the edge of a beam weakens it much more than holes in the middle, notches are even worse.

unless the bond between the two beams meets the definition of lamination (ie: the bond is as strong as the wood) holes near the edge of either beam must be avoided

Drill the holes as close to the center of the 2x8 as you can get without damaging the 2x6 (your maximum allowed size is 1-1/4 diameter). a scuff mark does not count a damage so where convenient I would start with my drill bit resting on the 2x6

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