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enter image description here enter image description here For scale, the holes in the bar are 2" apart and run end to end, despite not always being visible in the photo.

I've been trying to drill three holes in a line across three ceiling joists. But one joist in particular is very strange.

The center and right holes seem normal and go into joists that measure 16" apart from each other.

The left hole is the strange one. It measures 12" from the center joist/hole and bordering on its right edge is a 4" hole in the drywall for a lamp. It was the first hole drilled since looking into the lamp hole seemed to indicate there was a joist just to its left. When looking in the hole, I can see the right side of a 10" tall joist. There is no step in the joist that you would expect from an I-joist and probing the side of the joist through the lamp hole seems to indicate it runs all the way to the wall like a joist would. It was after when I started searching for the joists for the center and right holes I realize something was off about the left hole/joist.

Drill into that left "joist" is very strange. It's only very slightly more difficult to drill into than if I drill drywall and miss wood and nowhere near as difficult as if I hit actual solid, thick wood like with the other holes. If just drill through the dry wall and stop there does seem to be wood there. However, if I continue drilling and probe the hole, there is no end to the hole. It is as if I drilled clear through the joist when my drill is not long enough to do so.

Drilling an exploratory hole on the left side of the left joist, I can feel the side left side of the joist.

Does anyone know what is going on here? You can see in the photo that I have since drilled 16" left of center and did hit solid wood. But it still doesn't explain what it is that is running immediately left of the lamp hole.

EDIT: Could builders built something to mount the lamp 4" away from a joist normally spaced 16" apart and then placed the lamp hole there? That would explain why the joist seems to be unevenly spaced at 12"-16"-16". I can't think of a reason why you would do that though especially when the the so-called joist seems to run from wall-to-wall.

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  • What is a "lamp hole"? If it's a 4 inch round hole, you ought to be able to stick a flash light in there and see the joist or whatever it is. If it is an I-joist as in the answer, you must stop drilling its bottom flange! The flange is under tension along its length and you must not weaken it too much.
    – jay613
    Dec 11, 2021 at 16:34
  • @The answers just made me remember I left out some important details. Please see rewrite of the question.
    – DKNguyen
    Dec 11, 2021 at 18:34
  • I've read the updated information but it still remains a puzzle. What led you to drill the left hole 12" from the center joist, why not 16" as usually expected? The right side of the left hole borders a 4" lamp hole, which borders a 10" tall joist to its right, so this joist is 8" from the center joist, correct? Can you see what is to the left of the left Hole?
    – r13
    Dec 11, 2021 at 20:42
  • @r13 The first "joist "drilled was the left hole that was 12" from everything else because the lamp hole was right there so I could see the wood so it seemed like a sure thing. Then I realized something was off when I tried searching for the other joists. There is 12" the between the left hole and center, and 16" between the center hole and right hole. The 4" hole's edge sits flush with the right side of the left hole (it's in between the left and center holes closest to the center hole).
    – DKNguyen
    Dec 11, 2021 at 22:33
  • How about a picture and an explanation of what a lamp hole is? YES the lamp could have been mounted on something built between joists, possibly built expressly for the lamp. That is what my answer is about.
    – jay613
    Dec 11, 2021 at 22:36

2 Answers 2

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The lamp box may not have been hung from an actual joist, but perhaps from an I joist (other answer) or from blocking between joists. The blocking may be the same dimensions as the joists, as in the picture below, or it may be smaller, perhaps a 2x4 or even a 2x4 laid flat between joists just for hanging the lamp. You may be drilling where you think a joist should be but isn't, or you may be drilling through a 1.5 inch thick piece of blocking into a 10 inch space.

enter image description here

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  • @The answers just made me remember I left out some important details. Please see rewrite of the question.
    – DKNguyen
    Dec 11, 2021 at 18:34
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You might have drilled into the flange of an "I-Joist", which can be 2 1/2" wide, and 1 3/8 - 1 1/2" thick.

enter image description here

ADD: After the update and clarifications, I think you have drilled into the joist that has a hole for the electrical conduit. It is either an oversized hole or not used.

https://www.apawood.org/i-joist

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  • @The answers just made me remember I left out some important details. Please see rewrite of the question.
    – DKNguyen
    Dec 11, 2021 at 18:34

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