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.