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I'm finishing the interior of my garage and will be installing recessed lights in an angled part of the ceiling pictured. I wanted to know if it is OK to drill a 1/2" hole across each ceiling joists (red in photo) to run my wire to each can or should I run the wire to the wall, down through the top and across the wall studs then back up (blue in photo)? Thanks in advance.Roof joists

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    You call these both roof joists and ceiling joists, is there occupied area above these or is it just the roof (which I believe would make the correct term rafters)? – Jimmy Fix-it Apr 11 at 17:07
  • Yes, just the roof, no living space above. – Vincent Apr 11 at 17:09
  • I want to keep my wiring well clear of that crow bar. – Jasen Apr 12 at 4:46
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More information details would be needed to work out a definitive answer but in general the answer is yes if you keep the hole in the middle 1/3 of the joist depth. The worst thing to do is cut the bottom 1/3 where the joists are in tension and cutting it there will severally reduced the strength and create stress concentrations that will most likely cause cracking over time.

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  • Sure, what details do you need? – Vincent Apr 11 at 19:59
  • For now: span, member depth, snow load. Verify that the rafters are supported at the ridge with a beam. Were those sistered members for only nailing of the sheathing or was it an attempt to address sagging rafters? – Ack Apr 11 at 20:06
  • Yes, sistered to fix sag when roof was redone in 2014. Rafters are nailed at ridge to a beam then span approx 17ft where they are notched and supported by a beam (that is supported by a cement lally column which was also added to support sagging roof), then run another 58" where they are notched again and sit on the exterior wall framing. – Vincent Apr 11 at 20:45
  • You didn't include their size but that's ok. Well we know that they are inadequate as originally installed (sagging) and the sistering as done is not going to add a lot of additional support. More than likely, they wont' fail because on long spans such as yours the deflection controls over the strength. 'Bending' strength would be next in line to consider. An added hole would affect both the most at mid-span reducing to zero at the supports. I advise to not drill holes at all even if they won't change things much if placed mid rafter height near the supports – Ack Apr 11 at 21:12
  • I would be putting a 1/2" hole in the 58" portion between where it is notched and sitting on top of the beam and exterior wall. Is that still a problem? Seems like it is in an area with a lot of support. – Vincent Apr 11 at 21:31
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The rafter is weakest where the two different sizes of sistering meet. don't drill near that.

when drilling drill near the top of the middle third of the rafter so that you also hit in the the middle third of the sister. A half-inch hole is tiny and should not significantly reduce the strength of the timber.

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