Is it OK to attach a metal electric junction box and armored cable (MC) to underside of the subfloor in a basement with an exposed ceiling? Or does it have to be attached to the side of the joists in a parallel run?

  • Why is attaching the junction box to the studs an issue? Does it not locate the box correctly? – ThreePhaseEel Mar 3 '19 at 1:50
  • Trying to avoid plumbing pipes attached to the joists in the same space. Mounting to the "ceiling" (i.e. the underside of the subfloor would make the installation much simpler and easier to access. – 1962Ranch Mar 3 '19 at 22:28
  • How fat are these pipes, and how feasible would it be to route a blocking member around them? (A photo would help, for that matter) – ThreePhaseEel Mar 3 '19 at 23:52

Some potential problems could be:

  • You might not have much wood in the subfloor to screw into, so the box might not hold very much weight (if a ceiling fan or heavy light is possible in the future)
  • You might puncture the floor above with the box's screws
  • If someday the subfloor were to be replaced that would be a problem for the junction box
  • Long screws or nails from new flooring could puncture or be stopped by the junction box.

Why not just cut & attach a 2x4 support brace between the floor joists, and then attach the box to it? Then you can screw securely to the 2x4, either on it's side or bottom. Like one of these:

enter image description here [source - Framing to Add a Ceiling Fan]

Or like this view from above (an attic):

enter image description here [source - How to Replace a Light Fixture With a Ceiling Fan]

You could also buy a special metal ceiling fan support bar, like this one at Home Depot enter image description here but it's currently $14.28 US, a small piece of 2x4 is likely much cheaper, or even free if it's scrap.

  • 2x4 is a good thought. The space is below a small closet so little danger of future floor replacement. Not using the box to hang any fixtures, just branching off to install a new ceiling light and there are pipes in the way on the joists in the same space. Mounting on ceiling seemed to be simplest route but the same potential issues that you brought up made me wonder if there was some code that I might not be aware of prohibiting it or requiring a minimum depth below the floor. – 1962Ranch Mar 3 '19 at 22:38

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