Every other answer to a question that involves electrical boxes references the advantage of metal boxes over plastic (old work) boxes. It is claimed that they are very flexible to mount but I don’t see how.

If I have the choice , I’d go for metal. Right now, I have two holes open:

  1. A hole for a light switch where approximately 1/2”-1” under the hole is a horizontal wood beam. I could attach the box there but I would need a spacer between the wood and the box, such that the box is at the right height.
  2. A hole for a ceiling light with a beam next to it. But the light should be a few inches off the beam. Again, a spacer would be useful (I know the poles that attach in between two beams)

Are there “spacers” for mounting electrical boxes? Is it possible to install them in an “old work” setting as described above?

Note, my question is for lath & plaster which is very hard to patch so I went to avoid larger holes than necessary

  • 2
    They are called "shims" . They can be wood or a composite material. How they are mounted and how the box get mounted depends on the situation and the box.
    – RMDman
    Nov 15, 2023 at 11:59
  • Be aware that inspectors can frown upon metal boxes casually used in place of non-conductive boxes. You need to take additional steps to be sure they're grounded properly.
    – isherwood
    Nov 15, 2023 at 14:19
  • @isherwood can you elaborate? Of course, metal box needs to be bonded. Is this what you mean or is there an additional pre-caution?
    – divB
    Nov 15, 2023 at 22:36
  • That is what I mean, but the inspector in this case didn't even want metal boxes used.
    – isherwood
    Nov 16, 2023 at 13:48

1 Answer 1


Frame challenge: Make the hole larger and use a larger box. There are adapters (called mud rings) that will resize nearly any metal box down to a single- or double-gang opening. For the mount for the ceiling light, the base of the light is most likely big enough to cover a larger box.

This will allow you to screw the metal box directly to the existing wood without having to mess around with shims, additional blocking, etc. Besides, to get that in place, you may well have to enlarge the hole anyway.

The larger box also has the added advantage of being able to hold a Decora-style switch much more easily, and all new smart switches have very large bodies that don't fit well into small boxes. Even if you aren't installing a smart switch someone in the future (including, possibly, future you) will thank you for it.

  • I have two concerns here: For (1), the wood is 1” UNDER the hole, not next to the hole so using a larger box and a mud ring doesn’t help me if I want the the switch vertically (not horizontally). If I misunderstand, do you have a drawing what you mean? Second, my walls are lath & plaster so cutting a larger hole is extremely hard to patch so I would really like to avoid this
    – divB
    Nov 15, 2023 at 20:43
  • 2
    There are sheet metal clamps that slide between the box and the inside of the hole which clamp the box on the inside back of the wall, [reventing the box from moving outwards into the room. There are flanges on these metal boxes that clamp onto the outside (room side) of the wall, preventing the box from moving into the wall cavity . Together these will fix a metal box to the wall without it having to be attached to structural members inside the wall cavity. Nov 15, 2023 at 21:14
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  • Thank you, this is interesting. I'll consider this as well.
    – divB
    Nov 15, 2023 at 23:08
  • Another method of attaching a metal box would be to use only the tabs on the box using screws through the tabs. The best way to so that would be to reinforce the top and bottom of the opening by fastening strips of 3/8" or 1/2" plywood (e.g., 1" wide x 3" long) or other wood to the backside of the finished wall. Screws through the metal tabs and into these plywood strips would hold the box to the wall. The strips could be secured with quick drying glue or with brads or staples from the finish side of the wall. Nov 16, 2023 at 4:55

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