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I have an interior wall with a one-gang light switch; it contains a dual dimmer. I'm trying to change this to a single dimmer, and to add a new switch (to drive the other load) on the opposite side of the wall. Here's a sketch(-up) of what it looks like now:

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Some feeling around (as much as I can) seems to show the following geometry:

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In other words, the existing new-work box seems to be cut into the horizontal stud. The other side (where I want to install a new switch) looks something like this:

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So, finally, to get to my question: I want to install an old-work box about here:

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Is it feasible to remove some of the horizontal stud, ideally without having to do drywall repair? For example, I have a Sawzall; can I just use that to carve through the front part of the horizontal? Any other tips?

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My tip would be to save yourself the headache and just raise the box 4" or so to miss the horizontal wood. –  DA01 Nov 30 '12 at 21:58
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Yeah, I was wondering that, but was hoping that I could keep the two switches at the same height - but if nobody has any other suggestions I'll end up doing that. –  Geoff Nov 30 '12 at 22:15
    
I think it's possible to hack away at the wood through the small opening, but as DA01 says, it'll be a headache. I can't decide which will be worse, this, or patching drywall. You may not need to worry about compromising the fire block, they aren't required in recent codes provided the void is less than 10 feet, instead of the older 8 feet. –  bcworkz Dec 1 '12 at 18:38
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1 Answer 1

A horizontal board between two studs is typically fire blocking. The idea of fire blocking is to prevent a cavity in a wall more than 8' high where fire can quickly spread. You'll see this in any room with over 8' ceilings. For it to work properly, you can't have any gaps allowing air to go from the lower cavity to the upper one. This means any gaps need to be caulked with fire rated caulk (that's typically the red or orange caulk). Why the heck someone installed this blocking and electrical box in the same place is bizarre, either one could have been moved up or down a few inches.

To work around this, you have a few options. You can cut a gap into it, and fit your old work box, but from the small opening you'll have a very difficult time cutting out the back. The result will require a load of caulk on the 2x4 that you squeeze the old work box into. And you'll need to install it a few inches to the side so there is some material continuous from side to side.

You could also fix this the right way and open the wall up, and reinstall new blocking a few inches up or down. Then you can remove the old blocking and install a new work box.

And the easiest solution is as DA01 says, install the new switch a few inches higher so it's above the blocking. I'd probably go with this solution myself, unless you feel like doing some drywall repair.

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Have you ever tried fishing electric wire up to the second story of a house when fire blocking is in the way? It sucks. Perhaps the DIY'er notched the box into the fire blocking so that he had a junction box to fish wire to and from the receptacles upstairs. I am not saying it was a smart choice but certainly a reasonable explanation. –  maple_shaft Dec 1 '12 at 17:13
    
Thanks for the edit to identify the actual situation - the fire blocking explanation makes sense. I ended up moving the switch up as per DA01 and your suggestions, with the only issue being that I put the work box directly on top of the horizontal, resulting in the bottom pop-out not being useful - but the hole was tight enough that it's damn hard to pull out anyway. –  Geoff Dec 1 '12 at 19:18
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