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I'm trying to install a ceiling light in two rooms that didn't previously have hard-wired lighting. After cutting a 4" hole for a circular ceiling box (using a hole saw attached to my drill) I'm finding that the ceiling box I've got might not fit:

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The problem is that the ceiling is too thick. There is the original plaster & lath ceiling, and below that another layer of strapping and a fiberboard ceiling. All together, the thickness is 3 or 4". If I set the ceiling box in the hole from the attic, it sits very far from the ceiling surface you'd see from the room below. Because of the distance, I don't think it would be possible to mount a light fixture there.

So, how do I mount a ceiling box in this deep/thick ceiling? Is there a special box (or extension?) for situations like this? Do I need to somehow remove some of the upper layer of old ceiling in order to reduce the depth? Any special concerns for supporting the box?

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Are you putting in a simple light, or a ceiling fan? If you think you might put in a ceiling fan at any point, you might want to use a proper braced box for that. The braced box may allow you to get the box deeper in the hole, so it might be a good solution in this case even if you are just installing a simple light. Something like this electrical.hardwarestore.com/14-45-electrical-boxes-metal/… –  Tester101 Jan 24 '11 at 13:23
    
I'm only planning to put in a light, but would be happy to put in a fan box. Unfortunately, the joist bays in the ceiling are about 26" wide, so this brace wouldn't be long enough (at least not without some extra support). Still, using a box that is only secured from above might be a useful approach. –  Shimon Rura Jan 24 '11 at 13:37
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2 Answers

up vote 5 down vote accepted

I agree with answer #1 with the following exception: Do not use longer screws to attach the fixture. Find a suitable box with extensions as needed to make the box opening flush with the ceiling surface. You do not want a gap between the box and fixture the could allow heat and spark from a fault in the box to escape. Here are products that I would use in your situation. These may not be available at home improvement store but are available at electrical suppliers who service contractors.

This combination gives you a box 5 3/4" deep. Adjust the support on the attic studs to make this flush with ceiling.

Remember that it is important to not leave a gap that could cause a smoldering fire in the voids of the ceiling structure. This is the reason that most electrical codes require that box faces be flush with the finished surface.

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Great answer. In case anyone's curious, I am going to try and make do with what I have. I cut off the side nailing flanges of the circular PVC box with a hacksaw, and attached a block of wood to top of the box. In one spot, I then used the L-shaped bracket that was originally on top of the box (an extra brace for ceiling fan support) to mount the wooden block to a neighboring joist. The other spot isn't immediately next to a joist, so I'll be making a brace from 2x4. This ends up being pretty simple and allows the box to be positioned flush with the ceiling surface. –  Shimon Rura Jan 25 '11 at 3:53
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I'd recommend a braced box. You can mount a 2x4 on the inside of the joist to change the size from 26" to 24" inches. Matter of fact, you may want to put one on both joists. Make sure you give some length to the 2x4 to balance the potential use of a fan.

If the ceiling is really 3" to 4" thick, then you'll want to get the deepest box you can find and then buy some longer screws to mount the light. You need to check your local regulations, but I would guess that as long as the plaster and lath portion is covered by the box (since the wood is flamable, but the sheet rock is not), you should be ok.

Check your regs to be sure!

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Yes, the ceiling is really that deep; I'm having trouble finding a deep enough box. I'd really rather get it further down than use really long screws. Also, the ceiling is some kind of fiberboard, not sheetrock (unfortunately). –  Shimon Rura Jan 24 '11 at 18:13
    
Hugh : not to be overly pedantic -- but a modern 2x4 would only take a 26" bay down to 24.5"; if a box is sized to go between joists that are 24"OC, they might only extend to 22.5", so you might need as much as three 2x4s to make up the gap. –  Joe Jan 24 '11 at 18:59
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