Electrical Box in 2x6 T&G ceiling

[Count Fred]

I have a 2x6 car decking ceiling with a 2x4 cavity above through which to run electrical and plumbing. I'm puzzled by the best type of box to use to mount light fixtures in the room below. Obviously I'm going to want to cut into the ceiling to get the box flush with the bottom surface of the T&G, but I will then need to fasten the box somewhere in the ceiling. Most boxes only want to account for 1/2 wallboard, and I effectively have 1.5" of material to deal with.

Is my best bet going to be a ceiling fan type box, with a 2x4 between the cavity joists?

Thoughts,
JFS III

[Mikey]

Most light fixtures expect a 4" "ceiling box" which is a kind of rounded-off square box (also called an "octagon box"):

4000

I always mount them as you described -- on a 2x4 in the joist cavity. A similar box is available for about 3x the price called a "ceiling/chandelier box" and comes with hardware for mounting fixtures like fans or (!) chandeliers, but the weight of your fixtures will determine which you need.

Get the kind of box that has knockouts in the bottom (shown), since you won't be able to bring the cables in to the side of the box. Drill or saw a hole in the 2x4 to match up with the bushing in the box and pass the cable through the hole.

They make these boxes with a mounting tab attached, which would eliminate the requirement for the 2x4 (just mount it to the side of a joist), but I don't like them -- you're limited in where you can mount the box, and I wouldn't trust them with any weight:

4001

Do you have access to the space above the ceiling?

[frenchie]

They also make them with a hanger bars, like in this pic. The steel version (couldn't find a pic) will have 5 K/O's on the bottom, use the center one to attach to the hanger bar, leaves you 4 you can use.

(edit: "can't find a pic" - doh! - the top picture in Mikey's post is the kind of box I mean)

[Lakee911]

Why not use a remodel box and screw through the edge of the box in to the 2x6. I know screws through side of box may not be code compliant, but it would work. Other option could be a remodel box with longer screws for the tabs, or possibly with a box extension if they make an octogon extension.

Jason

[jadnashua]

I think you can use screws to attach the box, but can't have screws running through the box with exposed threads that could cut the insulation.

[Chris75]

Why not use a remodel box and screw through the edge of the box in to the 2x6. I know screws through side of box may not be code compliant, but it would work. Other option could be a remodel box with longer screws for the tabs, or possibly with a box extension if they make an octogon extension.

Jason

Thats why they make these...
http://www.contractor-books.com/SmartBox.htm