I am putting in a ceiling fan in a bedroom. It is a smaller one approx. 18 lbs.

The remodeling-type expander brace failed, so can I just screw one side of the box to the joist? (I cannot access the area through the attic.)

Well I know I can, but how bad would it be, really? Is there a type of box that is more suited for this purpose?

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    It'd help to see some photos taken inside the ceiling. What do you mean by 'failed' when you write: The remodeling-type expander brace failed? – mike May 9 '13 at 19:48

Check the markings on different electrical boxes. According to the National Electrical Code (NEC) (which is not applicable in all areas, so check local codes), boxes that support ceiling fans should be listed for the purpose.

National Electrical Code 2008

ARTICLE 314 Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings; and Handhole Enclosures

314.27 Outlet Boxes.
(D) Boxes at Ceiling-Suspended (Paddle) Fan Outlets. Outlet boxes or outlet box systems used as the sole support of a ceiling-suspended (paddle) fan shall be listed, shall be marked by their manufacturer as suitable for this purpose, and shall not support ceiling-suspended (paddle) fans that weigh more than 32 kg (70 lb). For outlet boxes or outlet box systems designed to support ceiling-suspended (paddle) fans that weigh more than 16 kg (35 lb), the required marking shall include the maximum weight to be supported.

A box like this 4 in. Octagon Box, is rated to hold 50lbs, however, that's only when it's properly installed. Since it sounds like you don't have much room to work, you'll probably have difficulty installing it properly.

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You're obviously not going to want to use a box like this Ceiling Box, which only attaches to the drywall.

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The best choice in this situation is a box supported by a brace.

enter image description here

I'm not sure what you meant when you said "The remodeling-type expander brace failed", but not all braces are created equal so it may be worth a second try. With this Westinghouse Saf-T-Brace (and others), installation is fairly easy even when you're working through a small 4" hole.

  1. Remove the electrical box and saddle from the brace (if it's not already).
  2. Slip the brace up into the ceiling, and rest the "legs" on the drywall.
    enter image description here
  3. Using an adjustable spanner or open ended wrench, rotate the center part of the brace. This will cause the ends to expand outward toward the joists. Continue until the brace is firmly in place.
    enter image description here
  4. Slip the saddle back over the brace.
  5. Affix the cable to the box, and mount the box to the saddle.
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It would amount to a pretty poor installation. Box walls are not very rigid, it doesn't take much force to make it flex. You are essentially cantilevering a mass of hardware off a flimsy piece of sheet metal. No fan is perfectly balanced, they all have at least a slight wobble. With a solid base, this would usually be invisible. A flimsy installation could amplify the effect so it appears worse than it is.

The old work brace that failed is the usual solution for this situation. Another solution is to remove a square of the ceiling and install a proper solid brace, then repair the opening. Depending on the ceiling finish this can be fairly straight forward or completely impossible. Yet another option is to install a piece of MDF on the ceiling surface that spans the joists. The edges and corners can have nice details cut into them so that once painted, the piece becomes a halfway decent decorative element.

Finally, you could apply a pancake box directly centered under a joist, but then you have to patch the old hole. If you can do that, the make a bigger hole and install a proper brace option is also realistic.

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