You can't tell from the picture of this Box, but there is a heavy metal plate that goes behind the area that screws to the ceiling joist and across the top of the box to support the fan itself. These are not your average plastic fan boxes by a longshot.
You can see it by clicking on the 'New Release' picture under the Box on page below:
http://www.licensedelectrician.com/Store/SmartBox-3.htm Bill
