Time for a little failure analysis. The photo above shows three screws and plastic mollies that failed in supporting a closet shelf yesterday. All three were attaching a single cantilever bracket to the gypsum board at the back of the closet; the shelf was supported on two such brackets, so it fell when the bracket failed. The failure was triggered by putting an empty suitcase – not a heavy load, but not negligible either – on the shelf. The bracket pulled out from the wall, with the top moving the farthest. All three mollies simply pulled out of the drilled holes in the gyp board, without any breakage of the board.
The cause of failure is quite clear: the mollies never expanded. When you put the screw into the hole in the molly and tighten it, the far end is supposed to pull back towards the exposed end, making those little arms on the side stick out and engage the back of the gyp board. That didn’t happen on all three of these, reducing the pull-out strength to the whatever grip the little teeth have on the gypsum at the sides of the hole. Not much, I expect. Repeated cycles of loading and unloading would work the molly back and forth and reduce the grip of the teeth, until the few pounds of an empty suitcase created enough tension to overload the top molly in tension, which threw the load to the middle one, which then failed.
Understand what happened does little to address the annoyance of having to reinstall the bracket with proper hollow-wall anchors.
