 If your light fixture was going to mount in between the joist instead of beside the joist or you weren't right on the joist with a pancake box, you would put this box on there and then you could slide it where you wanted it left and right between the joist, which was suitable to hold up a light fixture with a couple of bulbs in it with no problem. But when you get to a ceiling fan or like a heavy light, something with a lot of glass or a chandelier,