The load is supported by the outside walls and the roof doesn't fall because as it tries to fall it pushes against itself resulting in compression and the load being transferred to the walls. Steel wool will rust faster because it has more surface area for the oxygen to attack. No, iron is more dense than cork, but not necessarily heavier. A big enough piece of cork would be heavier than a small enough piece of iron. For example a cubic meter of cork would be heavier than a paperclip.
Water takes up more volume when it freezes so it becomes less dense.
Thus ice is less dense than water. A kilogram of aluminum has more volume. Since it is less dense, the same mass must take up more space. A liter of water is heavier than a liter of ice because the water is more dense than the ice. Thus the same volume of water has more mass than that volume of ice.