The joists are covered with a waterproof material such as asphalt, lead, zinc or copper, the three last materials being usually laid upon boarding, which stiffens the structure and forms a good surface to fix the weatherproof covering upon.
Simple roofs in general use with a double slope are the " coupled rafter roofs," the rafters meeting at the highest point upon a horizontal ridge-piece which stiffens the framework and gives a level ridge-line.
When pieces get covered in rust it stiffens up the joints and can cause a lot of damage.
Let the cake sit for a few minutes so that the buttercream stiffens.