When designing earthquake safe structures the first consideration is to make the highest bit, the roof, as light as possible. This is best done with profiled steel cladding on light gauge steel Zed purlins. This can also have double skin with spacers and insulation. It can have a roof slope between 3 and 15 degrees. If it is required to have a 'flat' roof, this could be made with a galvanised steel decking and solid insulation boards, and topped with a special membrane. Even a 'flat' roof should have a slope of about 2 degrees. If it is required to have a 'flat' concrete roof, then the best solution is to have steel joists at about 2m, 6", centres, and over these to have composite style roof decking. Then an RC slab can be poured over the roof, with no propping; the slab will only be say 110mm, 4 1/2", and will weigh only about 180 kg/sqm. Such a slab will be completely bonded to the frame and will not be able to slip off, or collapse.