Constructing a flat on a terrace

First and the foremost, this is a wonderful website. It has already helped me a lot.

We are about to purchase an open terrace on the second floor of a building about 1100 sq. ft. We plan to construct an apartment. Now I wish to have RCC roof(slab), but many have  recommended sheets (asbestos or any light material) so as to reduce weight. But I am worried about security and leakages with asbestos sheets. Also since the building has offices, constructing a slab later will be tedious.  Now, how do we decide if the building will bear the slab? The building is 15 years old.