All balconies and roof terraces are paved with pre-cast concrete slabs faced with exposed Pen Lee granite aggregate, the same granite which makes up the concrete of the buildings themselves.
The Barbican used to supply child guards to prevent young children straying out on to the balconies if the windows are left open. If you can get hold of one, it is slotted into the channel of the sliding door in the living rooms.
It is up to you whether to have window boxes. But if you do, your lease requires you to keep them cultivated. (This does not apply to Tower Block flats for some reason).
I am sure you won't be tempted, but just in case, you are not allowed to hang your washing out on the balconies.
The balconies combine to form a fire escape round the building, so you are not meant to leave anything out on the balconies.