Concrete
It is possible to fix joist directly on an existing slab or tiles. If needed, use wedge and ensure to respect the direction of rainwater evacuation.
A carrier (OM 65) will be fixed directly on the slab in the direction of the planks. So the joist will be perpendicular to the carrier and screxed on top of it.
Roof terrace on waterproofing.
In the case of a roof terrace with waterproofing, you can install the terrace on pedestal, wich permit the access to the slab if needed.
Bare ground
Before building the terrace, the ground must be stable and stabilized;
Caution : in the case of instable floor , it is better to call on soil analysis specialized company.
Yes but be careful to stabilize the ground before the installation.
Ground stabilization can as easy as complex depending on the ground type. That’s why it’s preferable to call on a specialized company.