ᐅ How to build and waterproof a balcony on a wooden ceiling?

Hello everyone,

I am planning to convert an old enclosed space on the first floor into a balcony/terrace in the coming months.

Currently, the situation is as follows:

It is an extension built with masonry and a wooden ceiling. Above it, there is a basic sloped roof that is already damaged in several places. This roof is to be removed and replaced with an open terrace. On the exterior wall of this extension, there is a connection point for water drainage, and a downpipe runs along it.

The wooden floor is loosely covered with tar mats that are not welded or sealed. These will likely be removed during the renovation as they do not provide proper waterproofing.

How could I build up the flooring on this wooden ceiling so that it remains watertight for many years? What would you recommend? This floor structure should have a slight slope away from the house wall to ensure proper water drainage. I plan to lay Bankirai boards or possibly composite decking made from a wood-plastic mix on this base. If rubber membranes are used as an underlayer, they would need to support the plastic pedestals of the decking substructure.

Using screed on a wooden ceiling might be difficult, although it is relatively stable.

The new terrace floor will cover approximately 30 square meters and will only be partially covered from above (for example, with a retractable awning).

Does anyone have any ideas?