ᐅ Inverted Roof and Drainage – Renovation Question

Hello. I recently had a balcony waterproofed with bituminous membrane sheets. Now I want to make sure the water drains properly, but I have a question. Under the balcony is a room, so I plan to insulate it with XPS boards. Google shows that for a so-called inverted roof, these XPS boards are placed directly on the bituminous membrane, with gravel or aggregate on top. For some reason, I would prefer to have gravel beneath the XPS boards to create drainage. However, I was told at the hardware store yesterday that the XPS boards wouldn’t like that. My planned build-up from bottom to top is:

- Bituminous membrane
- Separation fleece
- Gravel, crushed stone, aggregate ... some kind of drainage layer
- XPS boards
- Crushed stone
- Some kind of paving or tiles assembled like a puzzle.

I’m interested in your opinion on my idea of drainage beneath the XPS boards. Is it unnecessary? Useful? I’m concerned that if the XPS boards sit in water, it might prevent proper drainage.

Sandwich made of gravel? I would argue that the insulation effect of the XPS boards would then be virtually zero. How are the boards supposed to insulate the lower space if you create a thermal bridge out of gravel yourself?

If it becomes leaky, it’s leaky and will have to be redone.

There won’t be any leaks at first. I’m just wondering how the water is supposed to effectively reach the drain when the XPS boards are lying on top of the waterproofing membranes, with gravel or whatever else placed above them.

A slope of 1.5 - 2.5% is required.