What roof pitch is appropriate?

Hello, I would like to build with a roof structure visible in the upper floor, a flat pitched roof without an attic. Which roof pitch is sensible (with an intended covering of tiles), and why? Is the following correct, and are there further aspects (in NRW)?

[*]at least 15 degrees if roof windows are to be installed (writes Velux); [*]at least 22 degrees to achieve rain protection (writes dach.de) ("classical flat roof tile"); [*]30 - 45 degrees are optimal for photovoltaics or collectors (writes Thomas Königstein).

Are there significant cost differences depending on the roof pitch?

Hello, I would like to build with a roof structure visible on the upper floor, a flat gable roof without an attic. Which roof pitch is advisable (with the intended covering of tiles), and why? Is the following correct, and are there any other aspects (in NRW)?:

[*]at least 15 degrees if roof windows are to be installed (writes Velux); [*]at least 22 degrees to achieve rainproofing (writes dach.de) ("classic flat roof tile"); [*]30 - 45 degrees are optimal for photovoltaics or collectors (writes Thomas Königstein).

Are there greater cost differences depending on the roof pitch?

Is there a development plan? What does it say?

There are larger cost differences, for example, depending on whether you build (or have built) a classic rafter roof or a nail binder roof. Furthermore, this topic might not even be one if you plan to build with a [GU]. From personal experience, I can say that most [GU]s reject such things or charge a "pharmacy price" for them. However, if you build with individual contracts with an architect, that should be possible. A gable roof with a 45° pitch will probably be ruled out for purely aesthetic reasons. A 22° pitch gable roof comes to mind immediately.

25° combines the best of all worlds...

We are also building with 25 and I find it almost perfect. If we could have gone a bit higher, we would have taken 30 so that we had a bit more space on top. But 25 is fine too :)

at least 22 degrees to achieve rain protection (writes dach.de) ("classic flat roof tile");

You can already achieve rain protection with an underlay membrane at 18 or 20 degrees.

30 - 45 degrees are optimal for photovoltaic or collectors (writes Thomas Königstein).

Depending on how the roof surfaces are oriented. East/West even makes somewhat less sense.

You have advantages and disadvantages everywhere. You should clearly formulate for yourself what is important to you: aesthetics, photovoltaic self-consumption, photovoltaic full feed-in, …
I would be at about 20-22 degrees.