New construction flat roof: internal roof drainage

Dear forum,
for our single-family house, we are planning a flat roof (due to 2.5 floors). We have already discussed our planning with the GÜ we trust. To get an independent opinion, we have commissioned an external architect to review/optimize the planning. He has now made an internal flat roof drainage appealing to us. I initially strictly rejected the concept. Unfortunately, I must admit that it also offers (at least) aesthetic advantages. Perhaps there are also solid technical/structural advantages (no icing?)...
Now to my actual question: Do you have experience with internal roof drainage for flat roofs? Should we refrain from it or is proper execution not so difficult for a professional company?
In our case, the downpipes would be routed behind drywall partitions down into the basement. Alternatively, would it be permissible/possible to “hide” the downpipes in the external insulation (WDVS 140mm)?
Complicating matters in our case is that we have to discharge the surface water not into the sewer but into an above-ground channel on the street. Since OKE will lie only 20cm above the street, I wonder how we are supposed to lead the water from all four corners of the house to the street...? The rear two corners of the house are about 15-17m from the street, the front two corners only 3m. With external downpipes, I can at least roughly imagine this, since there is at least some slope. But how is something like this supposed to be implemented with internal downpipes? These only leave the house below the basement ceiling, so there is no gradient left to the gutter in the street...
By the way, the path to the street on the left side is “blocked” by a shed and promptly by a carport. I simply lack the imagination of how something like this is supposed to be implemented... Our flat roof will be greened so that rainwater is retained/delayed in release. Perhaps it would be better to aim directly for infiltration on our own property or in combination with a soakaway?
I am grateful for any creative tips!
Have a nice Sunday!
Best regards
Stephan

He has now made an internal flat roof drainage appealing to us. I strictly rejected the concept at first. Unfortunately, I must admit that it also offers (at least) aesthetic advantages.

Imho there are only aesthetic reasons and otherwise only risks.

also tangible technical/structural advantages (no icing?).

Not that I know of.
What I have always wondered is how an effective emergency overflow is implemented with an internal drainage. You can hardly let the roof slope run towards an internal point and at the same time realize an emergency overflow at the parapet. Or rather, this would require the water to already be held up quite high on the roof before the emergency overflow begins to work.

Do you have experience with internal roof drainage on flat roofs?

We have lots of flat roofs here and a few neighbors have done it internally. I haven’t heard anything negative. Imho those were mostly forced solutions because downpipes could hardly be accommodated on the facade (due to a lot of glass and the like).

Would it alternatively be permissible/possible to "hide" the downpipes in the external insulation (ETICS 140mm)?

That would significantly weaken the insulation effect, but it doesn’t have to be impossible. There are definitely solutions with integrated downpipes in double-shell masonry (in the plane of the clinker as square pipes).

Have a look around in settlements with flat roof buildings. There are definitely more than the standard rounded folded pipe with bulky inlet box.

Since OKE will only be 20cm above the street, I wonder how we are supposed to lead the water from all four corners of the house to the street...?

Why four corners? If you really want, you can easily drain a flat roof at a single point.

The rear two house corners are at least 15-17m away from the street, the front two corners only 3m

Then just drain the roof with 1-2 downpipes to the street :)

But how is something like this supposed to be implemented with internal downpipes? These only leave the house below the basement ceiling and thus there is no slope anymore to the gutter in the street...

The question seems justified. Ask the suggesting artist how it is to be implemented in practice.

On the left side, the path to the street is blocked by a shed and timely by a carport. I simply lack the imagination how something like this is supposed to be implemented...

I lack the imagination to picture the constellation myself.
Personally, however, I am of the opinion (and have implemented it this way at my own building) that a short overbuild of drainage is no apocalypse. Especially not with paved areas/carports/sheds – these are not permanent anyway and can be changed in case of the very worst damage.

Maybe then directly aim for infiltration on our own property or in combination with a soakaway?

Also an option. Cheaper would be a soakaway shaft, provided it can be approved. I still find an overflow into the public sewer sensible.

Unfortunately, I have to admit that it also offers (at least) aesthetic advantages.

It can look better, but doesn't have to. It's a matter of the whole.

Maybe there are also solid technical/structural advantages (no icing?)...

No further advantage.

Do you have experience with internal roof drainage on flat roofs?

48 years trouble-free in my parents' house - internal but pitched roof.

To get an independent opinion, we commissioned an external architect to review/optimize the planning. He has now made an internal flat roof drainage appealing to us.

The same one who already demonstrated his lack of spatial imagination with the stair layout, and who cannot successfully demonstrate to the builder how to more easily convert the staggered floor? – then one would have to ask whether this would still be courage or already madness to let him offer special extras.

Imho there are only aesthetic reasons and otherwise only risks.

48 years trouble-free in my parents' house – internal but pitched roof.

I do not personally know of such a case, neither trouble-free nor problematic ones. However, I am old enough to remember how short-lived this trend was back then.

I do not personally know of such a case, neither trouble-free ones nor those with problems. However, I am old enough to remember how short-lived that trend was back then.

The house was built in the early 70s. There were inspections and repairs of supply lines and drain pipes, but none of these had to do with leaks or problems with the internal downpipes, which functioned without issues until the sale at the beginning of the year; after that I have no information. By the way, drainage goes into the sewer system.

Now to my actual question: Do you have any experience with internal roof drainage systems on flat roofs?

Yes, we have the entire roof drainage system internally installed and have no problems with it, neither noise from running water in the walls nor issues with frost or the like. We chose this for aesthetic reasons, as the building is in Bauhaus style and we wanted smooth surfaces and not those ugly boxes on the walls.

What I have always wondered is how an effective emergency overflow can be realized with an internal drainage system.

The emergency overflow is not internal, it probably would not work otherwise. Unless you lay all pipes redundantly. But even then, there is an increased risk that both vertically downward pipes get clogged by leaves, etc. Our emergency overflows go outwards horizontally (at an angle). However, they are hardly noticeable in our case. Due to slight slopes on the flat roof, hidden behind the parapet, water flow on the roof towards the hidden emergency overflows is ensured. They discreetly exit at points where they are hardly noticeable from outside. If someone walks around the house, they probably would not even realize they are there. How a slope can be realized in your case should be determined by the planner. There are certainly many solutions; the worst would probably be a lifting system.