ᐅ Odor trap for drain on flat roof

Hello everyone,

We are currently building a roof terrace on top of our flat roof.
We have noticed that there is a smell like a sewer coming from the roof drain.
Is there any kind of valve insert or similar that can be retrofitted into the pipe?
It should be a standard 100mm (4 inch) PVC pipe.

The alternative would be to break open the wall on the ground floor and install a trap there, but that would be a lot of work...

Why a sewer? Have you connected your roof terrace to the wastewater system?

Yes, the rainwater from the flat roof flows (together with other connections such as kitchen / bathroom / toilet) into the sewage system.
It is difficult to change this retrospectively now; it should have been done 40 years ago... ;-)
At that time, it was not planned for the flat roof to be used and intended as a rooftop terrace.

That would be prohibited here.

Ah, why is that? What’s the background?
As mentioned, it has held up for the last 40 years (touch wood!), but if it’s prohibited, there must be a reason for it... Possibly the ammonia, which could damage the flat roof seal?

There are cities or municipalities that do not have separate blackwater and graywater systems.

Where there is only a combined sewer, only that can be used. And where there are two systems, they should be properly connected.