The siphon or shower drain empties when flushing the toilet.

Hello
could you please post a picture or link with ventilation next to the WC and directly at the bathtub.
Thank you very much.

Here is a diagram that I found with a quick Google search. I would say that the plumber has to install that for you or offer an alternative.

Now it annoys me that this was apparently forgotten. Let's wait and see what the plumber thinks. Of course, you can't reach anyone on Fridays at 3 PM anymore :)

Just went up to the attic to check the ventilation. Moisture is coming through there. That should be checked sometime.

Pipe branches that reach a length of 5 m must be vented. The entire pipe system must be vented over the roof at least once. Then the other branches can be equipped with a vacuum ventilator. The moisture on the pipe under the roof is very, very likely condensation; if the pipe is insulated here, the problem is solved. The plumber probably forgot this; he certainly knows the issue.

Hello everyone, unfortunately I have not received any feedback regarding my problem yet. Apparently the plumber doesn’t find time for me. I want to put a bit more pressure on the matter now. I also assume that the plumber will blame the “shower channel,” since it was installed by the tiler and not by the plumber. Therefore the question to be on the safe side: Is there any guideline/standard that deems a venting above the roof necessary in my case? I need a little leverage now, as there are other issues I am also not satisfied with.