ᐅ The siphon or shower drain empties when the toilet is flushed.

Hello everyone,

We have been living in the house for about 7 weeks now. During the first 3 weeks, the guest bathroom with a shower on the ground floor was our main bathroom, as the actual main bathroom in the attic wasn’t finished yet. For the past 4 weeks, the guest bathroom has mainly been used for toilet visits (too lazy to go upstairs) but rarely for showering. Now the bathroom sometimes smells a bit like sewage. The odor comes from the shower drain. After 2-3 toilet flushes, the drain and probably the shower trap are empty, allowing the smells to come through. Is it normal for shower water to be drained when flushing the toilet? Where should I start looking to find the problem?

Attached are 2 pictures. One shows the plumbing rough-in. Maybe there’s a visible mistake in the pipe layout. The other shows the almost finished shower drain.

Maybe someone has an idea.

Thanks and regards

Maybe a ventilation issue. However, I don't have enough knowledge about it, so this is just a guess.

Yep, I was just thinking the same thing. I don’t see any ventilation.

If you have a vent pipe on the outside of the house roof, like a 100mm (4 inch) pipe with a rain cap, if not, that could be the reason or it’s not connected.

Ok. Thanks, I could look into that. Although I suspect that the pipe is connected to the main line, including the ventilation?!?!. But that would be a starting point.
Everything is there and also connected. So the main ventilation is connected to the vent brick.

Okay, so where we currently live, the problem still occurs on some days, even though everything has been newly renovated. The specialist will have to come back and make adjustments to the ventilation system. I have done some research and there are several possible causes. However, the cause is most likely related to that.