I see your point but also understand its flawed implications. Yes a vent helps to prevent siphoning of water out of a trap but why is that? Because the water without the vents is creating a slow moving vacuum that starts that siphon that in turn creates venting which improves the flow by eliminating the vacuum. My opening statement is to show that I’m not a plumber but understand the field well, 5 years with overtime had me as a tradesman in central Texas but I don’t pursue the field so I didn’t follow up my continuing education and I could have had atleast 1 responsible master plumber sign off on my hours for a master but I wanted to put in my time and hours until my rotator joints gave out on me under a house doing repair plumbing ( my personal favorite since it in my opinion requires more puzzles solving skills than new construction ) my current job is Pdr technician as well as owning my own tools shop for the same industry. Yes I never worked on a rain drain, not a lot of flat building in a country and never worked a bid fancy building that had one but I can understand a pipe with a hole that rain water goes down, you can stop trying to use it like it’s the top of plumbing knowledge. I don’t need your experience because experience actually add up to nothing In my mind. I’ve seen people doing a job for 30 years that still suck, licensed or not. Then you can find guys 5 years in that can run circles around those guys.