There does not appear to be a roofing forum but this is a central air vent...

I discovered a leak on the outside of the central air vent pipe in the attic and traced it to the gap between the "hat" and pipe as shown in the pic. I asked a few people at the home stores and one person who seemed knowledgeable told me to lift up the hat and apply silicone between the flange and pipe. He told me that silicone should not be applied to the hat.

Unfortunately I did not get a pic with the hat raised but I noticed that there is a large gap (up to 1") between the flange fingers and vent pipe. I applied silicone and it worked but I'm skeptical that this will work long term because of the large gap and also I did not think to use high temp silicone.

I don't really understand how this system can prevent leaks as designed. I noticed that all of the other smaller vents on my roof have flanges that extend all the way to the top of the vent pipe and thus are 100% leakproof. I guess it's not practical on the taller vents but this arrangement seems guaranteed to leak, especially in a driving rain.

Is the hat supposed to be the primary leak preventer? If so how would this work with the pipe seam?

If the hat is not 100% leakproof then how will the flange prevent leaks?

What should I do to make this 100% leak proof?

Thanks