vents stink
what does the pipe actually look like??
Does it have some sort of hood cap on it (automatic studor vent) ??
or does it just literally have a wide open pipe just stinking up the attic???
If it is wide open, you really ought to have it taken out the roof...
both moisture , mold, sewer gas, and even sewer bugs, roaches ect will eventullly find their way to make their home in the attic.
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As far as getting the builder to make it right, most builders IMHO and in my experience, are total skumbags and only warranty their work for one full year and not a single day more. I honestly doubt he will do anything for you.
you might have a better chance calling the plumber who installed it in the first place...if he is still on good terms with the builder and got paid for doing the work the first time , he will most likely be your best shot.
Try the builder first and see what happens. Maybe he will be a good guy and just prove me wrong and fix it for you.
But , if he does not want to, or wont tell you who the plumber was,
its most likely because he never paid him. and if you let the origianl plumber back on the property, you could be looking down the barell of a Mechanics lien for the total plumbing installation.
I dont mean to scare you, but I have seen this happen to people so,.....
If you feel like any of this could be a possibility, (you know the builder) its best to just call another plumber and just pay to get done right .