the main drain should be a vertical stackpipe at the minimumventing either thru the roof or at 90 degrees as high as possible. All your fixture,water closet,basin,shower...etc,will have their own traps before they drain into the stackpipe.The more fixtures,the larger the main stack or drain.

I've always done them this way rather than the troublesome horizontal vents.

If you're using PVC you can clamp a saddle onto the stack as an afterthought or just cut it and installa proper tee.

__________________