Ah, now I see. Besides the fact you'd have to go below the sink (not a good thing) you'd have to rip out the wall all the way around the bathroom and replace it to do it that way. While I'd be as hesitant as you about cutting a hole in my roof, I'd probably do that before I'd rip out sheet rock from 3/4 the way around the bathroom. Sounds to me like you either need to cut into that 3" iron pipe and put in a tee (which sounds like more trouble that it's worth) or go through the roof. Anything else would be kind of going beyond what's reasonable.