Originally Posted by Cossack
Thank you for that explanation. So I am thinking I will have to vent the toilet and shower up into the attic and attach them to the Air Admittance Valve (AAV) along with the sinks and washing machine. Right?
I think what you mean about checking the pipes for proper air flow is that they are slanted UP toward the AAV. I think it is supposed to be ¼” per foot.
Depending on how its piped, you might be able to have a common vent for both fixtures and not have to vent each seperately. I really frown upon AAV's, they don't work as good as a vent that extends through the roof. An AAV allows air to enter the system, but it doesn't allow air to escape the system, so an AAV only does half the job a roof vent does.
When i was referring to airflow, I was refering to how the sink drain needs to connect to the toilet and tub drains at a certian location to allow airflow in the drainage line, not the vent line. You only have to pitch the vent pipe if its going through the roof to allow any residual rainwater to drain out of the vent, if your using an AAV, you don't really need to pitch the vent.