is it possible to vent a washing machine, shower, toilet and a lav into one 3 in. vent? For example, I have room to manifold the w/m vent into the shower vent then into the toilet vent using 2 in. Can I then go to 3 in. on the horizotal run, pick up the lav and then go vertical? The other two fixtures are the kitchen and the laundry sink which are in different locations of the house. My final question is: can I use a common 2 in. vent for these fixtures to satisfy the area requirements for a 4 in. main?