Hi guys, love this site!  I'm pretty handy around the house, but know very little about plumbing.  I have the basic concepts in hand okay, I just don't know any of the rules.

My question is this:

My current upstairs bathroom is tied into a 4" vertical waste stack that runs from my basement to the roof.  This stack services the kitchen sink and dishwasher on the first floor, and bathtub, sink, and toilet on the second floor.  The stack is cast iron and very old.  The corrosion is starting to split open, and waste is seeping out the side.  It's time to replace the whole thing.

I will replace this stack with a new 4"; PVC one, but when I do, I'm going to remodel the bathroom.  I'll be installing a large Jacuzzi tub, new shower, and double vanity.  The new fixtures will be on the opposite side of the bathroom from the stack, and I won't be able to sneak the drainage between the floor joists.  They run the wrong way.

This means I'll have to insert a secondary stack that runs through the first floor walls, and ties back to the main in the basement (for drainage) and the attic crawl space (for vent).  The wall it needs to travel through is not a "wet"; wall, so I don't have a lot of room.  The new pipe will service the tub and the shower.  

How big does that secondary pipe *need* to be.  Can I drop 1 1/2" for both fixtures or will I need something bigger?  Or, should I just go with the "as big as I can possibly fit" approach to plumbing this thing?  Do you see any flaws in this approach?  Is their a maximum length a vent can run horizontally (on a slight incline, of course)?

Thanks in advance,
-Tom

If you plan on major surgery to your plumbing you're going to have to go down to your Building Department and pick up a set of your local codes before you do anything. Go 2" on the drainage, 1 1/2" ain't going to cut it. I read you at 7 fixture units. That's your key. Find out how many fixture units your code calls for on 2" pipe. You should be able to wet vent your group and vent through the lavatorys. You are planing to revent back in the attic aren't you? No max limit for a horizontal vent but the run should have at least 1/8 " to the foot slope. For a job like this I realize that will be more questions. When you ask them please give me ALL the poop and info you have. Actually you guys do your own work and repair. All I do is take what you give me, put it together and toss it back to you. So I need all the information I can get. Good luck on a major project. Tom aka speedball1

Thanks speedball. That trip to the local code enforcement office has been long overdue. I'll let you know what they tell me.