Joining 3" waste lines in your home

[Nate S.]

I am removing a wall between my kitchen and dining room and it has 2 - 3" black waste lines running down the wall. They are coming from two bathrooms upstairs. I need to reroute them to an exterior wall and I was wondering if I could join them into a single 3" line?

 

[Jadnashua]

What drains into each line? The combination of things may exceed the capacity of a 3" line if they are combined. So, the answer is maybe without further information. Also note that on an exterior wall, depending on how cold it gets, and how much insulation there is, things could freeze up a bit, but that's unlikely. Regardless, with a big drainage pipe in the wall, there won't be much room for insulation, and you may have a cold stripe down the wall.

 

[Sponsor]

 

[Terry]

You can have up to three toilets on 3" horizontal.
You can combine them if it doesn't ruin the wood structure of the home.
No more than 1/3 of the joist and at least 2" of wood on the top of the joist and the bottom of the joist. The 1/3 allowed has to be somewhere in the middle of the joist.