DWV Sizing

I believe the main drain stack needs to be 3" minimum, would it be wise to use 4" instead?

Biggest issue I have found with 4" is that I cannot fit it in a 2x6 wall. This will be a new house construction, so if I need to I can build a bigger wall, but if my gain is minimal, I would bother with the hassel.

Just looking for thoughts on the best way to go. Let me know if there are other details that will help with this, below is what I could think of.

-2 Full bathrooms on 2nd floor, share same wet wall
- Laundry, 1/2bath and kitchen on main floor right below the 2nd floor bathrooms
- Basement unfinished, but will be stubed for full bath

Thanks!

IF 3" is the "proper size", not just the minimum size, then there is usually no advantage to using a larger size, but this is not an iron clad rule for smaller drain lines. WHY can't you fit a 4" pipe into a 2x6 wall? We do it all the time when we have the need for a 4" pipe.

Perhaps I read something wrong, but thought you must have at least 1 1/4" clearance from the edge of the stud to the edge of the pipe, if that is not true, then 4" becomes easy to install.

If you have any links to locations that will tell me how to properly size the drain (3", 4" or other), let me know, I am anxious to learn the proper way.

Thanks.

The 1.25" rule is for plumbing or wiring passing through studs. This is because that is where drywallers and homeowners would likely be drivng their screws/nails. In the stud bay, it is not an issue.

Thanks for the clarification, sounds like 4" is the way to go.

Sorry for my confusion.

Thanks!