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Waste Line Pipe Size

tpmcnichol's picture


Installing a new bath in a small outbuilding.  Starting from scratch, so can do anything.  Trying to determine waste line size.  Should a toilet be 3" or 4"?  What about a shower/tub and a vanity?  If I use 1 1/2" for a it o.k. to and a 2" vent?


(post #96217, reply #1 of 10)

Bump - any plumbers out there?

(post #96217, reply #2 of 10)

I'm not a plumber, but this will get you started:

A toilet is 3"

Here is an easy rule of thumb: For the main soil line leaving the building - 3 toilets = 3" line, 4 toilets = 4" line.

For sinks, generally, 1 bowl = 1 1/2" waste line, 2 bowls = 2" line.  Really though, it is more complicated than that once you add the venting into the equasion.

Terminology: pipe that carries Sh!+ is called a Soil pipe. Pipe that carries Water is called a Waste pipe.

Every structure with a toilet needs a 3" vent stack pipe that extends above the surface of the roof.


Edited 9/28/2004 9:39 am ET by DIRISHINME


(post #96217, reply #3 of 10)

Some codes reguire 4" waste lines once the 3" leaves the building.

Code Check Plumbing is a good basic book to have. Click on books and videos at the top of the page.

Call you local plumbing inspector for local code reguirements and inspection schedules. They are ussually very helpfull.



(post #96217, reply #4 of 10)

Every structure with a toilet needs a 3" vent stack pipe that extends above the surface of the roof.

Which, if I remember the rule-of-thumb right, jibes with vent the same size as drain, and biggest drain gets vented.

So, the example of a w/c and a lav would be 1 1/2 waste line connects to the 3" soil line from the w/c.  The vent would be 1 1/2" off the lav tied into the 3" vent off the w/c.

Since this would seem to be a simple powder room, a 3" soil pipe would likely be sufficient.

Occupational hazard of my occupation not being around (sorry Bubba)
I may not be able to help you Occupational hazard of my occupation not being around (sorry Bubba)

(post #96217, reply #5 of 10)

I scanned CodeCheck and don't see anything about a 3" vent required, but that don't mean it ain't so. I do know that the area of the vents must equal the area of the line leaving the building. A typical bath can be done with a 2" vent.

To the OP, get your hands on a copy of CodeCheck or something else with the vent sizing table... it lets you determine the vents you need based on the fixtures being installed. Worth having.

(post #96217, reply #6 of 10)

In the northern states you can vent with a 2" pipe, but before it termanates the roof  it must convert to a 3" pipe. This is so the pipe can not freeze over from the humidity/moisture coming out of the pipe and block up.


(post #96217, reply #7 of 10)

Talked to a plumber today.  He told me that receintly the plumbing code that we use here in NC was changed and the 3" stack on each house is no longer required.  The house he had just plumbed had two, 2" stacks.  One on the 2 bath grouping, and one on the kitchen.

Not meaning to be rude, but as far as "code check" type books I have no use for them.  Had one - threw it away.  I need to know the exact code requirements for where I build.  Generic statements may be OK for general discussion, but anyone needs to know what is required in their state/providence/etc. - unless they do work that is not inspected. 


(post #96217, reply #8 of 10)

I can see your point about Code Check. They are excerpts of the UBC/UPC/NEC/IRC and tell you what those codes say about common issues. I have seen several inspectors bring them to jobs and use them. Whether or not that says good things about those inspectors... who knows.

(post #96217, reply #9 of 10)

I'm shocked about what you said about your inspectors using Code Check.  The ones around here never carry a book, but if you ask them a real specific question, they always have a book in the truck.  Maybe inspectors where you live are less specialzed?  Here there are building inspectors, plumbing inspectors, electrical inspectors, mechanical inspectors, and a few elect/plumb/mech inspectors, but none do it all. 

I am only licensed to build, but not licensed to plumb/wire/install HVAC, so always use subs for that kind of stuff, and for the most part, they are responsible for their own inspections, so I only have a building code book.  Still, you learn stuff by talking to the various tadesmen and by looking at their rejection slips :-(


(post #96217, reply #10 of 10)

I have dealt mostly with combination inspectors, although once in a while there's a jurisdiction with specialists--seems like mainly big cities with a volume of commercial and highrise construction going on. When you talk to the combination guys about their background, it's usually one trade. The ones that stay in it become quite knowledgeable.