Drain pipe sizes 3" or 4"

Users who are viewing this thread

filipi

New Member
Messages
5
Reaction score
0
Points
0
Location
California
Hello

I am new here and have been reading different thread and learned a lot.
Great forum and very useful.
I am hoping to get some opinion from some of your experiences so I can make the right decision in the plumbing makeover that my house is going to get.
Basically my main concern at this time is to make sure the right size pipe is being used on the toilets and main drain.
They will be low flow (Toto) and I understand that 3" pipe is enough and may be even better although you can use 3" or 4" according to codes.
This remodel is for a one story house so several line are going to connect to the main drain underneath the house, which I was going to make a 4"(about 10 feet before exiting the house crawl space) but I am not sure if I even need to do that until I actually connect to the sewer line since it seems fine to run 3" all the way.
I will check with the plumbing inspector but was hoping to get some input from anybody willing to help.

Thanks.
 

Jadnashua

Retired Defense Industry Engineer xxx
Messages
32,770
Reaction score
1,190
Points
113
Location
New England
Some of it depends on where you live and the codes that apply. Each type of drain has a value called a fixture unit. The total fixture units determines the minimum drain line required.
 

filipi

New Member
Messages
5
Reaction score
0
Points
0
Location
California
Thanks, and I did calculate my total fixture units and came out to 24.
According to the California code a maximum of 35 is allowed on a 3" drain, but there is a note that I have a hard time interpreting,quote :
" and not to exceed 3 water closets or 6 unit-traps on any horizontal branch or drain "

If I read this correctly I need to go 4" on the last 10 feet before exiting the house because pretty much all fixture come together at that point.
 

Lakee911

I&C Engineer (mostly WWTP)
Messages
1,254
Reaction score
1
Points
0
Location
Columbus, OH
If one were to use a 4" coming off the WC, might that allow for less slope and therefore give you more flexibility in placement of the closet bend?
 

filipi

New Member
Messages
5
Reaction score
0
Points
0
Location
California
I did read somewhere , and I think it was a code , that you can go 1/8 of an inch slope on bigger pipe , but from what I've gathered from all the pros here and other references, it seems that 1/4 is the ideal slope no matter what size the pipe is , but obviously I am no plumber.

Thanks for the answer regarding my question.
I will go with a 3" drain pipe on both toilets , those will drain into another 3" main drain branch , 4 other 2" branch will drain into the main drain , and 3 other 2" branch will drain into one of the 3" toilet branch. All correctly vented of course.
 

filipi

New Member
Messages
5
Reaction score
0
Points
0
Location
California
The main sewer pipe I will be connecting to outside the house is 4".
But I thought it would be slightly easier to work with 3" underneath the house , and I was also under the impression after reading some quotes on this forum that 3" works as well or better than 4" specially with low flow toilet , ( "scourging effect") and that it might be even better in the long run to have 3".
 

Terry

The Plumbing Wizard
Staff member
Messages
29,942
Reaction score
3,459
Points
113
Location
Bothell, Washington
Website
terrylove.com
A 3" line does have a better drain line carry. If it's my home, I'm running 3"

4" handles more, but the average home has no need for it. Lines outside the home are typically 4", mainly I believe because it covers all expansion in the future. Once you hit "4" four toilets, it's required.
 

Lakee911

I&C Engineer (mostly WWTP)
Messages
1,254
Reaction score
1
Points
0
Location
Columbus, OH
Believe it or not, Terry, here in Columbus (at least the older parts) the line outside is 6 inches. Typically for even an older small one bath home to have a 4" and then it transitions to 6" just outside the footing. Overkill, no? :)
 
Top
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.
I've Disabled AdBlock    No Thanks