Return to the Plumbing Forum | Post a Follow-Up

Vent sizing 2 bathrooms

Posted by wease (My Page) on
Tue, Jan 4, 11 at 12:13

Hi all.

I'm in California. I'm remodeling and will be building 2 bathrooms (a powder room and kids bath) right next to each other. Total of 2 toilets, 3 sink basins, and a tub/shower combo. Wondering what size vent I'll need if I can share them all through one stack up.

Also, can I drain this setup with one 3" drain down to the central or do I need a 4 inch?


Follow-Up Postings:

RE: Vent sizing 2 bathrooms

3" drain. 1.5" vent for each of the lavs. Tub-shower can pick up wet venting from one of the lavs. You still need a DWV layout plan, with critical distances and slopes. Posting it will get you a lot of valuable feedback. I'd do a 2" vent stack. This is all off the top of my head; I'd read the California UPC code first before building, and also before posting DWV layout... There is a link to it in a previous thread somewhere. Stay tuned and someone will post it.


RE: Vent sizing 2 bathrooms

CAREFUL HERE- Under UPC vents are sized by the dfu load.

You have a WC (3dfu), tub-shwr(2dfu) and a lav (1dfu) for a total combined load of 6dfu for each bathroom.

Per UPC table T7-5 an 1-1/2" vent line is rated for up to a maximum of 6dfu's, however there is a note on the table that says "No W.C."

Given that both bathrooms have a WC you must use 2" for the vent.

2" is rated for a maximum of 24dfu's so you may combine the two vents into one and continue up with 2", but here again, you must be careful.

Under the UPC if you have a horizontal offset on a vent line that is greater than 3' you must increase the size of the entire vent by one nominal trade size.

Per code definition a horizontal line is any line that runs horizontal or rises at any angle up to 45deg above horizontal. If it rises at 45deg or an angle greater than 45deg it is said to be a vertical offset.

You could run 2" into the attic space then make the lines rise at a 45deg angle to a common point where you can combine them. In this manner all lines are still vertical lines and you will not need to increase the line size.

RE: Vent sizing 2 bathrooms

2" is always good.

Good to know: Table T7-5
And a note on the table that says "No W.C."
Note that WC's don't take venting.
If you need to fit 1.5", keep on digging and consult more.

In 2010 I saw a case of a respected plumber / plumbing instructor making a series of mistakes in a single internet discussion thread, and it all had to do with reading the notes and understanding how to apply them. And the notes were in these tables. What astounded me was that he would make these mistakes after all these decades as a "Master" plumber and plumbing instructor. He only admitted to making one mistake; the others he declined to comment on.

Here is the moral to the story: double check what you read. No matter how highly respected the source.

In PDF form the California UPC is about 35M. I just deleted my copy last week.

RE: Vent sizing 2 bathrooms

I'm not sure why it's a big deal to just put the 2" in instead of the 1 1/2". Seems like an easy decision. Right?

Also, are you saying WC's don't need to be vented?

RE: Vent sizing 2 bathrooms

2" is a big deal because 1-1/2"has an outside diameter of approximately 1-3/4" while 2" has an OD of approximately 2-1/4".

The problem is that code only allows a 2" diameter hole through a 2x4 therefore when we must install 2" pipe we need a 2x6 stud wet wall.

Contrary to what was previously stated, all fixtures including WC's require venting.

Under the UPC a WC is attached to a 3" drain line, and a 3" line may run a maximum of 6' from the WC to the vent opening.

The controversy arises from an exception in the IRC. Under the IRC a 3" line may run a maximum of 12' from the trap weir to the vent opening, but, there is an exception that states "if a WC is the only fixture served by the 3" line it may run an indefinite length from the WC to a vented stack, vented branch or vented main drain.

RE: Vent sizing 2 bathrooms

I did not know that. If code states only a 2" max hole through a 2x4, then my plumber has made many mistakes over the past ten have the inpectors.

RE: Vent sizing 2 bathrooms

Here's a crude drawing of what I'm trying to do. I left out one sink that will drain on the left side of the photo. See link.

Is it ok to vent one of the WC's upstream of the 3" pipe? Do I need the 1 1/4" pipe off the tub or can I use the 2" stack?

Here is a link that might be useful: Plumbing diagram

RE: Vent sizing 2 bathrooms

wease you wrote : " I'm not sure why it's a big deal to..."

Forget about the 2" or 1.5" vent question. Note I didn't say to undersize it.

But, feel free to go somewhere else to double check this: a flat or a 45 degree slant in your attic. (the latter mentioned above, because of the total DFU (total). This might be important on installation day, and you are best off knowing what's what before you go up there and start cutting.

You have 2 toilets, 3 sink basins, and a tub/shower combo. Do the arithmetic. Consult again.

hope this helps.

R2E: Vent sizing 2 bathrooms

wease I just looked at the diagram you posted. No good. 1./ Don't use right angles (you have four of them to replace / re-draw, before reposting). 2./ Don't use 1.25" for vents when you can go one size bigger and get a few advantages out of doing so. Finally, 3./ Depending on your response to points 1. and 2. you can get rid of the 2" vent you drew.

R3E: Vent sizing 2 bathrooms

and find a way to combine the 1.5" drains first before combining them with the 3" WC drain, or else find a way to create two of what is known as "bathroom groups".

RE: Vent sizing 2 bathrooms

Ok let me try this a different way. I made a new drawing with waste as I want it. I've labeled the pipe diameters and put some lengths on there. Can you give me some ideas as to where the vents should go and what sizes? Thanks.

Here is a link that might be useful: Revised Plumbing Diagram

R4E: Vent sizing 2 bathrooms

Are you trying to go perpendicular in order to stay inside a joist bay? Imagine you had only Wye's to combine arms, and only 135 degree turns (1/8th bend) to turn corners. Using those fittings makes distances smaller and corners smoother.

From here on in you will need a Master plumber on site who will see the real obstacles you are faced with.

RE: Vent sizing 2 bathrooms

Maybe you're assuming too much from the drawing. 2 rooms next to each other. No real joist bay issues other then the wall they share. Am I missing something?

R5E: Vent sizing 2 bathrooms

"... Imagine you had only Wye's to combine arms, and only 135 degree turns (1/8th bend) to turn corners. Using those fittings makes distances smaller and corners smoother."
Please respond to this point.

Forget I ever mentioned joist bay. The thing I could have said was 90 degree angles.

I've said enough. Someone else can take it from here.

RE: Vent sizing 2 bathrooms

Not sure why you're looking at the drawing and assuming 90 degree angles. The proper Wye fittings will be used which are sweeps.

Anyway, lazypup, are you still out there? Let me know what I can do to make my drawing a little more readable if it isn't already. Also, I have 2 bathrooms, 3 total lavs, 2 wc's, and 1 combo bath/shower. I calc 11 dfu's.


RE: Vent sizing 2 bathrooms

New drawing attached.

Here is a link that might be useful: New diagram and floor plan

RE: Vent sizing 2 bathrooms

I believe that in California the drain from your double fixture fitting for the dual vanities will have to be 2".

RE: Vent sizing 2 bathrooms

hi wease. I'm not sure if you meant to post that drawing which is now at your link, because it still has all the flaws that I mentioned and none of the adjustments that I mentioned and that I thought you were redrawing. See the link above which leads to a drawing, that has Wye's drawn very visibly as acute angles. Anyway, there is still time to change your drawing, so that the next person who clicks on it can see the new version you meant to post.

For double fixture fittings, see this thread:

Here is a link that might be useful: two bathrooms DWV drawn with angles clearly shown

 o Post a Follow-Up

Please Note: Only registered members are able to post messages to this forum.

    If you are a member, please log in.

    If you aren't yet a member, join now!

Return to the Plumbing Forum

Information about Posting

  • You must be logged in to post a message. Once you are logged in, a posting window will appear at the bottom of the messages. If you are not a member, please register for an account.
  • Please review our Rules of Play before posting.
  • Posting is a two-step process. Once you have composed your message, you will be taken to the preview page. You will then have a chance to review your post, make changes and upload photos.
  • After posting your message, you may need to refresh the forum page in order to see it.
  • Before posting copyrighted material, please read about Copyright and Fair Use.
  • We have a strict no-advertising policy!
  • If you would like to practice posting or uploading photos, please visit our Test forum.
  • If you need assistance, please Contact Us and we will be happy to help.

Learn more about in-text links on this page here