View Full Version : Adding new bath - need help with DWV please

02-05-2009, 06:13 PM
I'm adding a bathroom and need help running the DWV lines. I went under the house today to hook up the hot/cold supply for this new bathroom. I noticed that the current setup (built in '71) for the bathroom is as follows:

The main CI line 3" comes from the tank and goes through the crawl space and then straight up through the roof to vent (3" vent) between the bathroom walls (bathrooms are back-to-back). Below the bathroom floor is where this line turns upward to go through the roof as a vent. All drains tie into this line using a single 4-way connection to this waste/main vent line (each connection is @ 90 degees). Two toilets dump into this @ 180 degrees of each other (back-to-back on opposite sides of the wall). Then 90 degrees from the toilets one bathtub is plumbed in. 180 degrees from this bathtub (or 90 from each toilet) a single line in plumbed which combines: 1 bathub, laundry, and kitchen. At about 24" above the floor (approx) each sink dumps into this vent/soil stack. None of the bathroom items have additional vents, and they are all well within 5' of this 3" line (probably at most 3').

Noticing this, I am assuming that the drawing below would be correct as it mimicks the current setup that I have. The 3" line shown would tie into the main line using a 3" wye. The shower would connect to the vertical part of the vent using a 3"x2" wye. The lavatory would also connect to the vertical part of the vent using a 3"x1.5" wye. This vent would basically come down vertical from the roof with these 2 wye connections at the bottom of this vent (on the vertical part), then this line would make a 1/4 bend to become horizontal with the watercloset connecting in a couple of feet with a 3"x3" wye (towards the main line).

Would this be a correct assumption? What was code 30 years ago may not be code today...If this is not correct, what would be a correct way?

I am assuming that the shower and lavatory do not need additional venting as they are within the 5' requirement for a 2" fixture arm (1.5 for the lavatory).

If I post correctly...here is the picture:


02-06-2009, 07:08 AM
If the line to the lavatory is under the floor, it needs its own vent, and if it is above the floor, where it would not need a separate vent, you are going to have to step over it every time you go into the shower. In other words, just drawing lines on a paper does not tell us HOW you are going to install the piping, and THAT is what determines whether they are correct or not.

02-06-2009, 09:56 AM
"If the line to the lavatory is under the floor, it needs its own vent, and if it is above the floor, where it would not need a separate vent, you are going to have to step over it every time you go into the shower."

From what I have read, under the IRC the maximum length of a fixture arm expressed in feet can be computed by dividing the diameter of the line by the pitch. 1-1/2" requires a minimum of 1/4" per foot pitch so the maximum length of the waste arm. 1.5" / .25" = 6'. This means the maximum length of your waste arm may not exceed 6 feet when measured on a line on the top of the pipe from the trap weir(point where water actually spills out of the trap's inverted U) to the vent. Since my lines are shorter than this, I should be well within code. Please correct me if this is incorrect...

Does anyone have any ideas how I might be able to correctly plumb this?

02-06-2009, 02:49 PM
Someone has given me another way, but I wanted to see what any pros out there think (explanation below so that nobody will think that I have pipes running on the floor ;-)


The bath sink dumps into the wet vent. The vent continues down to a quarter bend where it continues toward the toilet. The shower connects to this wet vent via a 1.5"x2"x2" wye. It continues to a wye at the 3" pipe that also connects to the toilet upstream. Therefore the entire bathroom is wet vented through the stack coming up from the lavatory...what do you think?

02-06-2009, 05:34 PM
Excuse the freehand. hope this helps


02-06-2009, 06:13 PM
Now it all makes sense (my last reply, what someone told me). Pictures DO say 1000 words! Thanks so much for the reply and the drawing! I will put it into action this weekend. Again, a big THANK YOU !!!!

02-07-2009, 09:06 AM
I now have 1 more question. Starting at the 3" pipe, the lavatory/shower drain connects via 2" pipe (3"x3"x2" wye). It travels as 2" to the shower wye (2"x2"x2"). It then continues as a 2" pipe and then uses a 1/4 turn to go vertical up the middle of the wall (the wall that the lavatory is on). Can this vertical pipe (the one that the lavatory spills into) be 1 1/2" from this 1/4 turn to the roof vent...or does it have to be 2" up to the lavatory tee and then become 1 1/2" vent?

The reason I'm asking is that there is not enough room to run the 2" pipe through the base of the stud wall with the water supply that runs along the same wall.


02-07-2009, 09:59 AM
Normally the 2" runs from the 1/4 bend up to a 2x11/2x11/2 sanity tee. You should be able to offset the water around the 2" or raise the lines higher to where the 1 1/2 starts.

02-08-2009, 08:14 AM
Since that pipe is the toilet's vent, its size would be governed by YOUR plumbing code. Most would require 2" but many places have adopted a lesser code which allows 1 1/2".

02-12-2009, 07:16 AM
I saw a similar diagram on another forum: make sure you note that the three Wyes are the key: straight pipe from each Wye back to WC / shower P trap / quarter bend for lav.

The "vertical" pipe going to the lave can be "not plumb" slanted if that helps.


02-26-2009, 07:19 PM
I got it finished last weekend...the crawl space is between 24" and 32"...that sucks! I was able to get the 2" all the way up to the lavatory drain.

Thanks to everyone for their help and advice! I will try to post pictures a bit later.