Changing Building Drain Size

[Fein]

Does a building drain line that vents to the roof have to be the same size through the roof?

I have a two story home with a basement and 4" CI main drain that vents to the roof. There's a WC in the basement and first floor kitchen sink that all connect to a separate 2" VTR.

I would like to cut out the vertical portion of the 4" CI and replace with 3" PVC to pick up drains for two new full baths (used to be only one).

With the help I have received from you all, my project is humming along (ok- it's been over six months for two baths- but I'm definitely moving forward).
Thanks.

 

[Jimbo]

That varies a lot. Some areas do require a full size vent to roof. CA only requires that the total of all the vents add up to the proper amount for your DFU.s and size of main. Toilet requires min. 2" VTR. Very common to see one 2" and a couple of 1½" vents on a 3 br house.

 

[Sponsor]

 

[SewerRatz]

Your best bet is to check with your city building department, and see what the codes allow. Here in Illinois it must remain full size to the roof.

 

[Fein]

Ugh. I don't know why I didn't think of this before cutting...
I'm under 2006 NSPC if anyone happens to know the section to review. I already have all the DWV lines joined- so if there is anything that I can use to persuade the inspector I would be happy to hear it. thanks.

 

[SewerRatz]

Ugh. I don't know why I didn't think of this before cutting...
I'm under 2006 NSPC if anyone happens to know the section to review. I already have all the DWV lines joined- so if there is anything that I can use to persuade the inspector I would be happy to hear it. thanks.

What city and state are you in? This will help since lots of cities have their own amendments to the plumbing code.

 

[Fein]

I may have found my answer:
11.2.3 says building drain size is determined by DFU.
Any thoughts still appreciated...

 

[Fein]

Maplewood, NJ

 

[NHmaster]

IPC requires one vent, 1/2 the size of the building drain, through the roof. Since the building drain in most residences is 4", that means 2"

 

[hj]

vent

Some areas require a full sized vent through the roof. Others require that the aggregate area of the vents through the roof equal the area of the main line. And still others apparently base it on the fixture units, but since that could change over time as revisions are made it would be the least accurate method.