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Old 08-14-2012, 02:20 AM   #1
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Can I split a main drain vent stack? --


Hi Everyone,

I live in Texas. I've got all cast iron plumbing under & throughout my pier & beam house, built in 1955. I am expanding a tiny Jack & Jill bathroom into a large adjoining closet. There is 1 toilet, 1 sink, and 1 shower. This is the only plumbing on this end of the house.

I've torn down the "shared" wall that concealed a 4-5" vent stack that goes straight up from the main drain line (same size) under the house, through the floor, up through the ceiling and terminates above the roof. So now the vent stack is in the middle of the now larger bathroom... and I need to move it to a wall(s) to hide it once more.

Obviously, I will be reworking this vent with PVC, so the OD of the pipe will be smaller than the OD of the original CI pipe in the middle of the room.

I need to relocate this vent to any of the 2x4 bathroom walls (not currently possible with the size of the original pipe). My question:
  1. What is the smallest single vent I can use to properly vent this drain line? There will be no additional plumbing fixtures added to the bathroom.
  2. If I cannot do #1 above (downsize the vent to a size that will fit into a 2x4 wall), can I split this large vent under the house and plumb 2-3 smaller vent stacks to go up through the wall, and then tie them together in the attic to the original roof stack?
Please forgive me if I have ommitted any relevant information. I'll do my best to clear up any issues.

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Old 08-14-2012, 05:27 AM   #2
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Can I split a main drain vent stack? --


You could use a 2" vent for the toilet and another 2" vent for the shower/sink---then merge into a 3" to go through the roof.

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Old 08-14-2012, 07:12 AM   #3
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Can I split a main drain vent stack? --


everywhere i have did plumbing you were required to have at least one minium 3 inch vent continuous from the main sewer through the roof.
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Old 08-14-2012, 09:21 AM   #4
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Can I split a main drain vent stack? --


http://www.kingcounty.gov/healthservices/health/ehs/plumbing/unitvalues.aspx

This will give you the fixture value for various size pipes and fixtures. This chart is from King County (Seattle) WA, but is derived from international standards. By my calculations, you could vent your bathroom with 1 1/2" DWV piping. Local codes and considerations could vary. For instance, in many snowy areas, all roof penetrations must be 3" minimum because of internal frosting
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Old 08-14-2012, 09:51 AM   #5
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Can I split a main drain vent stack? --


Do you know which code applies where you live? If its the IRC (as where I live), the toilet does not require a separate vent.
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Old 08-14-2012, 10:12 AM   #6
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Can I split a main drain vent stack? --


Our code requires the total venting to be at least half the area as the drain exiting the building and they have to be minimum 3" exiting the roof(frost)
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Old 08-14-2012, 10:12 AM   #7
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Can I split a main drain vent stack? --


Quote:
Originally Posted by DannyT View Post
everywhere i have did plumbing you were required to have at least one minium 3 inch vent continuous from the main sewer through the roof.
Mine requires this too
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Old 08-14-2012, 10:23 AM   #8
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Can I split a main drain vent stack? --


Quote:
Originally Posted by DannyT View Post
everywhere i have did plumbing you were required to have at least one minium 3 inch vent continuous from the main sewer through the roof.
Quote:
Originally Posted by Canucker View Post
Mine requires this too

For all we know, there is another vent at the other end or the house, which would allow this run to be treated as a branch, and not need a 3" statck at this location

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Originally Posted by SpinFly View Post
Hi Everyone,

This is the only plumbing on this end of the house
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Old 08-14-2012, 11:14 AM   #9
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Can I split a main drain vent stack? --


Quote:
Originally Posted by Anti-wingnut View Post
For all we know, there is another vent at the other end or the house, which would allow this run to be treated as a branch, and not need a 3" statck at this location
Thanks for all the responses so far! I will look under the house and see if this main line is vented somewhere else with at least a 3" stack.

Our city abides by the International Plumbing Code.

I'll let you know what I find!
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Old 08-14-2012, 02:13 PM   #10
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Can I split a main drain vent stack? --


It looks like the other vents in my house are just 2 inch

Anybody have any creative suggestions for concealing a 3 inch vent in a 2 x4 wall? Technically, a 3 inch PVC vent might fit (?), but I'm concerned about compromising the top plate of the wall that the vent would go through....

Thanks everyone!
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Old 08-14-2012, 05:28 PM   #11
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Can I split a main drain vent stack? --


I would just add some furring strips to the side of the studs to make it wide enough for the 3" PVC.
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Old 08-14-2012, 07:56 PM   #12
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Can I split a main drain vent stack? --


3" PVC will fit into a 2x4 wall----od 3 1/4"
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Old 08-14-2012, 08:12 PM   #13
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Can I split a main drain vent stack? --


Yeah, I figured it would fit but it doesn't leave any meat (less than 1/4"; the wall is load-bearing) in the top plate to hold the wall together. I will probably have to add furring strips to the shortest wall in the room.

Unless somebody can suggest a way to tie the top plate of the wall together around the vent stack..?

Last edited by SpinFly; 08-14-2012 at 08:13 PM. Reason: add info
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Old 08-14-2012, 08:41 PM   #14
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Can I split a main drain vent stack? --


you can reduce pipe size according to irc read up on it
http://publicecodes.cyberregs.com/ic...2_9_sec006.htm
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Old 08-15-2012, 10:32 AM   #15
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Can I split a main drain vent stack? --


like mike said, 3 inch pipe will fit in a 2/4 wall. the fittings won't tho but furring out to cover the fittings would be minor. if they are only at the top and bottom you could cut a recess in the back of the drywall about 1/4 inch so the drywall will lay flat.

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