I originally posted a question about the builder of my new home moving one of my plumbing vent pipes to add it to the vent stack so that I only had one protrusion from the roof. Now I have a follow up question about vent sizes.

Our drainage pipe for our two story home is 3 inches. We have laundry, kitchen sink, half bath (sink and toilet), master bath (separate tub and shower, toilet, and double sinks) and guest bath (double sink, combined shower/tub, toliet). There is a 2 inch vent stack, as that stack reaches the roof two more 2 inch vents connect in at an angle (I know one is the guest bath and I believe the second connection is the master). The vent coming out of the roof is 2 inches. I know now that it is fine to combine vents, but now I'm severely concerned about the vent stack only being 2 inches. I'm confused when I read the IRC on whether the total fixtures is for each vent connected into the stack, or looking at every fixture unit in the entire home being on one stack? I'm worried about drainage issues and curious if I'm over complicating it. I have asked 3 different plumbers and two different inspectors and I get different answers from all of them.





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Some codes require a full size vent, 3" in your case, protrude through the roof, others state the combined area of all the vents protruding through the roof shall equal or exceed the area of the main drain pipe. Have you contacted your plumbing inspector and see what he/she says is required?

When we contacted the county inspector (who had not reinspected the home prior to drywall starting today) he stated 2 inches was more than sufficient. Everything I've read is how you stated and contradicts his statement. This is my reason for continuing research.

that would not pass MA stringent plumbing code but then not much in the rest of the US would pass either.
having said that, 2 inch has enough air running thru it to properly vent quite a few fixtures.
given that you are in a single family residence and the chance of everything being flushed at once is very remote, you should be fine.
you should ask the plumber to tell you the location of any and all AAV's so will know where they are for future replacement (should they need it)..
AAV = automatic air admitance valve/s.

What size is the pipe through the roof? If it is 3" than it should satisfy ALL the requirements, if it is 2", then it does not.

It is 2". Does it make a difference that the kitchen sink and dishwasher are not vented through the roof because they are in an island?

No. All that matters is the size of the vent through the roof and HOW the smaller vents connect to it. In other words, they CANNOT just put a 3x2 reducer on the line before it goes through the roof and say "Now it is legal".



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South Carolina is under UPC code. The 2" vent is perfectly fine, functionally and code compliant.

Can someone explain to me why different codes vary so widely? Originally I thought it was just commercial public use vs private residential but I'm confused why what is okay in SC wouldn't be okay in, say, Georgia? I know up north vents call to be larger I believe because of ice and snow, but other than that I'm confused.

michelle, i wish i had an answer for you but i don't.
here in massachusetts we MUST use copper or cast iron for drains in commercial applications.
right next door to us, new hampshire allows PVC for commercial applications.
so, 2 neighboring towns on the state line ( say seabrook NH and salisbury MA) can have a small restaurant right next to eachother. one is in MA, next door is in NH.
the restaurant in MA must have copper or cast iron drains while the restaurant in NH can have PVC drains.
no one who eats in the NH restaurant has contacted the bubonic plague.
good luck and enjoy your new home..

One of many reasons I disagree with UPC guidelines and regulations. UPC used to be the foundation for plumbing codes across the US, but now it seems to have strayed too far from the important basics of plumbing.