Change in DWV Size Before Roof Termination

Good morning all. Happy Sunday! I was wondering if some of you could weigh in on this. I did an inspection yesterday morning and when I was in the attic, I came across the vent pipe for the hall bath. This is the only vent to be located for the hall bath. It was being run across the attic floor to the back side of the roof where it terminated. The issue is that it starts out as a 2" pipe, but then transitions into a 3" pipe, in the middle of the run, which is what it terminates as. What is the reason for this? I have never come across something like this. Thanks in advance. FYI, this is a new build in upstate SC.

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I believe it is so that frost doesn’t buildup inside pipe and block it.

That would be true for us up here, but not necessarily for South Carolina. Hopefully that is not the only VTR.

The floor plan is a split plan, so the master bedroom is on the other side of the house. The attic provided access only around the pull-down stairs, as is typical, and with cathedral ceilings I couldn’t even see the DWV for the master or kitchen/ laundry. It is the only vent for the hall bath that I could see. Do you see an issue with this being the only vent for this bath?

We do get cold weather in the teens and singles for only a few days at a time consistently, but usually the days will warm to about 40’s-50’s or at least above freezing, so heavy frost buildup should not be an issue.

Nothing wrong there… it was done just in case or by plumber from outside the area:

SC IRC 2018:
Where the 97.5-percent value for outside design temperature is 0°F (-18°C) or less, [vent] extensions through a roof or wall shall be not less than 3 inches (76 mm) in [diameter]. Any increase in the size of the [vent] shall be made not less than 1 foot (304.8 mm) inside the thermal envelope of the building.


Thanks all. I just haven’t seen that before. I learn something on almost every inspection I do. Unless anyone else has a concern with this and wants to provide thoughts, that will be it for me. I hope all of you have a great weekend…or what’s left of it. Stay safe and keep being awesome!

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Assuming the 2" was sized correctly, going from 2" to 1" would be a concern, not 2 to 3. It may not be necessary in your area but otherwise not a concern. Tradesmen move around the country all the time and sometimes bring with them things that are not necessary in the new area :slight_smile: Also, sometimes you run out of 2" pipe and all that’s left is 3" You need to finish the job… guess what happens :slight_smile:

Agree… …

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