Plumbing vent through roof
I live in zone 4c. I’m wondering if there is any benefit to putting a 2” 3” reducer on each of my existing 2” vent pipes (there’s 3 total for the house that poke through the roof, all 2”). I have the opportunity now as we’re remodeling to replace all the 2” pipes that go through the roof, the last couple feet or so, and transition them all to their own 3” in the attic and then out through the roof as 3” (or 4”).
My main goal here being to improve my overall plumbing venting and not so much snow/ice buildup because that really has never been an issue around here on our vent pipes. Is doing what I’m suggesting at all going to help improve venting?
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What problem with your existing venting are you trying to solve? If there are issues all the time, simply increasing the diameter of the last foot or two of the vent termination is unlikely to make any difference -- it'd be more likely that something in the rest of the DWV system is configured improperly. If you're concerned about frost / snow blocking the vent terminal, increasing the diameter would help -- code in my state (which may be different than your code) has this to say:
"903.2 Frost Closure:
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 (305 mm) inside the thermal envelope of the building."
If I'm understanding correctly, you're looking to consolidate vent piping in order to reduce roof penetrations, is that correct? If so...
As long as all connecting vent pipes adhere to proper sloping as they traverse horizontally (and, the right connectors are used) AND you have the minimum pipe size based on your system then I don't see there being any issues. And of course, proper support/bracing is to be practiced.
I have no idea on allowable pipe size. Generally one doesn't decrease sizing. I'm wanting to say that the vent pipe (if one is not in a cold/freezing climate where there could be icing concerns) is good enough if it matches the final drain pipe size.
Venting usually either works or it doesn't, people don't usually talk about "improving" venting they talk about fixing it if it doesn't work.