I’ve recently realized that air admittance valves have generally become
more permitted by code and help reduce the number of roof penetrations,
but they likely still won’t do for a sealed system [on-property septic]
as any positive pressure has to have *somewhere* to go. Thus, as far
as I know code only permits AAVs for branches but not the main stack;
there has to be *someplace* for the sewer gas and its attendant
condensation to escape. [Is that also true for systems piped into
city sewers, which would eliminate the pressure problem?]

I did some searching here, without finding much. One or two projects
sent the stack through a gable wall, which might mitigate the roof problem
but is often a pretty inconvenient way to run it, not to mention being
looked on with much skepticism on the part of the local AHJ. One of
theGBA postsseemed to imply a system based solely around an AAV, but
didn’t clarify if it was for the main stack and/or the home was connected
to city sewer instead.

So I’m just trying to clarify what’s legal, sensible, and safe to do in a
building with septic that wants to be as tight as possible and have the
least amount of stuff poking through the roof. No immediate need here,
although I recently did a bunch of noodling to design a little protective
“snow splitter” for my own stack which is about midway down a standing-seam
panel.

_H*