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Not sure what California code is, but Oregon requires the same cross sectional area to be going out through the roof as what the minimum required building drain is.
For example, if you have a 3" drain, Oregon would require you to have either :
1 3" vent
2 2" and 1 1-1/2" vent
or any other combination where the area of each meets or exceeds the area of the 3" drain.
If your fixture load requires a 3" drain and you have a 4" drain, then you still go by the surface area for a 3" drain.
For example, if you have a 3" drain, Oregon would require you to have either :
1 3" vent
2 2" and 1 1-1/2" vent
or any other combination where the area of each meets or exceeds the area of the 3" drain.
If your fixture load requires a 3" drain and you have a 4" drain, then you still go by the surface area for a 3" drain.