Vertical direction of PVC tee used for vent only
If you are using a sanitary tee to go from a horizontal vent pipe into a vertical vent the correct orientation is &quot;upside down&quot; from its normal usage.  The curve of the tee should go from the horizontal direction curving up toward the roof.  Air flows into the vent from outside to replace air pushed downstream by the falling waste, so the vent is oriented to allow smooth airflow from the roof side into the horizontal vent.