Incorrect use of sanitary tee fittings in drainage systems is a common problem. What is the proper use of sanitary tee fittings in plumbing DWV piping? Table 706.3 of the IPC provides the code requirements for correctly using plumbing fittings.
On sanitary tees, the branch connections have a short radius. They can only be used to change the direction of drainage flow in the horizontal-to-vertical orientations. They cannot be used to change the direction of drainage flow in a vertical-to-horizontal direction or a horizontal-to-horizontal direction.
As shown above, the branch connection of a sanitary tee lying on its back can be used for a dry vent. In this sanitary tee orientation, the branch connection cannot receive drainage flow, as this is a vertical-to-horizontal change in direction, which is prohibited.