how much fall is needed for a toilet drain
A toilet's drain must be run at 1/4 per foot minimum grade when horizontal. A toilet cannot be installed upstream of any wet vented fixture. The maximum vertical fall of the drain is 1 meter(39 inches) and the maximum horizontal run is 3 meters before it connects to a vent. All downstream drainage after the connection of a third toilet on the same horizontal pipe must be 4 diameter or larger.