The drainage non return valve can be inserted into the foul pipe inside the inspection chamber. The valve has a single flap, which is in the closed position most of the time. When water leaves the property, the flap opens and will close again once the pressure has rescinded. The valve is designed open with just the pressure from the flush of a toilet. In a flood event, or a back up of water due to blocked rains, the valve is closed shut to prevent back flow. In addition, a stainless steel flap stops rats in the drains.