The anti-flooding flap valve prevents dirty water from flowing into the rainwater storage tank in the event of drain or sewer flooding. The valve is installed downstream of the filter in the storm drain outlet of the storage tank. The valve can either be fitted inside or outside the tank. If it is installed outside the tank, a small concrete shaft must be provided to allow access to the locking lever. If water in the storm drain rises against the normal direction of flow, the stainless steel sealing flap suspended inside the anti-flooding valve automatically closes to prevent the storm water from entering the tank. The valve can also be closed permanently by means of a manually-operated lever.