The backflow valve protects basements and ground floor rooms against flooding caused by return flow of sewage waters. Such risk occurs most often in sanitary drainage systems connected to combined sewerage systems and in rainwater drainage systems. It may be caused by intense rainfall, thaw or flood. In normal operation the system is fully open allowing solids to flow freely and unrestricted through the valve, the efficiency of movement is also improved by in-built flow wings in the valve body. Because of its fully open design the valve offers no restriction to cleaning rods, which facilitates drain maintenance and the clearing of blockages further down the branch. Air is allowed to circulate freely through the valve and drainage system eliminating the risk of gas build-up or smells. In the event of water surging back through the pipes or in flood conditions, the backflow automatically lifts a gate within the valve which shuts against a neoprene gasket seal and prevents backflow past the valve.