A floor drain valve (10) in the form of a flapper valve, which includes a pressure relief conduit (34) to equalize pressure across the valve and permit unrestrained draining of fluid through the valve. The pressure relief conduit (34) is equipped with a valve member (30) which opens upon a predetermined pressure on the downstream side of the valve (10), and may further comprise a ball valve (24) to ensure that gas can escape through the conduit but liquids are prevented from it. The valve (10) may be fixed into a drain pipe by means of an expansion ring (80, 82, 84)