A stack fitting (10) and coupling (102) combination adapted to be mounted through a floor (104) in a building to prevent the spread of fire through pipe (100, 103) from below the floor is described. The fitting includes a thermally releasable plug (13) mounted in an extension conduit (12) from a main conduit (11) of the fitting which is activated by a fire to allow the plug to move into the main conduit. The fitting is particularly adapted for multi floor buildings where plastic pipe is to be used.