Sliding skirt valves for waterless urinals and method of controlling fluid flow through the sliding skirt valve are presented. The sliding skirt valve has a valve body, which includes an inlet, an outlet and a sealing surface. The sliding skirt valve also has a central shaft with an upper stop and a lower stop and a barrier interface. Between the upper and the lower stops, a conical barrier is slidingly engaged with the barrier interface. In a normal pressure state, the conical barrier lies against the lower stop of the central shaft. In a back pressure state, the conical barrier moves upwardly to lie against the upper stop of the central shaft and block the vent to upward fluid flow from the downstream of the sliding skirt valve.