In this system, sometimes referred to as the combined system, all discharge pipes conveying soil from water closets, urinals, slop hoppers, etc. and waste from wash basins, sinks, baths, bidets, etc. are connected to one common vertical stack which is connected at the foot directly to the drain similarly to the separate soil pipe in the two pipe system. The provision of ventilating pipes is in accordance with the size of the installation. It should be noted that CP304 recommends the use of the term discharge pipe: ‘as a comprehensive all-embracing description in place of the traditional soil and waste terms.’ Notes on the Single Stack System. This system was introduced into this country from the USA about 40 years ago. The idea of discharging all sanitary appliances into one main stack, relying solely upon appliance trap seals to act as foul air barrier, did not meet with the approval of many designers and certain byelaws did not even permit the connecting of a waste pipe to a soil pipe. The one-pipe system had certain advantages over the two pipe system but the risk of trap seal loss had to be overcome. This was achieved by insisting upon deeper seal traps (where possible) and fully ventilating the systems. Eventually, byelaws were amended to permit the use of the one-pipe system. To be an economical proposition at all, close grouping of appliances around the main stack is essential. Therefore, the one-pipe system is suitable for multi-storey dwellings, offi ce buildings, etc. where close grouping is practicable and often repeated at various fl oor levels. Economy is possible with the larger installation but for 2 storey dwellings, the two-pipe system was less costly when using hopper heads and external downpipes to gullies. As soil and waste pipes must be internal the cost of two-pipe systems for housing has increased. Most designers would now prefer the simplicity and economy of a one-pipe system without anti-siphon pipes if possible, referred to as the single stack system.