This illustration, Fig. 23, should be compared with the illustration of a two-pipe system, Fig. 18. In the one-pipe system non-siphon traps are used without vent pipes. This method of piping reduces the cost of roughing to almost one-half the cost of the two-pipe system, and necessitates less cutting of walls and floors. From a sanitary standpoint the one-pipe system, with approved non-siphon traps, is equal, if not superior, to the two-pipe system. It is open, however, to the objection that a slight gurgling noise might be heard in the waste pipes, due to siphonic action of the trap when a fixture is flushed. Closet traps are not made non-siphon, hence to protect their seals from being lost it is necessary to back vent the closet bend below the floor. This can be done as shown in the illustration, by connecting the back vent pipe to the main soil stack at least one foot above the level of the closet seat. If siphon-jet or wash-down water closets are used the top closet need not be vented. It is not to protect the closets from self-siphonage, but from siphonage by aspiration that in this case the back venting is resorted to. Expansion of Soil and Waste Lines - When soil and waste stacks are installed in high buildings, local conditions might be such that allowance should be made for expansion and contraction; under ordinary conditions, however, in buildings of moderate height no special provision need be made for expansion; the spring of wrought-iron pipe and flexible joints to cast-iron pipe compensating for any variation of length due to temperature. As a matter of fact, the range of temperature during the year should not vary 40 degrees Fahr. It is doubtful if it would vary half that much, but assuming a variation of temperature during the year of 40 degrees Fahr., the expansion of a wrought iron pipe in a building 300 feet high would be less than one inch. The vertical parts of a building, however, are subject to very much the same range of temperature as the pipes, and the entire building will expand correspondingly with the pipes. While special provision need seldom be made for expansion, some provision should be made to allow for settlement of a building without damage to the drainage system. Pipes should be kept free from structural beams and connections to stacks should be made with swing joints to allow for settlement.

Fig. 21

Fig. 22