There are two systems of stacks and branches in use at the present time, the two-pipe system and the single-pipe system. In the two-pipe system siphon traps are used, and their seals are protected from siphon-age by a system of vent pipes. The principles of installation of the two-pipe system are shown in Fig. 18. In this system a vent stack intersects the soil stack at an angle of 45 degrees at a point below where the lowest fixture discharges into the soil stack. The object of connecting the soil and vent stacks together at this point is to provide an outlet from the vent stack into the soil stack for rust scales or other foreign matter that might enter it. In the illustration the vent stack rejoins the soil stack at a point above the level of the highest fixture discharging into it. This connection is made only for economical reasons, however. The vent stack may be extended separately through the roof, and in buildings over six stories in height it is better to do so. On stacks three or more stories in height the waste pipe from the wash basin and bath tub on the first floor is generally connected to the heel of the vent stack to wash out any foreign matter that might lodge there. This provision, however, is of more importance in wrought iron systems than in cast iron systems, owing to the greater possibility of wrought iron stacks being stopped at this point by rust scales. Connections to vent stacks are made at a point above the level of the outlet from the highest fixture in the group; and all the vent pipes drain toward their respective traps so that water of condensation or sewage that backs up in the vent pipes, during stoppage of the waste pipe, will drain out again when the waste pipe is clear. Should the connection to the vent stack be made at a level lower than the outlet from the fixtures, the waste pipe might become stopped up and the fixture would waste through the vent pipe without the stoppage becoming known. The principles and practice of the two-pipe system are fully explained in the foregoing description. More fixtures may be connected to a stack or more stacks may be installed in a building, but they are only a multiplication of units of which Fig. 18 is an example. In the two-pipe system of plumbing, as in all systems of piping, the more direct the pipes are run and the fewer fittings used the better the work will be; however, as the tendency of the times is towards a multiplicity of pipes and fittings, the following examples of how the two-pipe system should be installed are subjoined. When two fixtures on one floor are located on opposite sides of a partition, and there are no other fixtures above that floor, they can be cheaply, neatly and properly connected as shown in

Fig. 19. By this method of installation no pipes are exposed outside of the partitions and each trap is effectively ventilated. It should be borne in mind, however, that a sanitary cross (Fig. 20) must be used with this method of installation. If a double Y fitting (a, Fig. 19) were used instead, the waste pipes b, b (indicated by dotted lines) from the traps would form long legs of siphons that would empty the traps at each discharge from a fixture. When fixtures on two floors are located on opposite sides of a partition they can be properly and economically connected as shown in Fig. 21. In this method of installation separate waste stacks are run to the fixtures, on the different floors, and the stacks continued up to the roof to serve as vent pipes. It is simply applying the principles of Fig. 19 to fixtures on two floors. In Fig. 22 fixtures are located on opposite sides of a partition on three or more floors. It would be too cumbersome and expensive to carry separate stacks to each floor as is done in Fig. 21, so a soil stack and vent stack are run, and between the walls of the partition, at the intermediate floors, the vent stack is connected to the waste stack and a sanitary cross in the connecting branches used on the same principle as in Fig. 19. On the first floor both fixture wastes are connected direct to the vent stack. This is to simplify the construction and wash out of the vent stack any rust scales or other foreign matter that might lodge there. On the top floor both fixture wastes are direct connected to the waste stack. This makes a perfectly sanitary connection and simplifies the construction. All of the examples of roughing illustrated possess the additional advantage of having concealed in the partition all waste and vent pipes except the short lengths of waste pipe from the fixture traps to the wall; furthermore, when the waste pipe to the fixtures is brought back through the wall, it leaves the floor space beneath the fixtures free from pipes.

Fig. 18

Fig. 19

Fig. 20