The size of rain leaders should never be less than 3 in., and as much larger as the roof area which is drained should require.
Plumbing ordinances differ in trap requirements for rain leaders, some requiring no leader trap when the main trap is used, others demanding leader traps even though the system is protected by the main trap. It goes without saying that each rain leader should be trapped on the system which has no main trap. It would appear wise to use the trap also on systems provided with main trap. There is no danger in this case of air lock from double trapping, for this trouble is obviated by the presence of the fresh-air inlet. The use of the trap prevents foul odors from the house drainage system, and possible back pressure from the sewer, from finding their way through the rain leaders and conductor pipes and escaping through joints and defects in the latter into the rooms of the house through open windows. The usual method is to run the rain leader, of cast or wrought iron, from its connection with the house drain to a point outside the foundation wall, where the galvanized iron conductor enters it. The iron pipe connection should end not less than 5 ft. above the grade level. When run entirely inside the building, they must be of cast or wrought iron, and connected at the roof by means of lead or copper pipe wiped to a brass ferrule and caulked into the top of the leader, the opening being protected by a wire guard or basket. Whenever possible, it is better practice to connect two or more branch rain leaders into one main, and place a trap on this main, rather than to separately trap each leader. This method guards the piping better, for the reason that a trap thus located is more certain of maintaining its seal. In the same way, and for the same reason, the rain leader may be connected into a yard drain, the two lines being protected by one trap.
Conductors run outside should be one size larger than required for a conductor draining the same area when run inside.
When rain leaders pass through the foundation close to a driveway, or where there is danger of being harmed by passing teams, they should be run up in recesses made in the walls, and should not pass through the side of the building at a point lower than 12 ft. above the grade.
If there is no sewer in the street on which the building is located, its roof drainage should be conducted from the leaders into a pipe running below the sidewalk to the street gutter.
If the street is provided with a public surface sewage system, the rain leaders should connect into the surface house drain, and not into the house drainage system. If desired, it is proper to carry the rain leaders outside the house and enter them outside the main trap into the house sewer. When so run, they may be of either extra-heavy cast-iron or glazed-earthenware pipe, and should be provided with traps made accessible by being located in brick or stone wells or manholes. The chief danger that confronts the rain-leader trap is the loss of its seal during a long-continued drought. In traps having only a 3/4-in. seal or thereabouts, it can be imagined that evaporation will not be long in causing its destruction. It would be a good idea to construct on all rain leaders, deep seal traps made of quarter-bends, in order that a sufficient depth may be obtained.
The evils of evaporation thus far have been almost impossible to remedy, and the only safe course is to take every possible precaution against it. There is one point that may be advanced in favor of connecting the rain leaders inside the cellar wall with the house drain, instead of running them outside the cellar wall and connecting them into the house sewer. When connected inside, the rain water during a storm enters the house drain in sufficient quantity to thoroughly scour and cleanse the piping.