The size of traps should conform to the size of waste-pipes, and usually the size of the branch vents is about the same size as the waste-lines. However, there are special conditions where this varies. For venting the water-closet trap, it should be noted that the vent is not taken from the trap which is contained within the fixture itself, but is taken from the upper side of the bend (usually of lead) where the fixture is joined with the piping system, and is 2 inches in diameter.
PLUMBING SYSTEM USING BACK-VENTING
Where there are two fixtures, such as the lavatory and the bathtub, with 1½-inch branch vents coming from the traps, these may be joined into one main branch vent, which need not be more than 1½ inches in diameter. The pitch of the branch vents entering into the main vent should be at an angle of about 45 degrees, so that all rust scale will drop down into the fixture outlet and be washed away.
The main vent, which runs parallel with the main soil-line, needs to be only 2 inches in diameter, and should be branched in at the bottom and the top to the main soil-line, as shown in the drawings. The material of which both main vent and branch vent is made should be galvanized-iron piping.
The fresh-air inlet, the house-trap, the clean-outs, and all other parts of the system are the same as was shown for the simpler method of plumbing.
Rain-Water Drainage
The small house need not drain off its roof-water into the plumbing system, if the plumbing code does not require it. The simplest and easiest method to dispose of it is to collect the water in gutters, lead it down the waterspouts into pipes which terminate in a dry well in the ground. Small roofs over porches and back doors need not even have the leaders, but spill the roof-water out onto the ground, where a stone has been placed to prevent the undermining of the surface of the lawn by the wearing action of the water stream.
In outlying city districts where the sewers have not yet been installed it is customary to carry the roof-water in pipes below the level of the sidewalk to the gutters of the street or to a leaching cesspool which is independent of the cesspool used for sewage disposal, and which is practically the same thing as a dry well, for the bottom is made with gravel through which the rain-water seeps off into the surrounding soil.
Wherever the rain-leaders must be connected to the drainage system of the house, the sheet-metal leaders are inserted into cast-iron pipes called shoes at the base, which in turn are trapped on the inside of the cellar wall and connected with the house-drain. It is always best to try to trap a group of leaders to one trap rather than use a separate trap for each leader.