29. Every water-closet, urinal, sink, basin, wash-tray, bath, and every tub or set of tubs, must be separately and effectively trapped, except where a sink and wash-tubs immediately adjoin each other, in which case the waste-pipe from the tubs may be connected with the inlet side of the sink-trap; in such a case the tub waste-pipe is not required to be separately trapped. 30. Traps must be placed as near the fixtures as practicable, and in no case shall a trap be more than two feet from the fixture. 31. All exit-pipes must be provided with strong metallic strainers. 32. In no case shall the waste from a bath-tub or other fixture be connected with a water-closet trap. 33. Traps must be protected from siphonage, and the waste-pipe leading from them ventilated, by a special air-pipe, in no case less than two inches in diameter for water-closet traps, and one inch and a half for other traps. Except in private dwellings, the vertical vent-pipes for traps of water-closets in buildings more than four stories in height must be at least three inches in diameter, with two-inch branches to each trap, and for traps of other fixtures not less than two inches in diameter, with branches one and a half inches in diameter, unless the trap is smaller, in which case the diameter of branch vent-pipe must be at least equal to the diameter of the trap. In all cases vertical vent-pipes must be of cast or wrought iron. 34. These pipes must either extend two feet above the highest part of the roof or coping, the extension to be not less than four inches in diameter to avoid obstruction from frost, or they may be branched into a soil-pipe above the inlet from the highest fixture. They may be combined by branching together those which serve several traps. These air-pipes must always have a continuous slope, to avoid collecting water by condensation. 35. Traps of fixtures near the fresh-air inlet may be ventilated by being connected with it. 36. No trap vent-pipe shall be used as a waste or soil pipe. 37. Overflow-pipes from fixtures must, in each case, be connected on the inlet side of the trap. 38. Every safe under a wash-basin, bath, urinal, water-closet, or other fixture, must be drained by a special pipe not directly connected with any soil-pipe, waste-pipe, drain, or sewer, but discharging into an open sink, upon the cellar-floor, or outside the house. 39. The waste-pipe from a refrigerator shall not be directly connected with the soil or waste pipe, or with the drain or sewer, or discharge into the soil; it should discharge into an open sink. Such waste-pipes should be so arranged as to admit of frequent flushing, and should be as short as possible, and disconnected from the refrigerator. 40. The sediment-pipe from kitchen boilers must be connected on the inlet side of the sink-trap.