In hotels and large institutions, and in some large homes, a grease trap is built in the sink which is so constructed as to separate the grease from the water, which obviates clogging of the pipes and which amasses the grease which is sold to soap makers for soap. FRESH AIR INLETS AND MAIN TRAPS Fresh air inlets and main traps are also for the prevention of odors and gases coming directly from the sewer. The entrance of these gases often takes place, even though the plumbing is excellent, by the settling of the floors and foundation rendering the soil pipes defective. The question of soil pipes, etc., is sufficiently covered by the plumbing regulations so far as not to need any explanations here. Every sink, of course, must have its own trap. The following are a few excerpts from the law: SEWERS, DRAINS AND TRAPS must be of extra heavy cast-iron. When found in a leaky or defective condition, shall not be repaired or replaced except with heavy cast-iron pipe. The house drain and its branches must be of extra heavy cast-iron when underground, and of extra heavy cast-iron or galvanized wrought iron or steel when above ground. The house-drain must properly connect with the house sewer at a point two feet outside of the outer front vault or area wall of the building. An arched or other proper opening must be provided for the drain in the wall to prevent damage by settlement. No steam-exhaust, boiler blow-off or drip-pipe shall be connected with the house-drain. Such pipes must first discharge into a proper condensing tank, and from this a proper outlet to the house sewer outside of the building must be provided. In low pressure steam systems the condensing tank may be omitted, but the waste connections must be otherwise as above required. SOIL AND WASTE LINES All main, soil, waste or vent pipes must be of iron, steel or brass. Soil and waste pipes must have proper Y or TY branches for all fixture connections. The diameters of soil and waste pipes must not be less than those given in the following table: Main soil stacks 4″ Main waste stacks 2″ Branch wastes for slop sinks 3″ Branch waste for kitchen sinks 2″ VENT PIPES All vent pipe lines and main branches must be of iron, steel or brass. They must be increased in diameter and extended above the roof as required for waste-pipes. They may be connected with the adjoining soil or waste line well above the highest fixture, but this will not be permitted when there are fixtures on more than six floors. Branch vent pipes shall be kept above the top of all connecting fixtures, so as to prevent the use of vent pipes as soil pipes or waste-pipes. Branch vent pipes should be connected not less than six inches nor more than two feet from crown of trap or side of lead bend. No form of trap will be permitted to be used unless it has been approved by the Superintendent of Buildings or the Board of Standards and Appeals.