The total fall in a fixture drain due to pipe slope A single trap can service more than one sink, but adjacent drains cannot be farther apart than 30 inches. When I’m performing a plumbing inspection, I don’t often bother with measuring the exact drain length, unless it seems to be unusually long or is clearly at a shallow slope. If the entire cross-section of a drain is able to fill with water downstream of the trap, a siphon can be created for a sufficient period of time to pull enough water out of the trap to render it ineffective. The section of the code that pertains to the maximum trap-arm (also called a “dirty arm”) length has strict provisions designed to prevent a siphon from forming in a drain. For more on plumbing traps, drains and vents, visit the JLC Field Guide. Using Table 1002.2. if the fixture requires a 1 ¼ -in. •906.2 Venting of fixture drains. They may also see that the slope column references 1/4 inch per foot, but they are likely to assume that it’s just reiterating the minimum allowable slope for a 2-inch drain. Would I get a vacuum and thus draw the water from the trap. It’s the height at which water will no longer flow from the trap via gravity—the rim of the cup, so to speak (see “How Siphons Are Created,” below). A bathtub trap begins at the end of the tub drain assembly, where it is glued onto a fitting screwed to the drain outlet. For instance, a 1-1/4 inch pipe can have a trap-arm length (from trap to vent) of 30 inches. So the additional slope of just 1/8 inch per foot reduces the allowable drain length between the trap and vent by a whopping 2.66 feet, or 33%. 1/4" * 8' = 2" If the distance is more than a couple of feet is it better to put in a separate vent / drain ? Its not the acceleration of water that makes it overflow into the drain, the water itself seeks level. A running trap may be used remotely which uses less space at the shower drain. It may be 3" if it serves as a clean out also. Cast-iron cleanout sizing shall be in accordance with referenced standards in Table 702.4, ASTM A 74 for hub and spigot fittings or ASTM A 888 or CISPI 301 for hubless fittings. If the distance is more than a couple of feet is it better to put in a separate vent / drain ? An open vent terminal from a drainage system shall not be located directly beneath any door, openable window, or other air intake opening of the building or of an adjacent building, and any such vent terminal shall not be within 10 feet (3048 mm) horizontally of such an opening unless it is at least 2 feet (610 mm) above the top of such opening. Glenn Mathewson is a consultant and educator with BuildingCodeCollege.com, and a frequent presenter at JLC Live. For simplicity, the maximum distance between a trap and its protecting vent has been presented in the form of a table for decades. It’s stating that you can only use up to an 8-foot length if the pipe is run at the minimum allowable slope of 1/4 inch per foot. But if the drain is sloped at 3/8 inch per foot, the maximum length is less than what someone might assume with just a quick glance at the table—it is actually 5.33 feet: 3/8 inch x 5.33 = 2 inches. The greater the velocity of the water rushing through the plumb fixture drain, the more likely the drain will siphon the trap seal. 908.1 Distance of Trap from Vent Each fixture trap shall have a protecting vent so located that the slope and the developed length in the fixture drain from the trap weir to the vent fitting are within the requirements set forth in Table 908.1, except as permitted by 913.1. Basically, it’s a simple cup of water that keeps sewer air from communicating with the air you breathe. A horizontal drain pipe has an existing load of 30 fixture units. Table 908.1 Distance of Fixture Trap from Vent within the distance given in Table 10 - 1, but in no case less than two times the diameter of the trap arm. You should see a 90 (no trap) just below the slab, some times its real shallow, other times its deeper. 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Each fixture trap shall have a protecting vent located so that the slope and the developed length in the fixture drain from the trap weir to the vent fitting are within the requirements set forth in Table 906.1. 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