. Principles and practice of plumbing . f the pipe to a level line a-b is marked3 inches, which represents a depth of seal in the trap of li/oinches, and the inside bore of the trap, which is IVa inches,making a total of 3 inches in all. Now the trap for a fix-ture can be set at such a distance from the stack that thewaste pipe will not have more than 3 inches fall. With afall of 3 inches, as shown in the illustration, the long leg ofthe trap of siphon is just on a line with the line a-b; anyfurther dip would make the connection a true siphon anddraw the water out of the trap every time the fi
. Principles and practice of plumbing . f the pipe to a level line a-b is marked3 inches, which represents a depth of seal in the trap of li/oinches, and the inside bore of the trap, which is IVa inches,making a total of 3 inches in all. Now the trap for a fix-ture can be set at such a distance from the stack that thewaste pipe will not have more than 3 inches fall. With afall of 3 inches, as shown in the illustration, the long leg ofthe trap of siphon is just on a line with the line a-b; anyfurther dip would make the connection a true siphon anddraw the water out of the trap every time the fixture was Digitized by Microsoft® Principles and Practice of Plumbing 65 discharged. As a matter of fact, the top of the pipe shouldnever be carried down clear to the bottom line in practice,for the seal in the trap is then so nicely balanced that it iseasily siphoned. The total distance the waste pipe from anordinary ll/2-inch siphon trap can fall is 31/4 inches, and21/^ inches is the extreme amount that should be allowed Fig. 32Simplified Roughing for Single Bath Room With a possible fall of 21/2 inches, the distance the trapcan be located from the stack will depend on the fall perfoot allowed, and the fall per foot will depend on the loca-tion of the pipe. For example, the waste pipe to the lava-tory could be run at a pitch of i/^ inch per foot, or even less,so that five, six, or even seven feet at a pinch could bereached. When the pipe is run under the floor in woodenjoist construction, on the other hand, there is another limit-ing factor. Twelve-inch joists, the size commonly used inresidence work, shrink upon seasoning or drying, about 1/2inch. On this account, whenever a waste pipe is run undera wooden floor, it should have a fall of at least % inch, nomatter how short it may be. Suppose for instance, thewaste pipe is only 1 foot long, and has a fall of % , when the shrinkage takes place and the weight of thetub has pressed down the waste pipe, it
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