Drain Line Fall from Slope and Run

Gravity drainage lives or dies on a slope quoted as a plain ratio. The North American convention is a quarter inch per foot, which is 1 in 48 or about 2.1 percent; the British and European convention for 100 mm foul drain is 1 in 40, or 2.5 percent. Multiply by the run and you get the fall, which is what you actually set with a level and a story pole. A quarter inch per foot over a 40 ft run drops 10 inches, and that drop has to exist between your fixture and your connection or the layout does not work.

Too flat is the obvious failure, but too steep is a real one. If the pipe runs much beyond about 1 in 20, the water outruns the solids, the liquid drains away and the solids are left behind to build a blockage. Plumbers call it "the water leaves the paper behind", and it is why codes set a maximum slope as well as a minimum. Larger pipe wants a gentler grade, not a steeper one: 1/8 in per foot is standard for 4 in and larger building drains.

Measure the run horizontally, not along the pipe. Over the distances a drain covers the difference is small, but the habit matters the moment you start working with real grades on a site.