Gradient Drainage for Floor Drains

Drains and Sewers

• In both common parlance and technical terminology, drains and sewers are different. Drains are the property of building owners, and typically they run under private land. Sewers are the property of civic entities such as cities, and typically they run under publicly owned land. Although drains are the property and the responsibility of private individuals and companies, it is probable that the local municipal authority mandates minimum and maximum gradients.

Drainage Gradient

• Pipe gradients, including drainpipe gradients, are arrived at by dividing the length of the run of pipe by the fall. The fall is the vertical distance between the inlet and the outlet. The ratio usually is expressed with the numeral “1” defining the fall and the other number representing the length. For example, a drainage pipe that falls 1 foot every 100 feet would have a gradient of 1-to-100.

  The ratio that a drainage pipe must fall is determined by the volume of waste water it can carry. Pipes with a large bore, or diameter, require less fall to transition the same amount of water as smaller pipes: A 4-inch diameter pipe would drain well at a fall ratio of 1-to-60, whereas a 1 1/4-inch diameter pipe transitioning the same volume would need a fall ratio of 1-to-20.

Interior Drains

• Most prefabricated shower stall floor pans have gradient falls built in by their manufacturer. So installers have to apply only the finishing surfaces and attach the drains. For other shower stall floors, however, using a gradient template is helpful. One company manufactures gradient wedges, or tapers, for use under floors that drain into its equipment; the wedges increase elevation about 3/8 inch every 3 feet. The minimum acceptable fall for 4-inch pipes carrying water from items such as appliances, sinks and toilets is considered 1-to-40 by some companies, but a fall with a higher ratio, such as 1-to-110, may be necessary.

Exterior Drains

• Exterior waste-water removal is referred to in two ways: surface drainage and subsurface drainage. Disposing of water from an open area across the top of that area is called surface drainage. Removing water through underground pipes and channels is called subsurface drainage. Exterior drainage is presumed to carry only runoff from precipitation and deliberate watering, with no solids involved. For this reason, the diameter of exterior drainage pipes tends to be smaller than the diameter of pipes that dispose of waste water from buildings, which can include sewage. Drainage gradients tend to be greater for small pipes.

Additional Considerations

• Drainpipe diameter must never increase in the direction of flow. Drains must be laid at an even gradient, without humps or troughs. Because a city, county or state may mandate drainage pipe diameters and ratios of fall, always contact your area's building code enforcement office before starting a project.