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For horizontal sanitary drainage, the slope ensures wastewater moves under gravity without stagnation. Plumbing codes (e.g., Uniform Plumbing Code and local adaptations) recommend a minimum slope of 2% (2 cm per meter) for pipes with a diameter of 100 mm (4 inches). A slope less than this may result in insufficient velocity, causing solids to settle and eventually leading to clogs. Excessive slope, conversely, can cause liquids to outrun solids, leaving residues. Proper slope is a vital consideration in sanitary plumbing design, as it maintains self-cleansing velocity of around 0.6–0.9 m/s. During installation, plumbers use spirit levels, slope indicators, or laser levels to ensure compliance. Understanding this standard is critical not just for passing the licensure exam, but also for preventing sanitary hazards such as blockages, odors, and pipe failures. The principle also emphasizes how theory translates into hands-on work, as incorrect slope is one of the most common failures in poorly designed drainage systems.