The maximum slope for a horizontal drain and vent combination is _____ every 12 inches.

The maximum slope for a horizontal drain and vent combination being 1/2 inch every 12 inches is established to ensure proper drainage and venting without causing obstruction or insufficient flow. A slope of 1/2 inch per foot is typically adequate to facilitate the movement of wastewater while allowing air to enter the system for ventilation, preventing siphoning effects that can lead to drainage issues.

This specification helps maintain the integrity of the plumbing system by striking a balance between efficient waste removal and preventing the accumulation of solids, which could happen with steeper slopes. Additionally, if the slope were greater than 1/2 inch, it might lead to excessively fast flow, reducing the effectiveness of ventilation, and could create noise and pressure fluctuations within the system. Maintaining this specific slope is crucial for compliance with plumbing codes and for the overall function of drainage and venting systems.