What is the maximum slope for a horizontal pipe in a combination waste vent system?

In a combination waste vent system, the maximum slope for a horizontal pipe is established to ensure effective drainage while preventing the potential for trap siphoning or blockage due to solids settling. A slope of 4 percent is considered optimal because it strikes a balance between minimizing the risk of clogs and allowing sufficient flow of wastewater. This slope equates to a drop of 4 inches per 100 inches of pipe length, facilitating the movement of waste without excessively accelerating the flow, which could cause splashing and disrupt the venting function.

By adhering to a maximum slope of 4 percent, inspectors can help ensure that all elements of the plumbing system work effectively and efficiently, maintaining proper drainage and preventing issues that could arise from steeper slopes, such as inadequate venting or solids accumulation.