The maximum allowable slope for a combination drain and vent pipe is?

The maximum allowable slope for a combination drain and vent pipe is 1:12. This means that for every 12 units of horizontal distance, the pipe can slope down by a maximum of 1 unit. This specific slope ensures that the wastewater flows adequately without causing stagnation while preventing the potential for venting issues.

A steeper slope, such as 1:4, would lead to rapid flow that could cause the solids to separate from the liquids, which might hinder proper drainage and create blockages. Conversely, a slope of 1:10 or 1:6, while potentially allowing for drainage, exceeds the standard and can lead to similar issues with separation or inadequate venting. Therefore, the 1:12 ratio strikes an optimal balance between efficient drainage and maintaining proper venting functionality within the plumbing system, making it the correct choice.