A DWV piping system relies on gravity at a fall of how many inches per foot?

• A. 1/16 to 1/8
• B. 1/8 to 1/2
• C. 1/2 to 1
• D. 1 to 2

Answer: (B)

DWV systems rely on gravity to move waste through the pipes, so the horizontal runs must have a small but consistent fall to keep the liquid and solids flowing. The recommended fall range is 1/8 inch per foot to 1/2 inch per foot. This keeps the slope gentle enough to prevent sluggish drainage while avoiding a flow that’s so steep it carries solids ahead too quickly or causes other flow problems. In practice, many residential drains use about 1/4 inch per foot, which sits comfortably inside this range. Falls shallower than 1/8 inch per foot can lead to standing water and poor drainage, while slopes steeper than 1/2 inch per foot can cause excessive velocity and flow issues downstream.