Which measurement is the desired fall for a 3-inch house sewer?

The desired fall for a 3-inch house sewer is typically 1/4 inch per foot. This measurement is considered optimal because it provides an effective slope that ensures adequate drainage while minimizing the risk of clogging.

A slope of 1/4 inch per foot allows wastewater to flow rapidly enough to transport solids through the sewer line effectively. If the fall is too shallow, such as 1/8 inch per foot, there may not be enough movement to carry debris and solids, which can lead to blockages. Conversely, a steeper slope, such as 1/2 inch per foot, can increase the speed of the water flow excessively, potentially causing solids to be carried too quickly and not allowing enough time for sedimentation, which can also lead to issues.

Thus, the 1/4 inch per foot fall strikes a balance between efficient drainage and preventing clogs, making it the recommended standard for residential sewage systems.