Drainage gradient, often called the fall, is the slope at which a drain pipe is laid so that wastewater flows under gravity at a self-cleansing velocity. UK Building Regulations Part H1 sets out typical falls for foul drains, generally between 1 in 40 and 1 in 110, depending on pipe diameter and expected discharge. Too steep a gradient can cause water to outrun solids, leaving deposits in the pipe, while too shallow a fall risks settlement and blockage. Surveyors and engineers measure gradient using laser levels or by relating invert depths between chambers. In London, where original Victorian drains often follow historic street levels, gradients can be irregular and contribute to recurring blockages. Correct fall is one of the most important design parameters in any drainage installation.
Drainage gradient (fall)
Slope of a drain pipe expressed as the vertical drop per horizontal metre length.
