Pipe invert level

The pipe invert level is the lowest point of the internal surface of a drainage pipe at a given location, typically measured to ordnance datum or relative to a fixed benchmark. Invert levels are recorded at every manhole and inspection chamber on a drainage drawing and are used to calculate gradients, cover depths and connection geometry. When designing extensions or new connections, engineers check that the proposed branch invert sits above the existing main invert so that flows fall naturally into the larger pipe. Surveys carried out for property purchases or commercial developments usually include invert levels in line with the WRc Manual of Sewer Condition Classification. Accurate invert measurements are essential when planning relining works or replacement of damaged pipe runs.