Why cast iron fails, and why it fails quietly.
Cast iron drainage pipe corrodes from the inside out at the surface exposed to sanitary discharge. As corrosion progresses, scale and tuberculation roughen the interior, choke the bore, and trap the grease and solids the system carries.
Because the piping is concealed within walls, floors, and ceilings across occupied units, a breach can discharge into the building assembly long before it appears at a finished surface.