Absolute Roughness of Cast Iron Pipe

New uncoated cast iron is quoted at 0.26 mm, asphalt-dipped cast iron at about 0.12 mm, and cement-mortar-lined ductile iron — the modern standard for water mains — behaves closer to 0.10 mm when new. The lining is the point: it isolates the iron from the water and stops the tuberculation that made unlined cast iron mains notorious.

Aged unlined cast iron is the real lesson in this shard. Decades of tuberculation build nodules that both roughen the wall and constrict the bore; measured effective roughness of 1–3 mm is common, and hydrant-flow tests on century-old mains routinely back-calculate Hazen-Williams C values of 60–80 against the 130 the pipe left the foundry with. Since head loss goes as roughly D⁻⁵, the diameter loss usually hurts more than the roughness. Never size or model an existing distribution system on new-pipe roughness — get a field C-factor test.