Short definition
Tuberculation is the buildup of nodular rust deposits (“tubercles”) on the inside walls of iron and galvanized-iron supply pipe. Over decades, the zinc coating wears off, the underlying steel oxidizes, and bumpy iron-oxide nodules grow inward — slowly choking the effective inside diameter of the pipe even when the outer wall still looks intact. It’s the defining failure mode of pre-1970 Seattle and Tacoma plumbing.
What it is
Galvanized steel pipe (the dominant residential supply material from roughly 1920 to 1970) is steel coated in zinc to resist corrosion. The zinc is sacrificial — it corrodes first to protect the steel underneath. After 40 to 60 years, the zinc layer is gone in patches, and the exposed steel oxidizes. Iron oxide (rust) grows as bumpy nodules on the inside wall instead of spreading uniformly the way some metals do.
The effect on flow is much larger than the dimensional change suggests. The Hazen-Williams “C” factor — a measure of the pipe’s smoothness and effective diameter — drops from around 140 in new copper or PVC to 64–83 in 40-year-old cast iron. That’s roughly a 50% drop in flow capacity at the same pressure. A pre-1970 galvanized house with otherwise normal mains pressure can deliver fixture flows that feel weak at one fixture and worse at two.
Tuberculation also sheds particles. The brown or red water you sometimes see on first draw after a long sit is rust scale falling into the water column.
Why it matters to a homeowner
If you live in a Seattle, Tacoma, or older Spokane neighborhood with original galvanized supply piping, tuberculation explains nearly every progressive low-pressure complaint:
- Whole-house pressure that’s been getting worse for years. Not a sudden drop, just slow strangulation.
- Worst pressure at upper-floor fixtures. The highest fixtures see the friction loss accumulated through the longest path.
- Hot side worse than cold side. Higher water temperature accelerates corrosion, so hot lines often tubercolate faster.
- Brown first-draw water. Rust shedding from the internal nodules.
- A cut-out section of the pipe shows knobby internal scale. This is the visual confirmation when a plumber opens a wall.
The fix is a repipe — replacement of the galvanized supply lines with PEX (more common, $4,000–$12,000 for a typical WA home) or copper (more expensive, $8,000–$20,000+). Pipe relining (cured-in-place epoxy) is occasionally used on water mains but rarely on residential supply lines because of the small diameters. Partial repipes (replace the worst sections only) buy time but don’t solve the underlying problem.
Common variants and what tuberculation isn’t
- Tuberculation vs. mineral scale. Tuberculation is corrosion-based — rust nodules. Mineral scale (limescale) is calcium and magnesium precipitation. Both reduce flow but show up in different homes: tuberculation in pre-1970 galvanized pipe; scale in hard-water areas like Spokane.
- Tuberculation vs. internal corrosion (smooth wall thinning). Tuberculation has knobby growth that adds material inward; internal corrosion thins the wall outward without significant bulk inward.
Washington note
Pre-1970 housing stock in Seattle, Tacoma, and parts of Bellevue, Renton, Everett, and Spokane was typically built with galvanized steel supply pipe. Most of those homes that haven’t been repiped now have substantial tuberculation. The classic WA real-estate pattern: a 1925 Seattle Craftsman with original galvanized supply, low pressure that the owners describe as “always like that,” and a repipe quote in the inspection report.
The remediation is well-established locally — most WA plumbers offer flat-rate or square-foot pricing for whole-house PEX repipes, and the work doesn’t usually require a full permit if it’s like-for-like in existing walls. A pre-purchase inspection on any pre-1970 home that hasn’t been repiped should treat the supply piping as a budget line item, not a question.