Short definition

Tuberculation is the buildup of rust nodules (“tubercles”) on the interior wall of galvanized steel pipe over decades. The nodules and the rough interior surface dramatically increase friction loss, halving or worse the effective flow capacity. In Washington, pre-1960 Seattle and Tacoma supply lines are the dominant tuberculated population.

What it is

Galvanized steel pipe rusts from the inside out as the protective zinc coating depletes. The rust doesn’t just stick to the wall — it forms small, hard, hemispherical nodules (“tubercles”) that protrude into the flow path. Calcium and iron deposits build up around the nodules, and the once-smooth pipe interior becomes a rough, restricted surface that water has to fight to flow through.

The flow effect is dramatic. Engineers measure pipe smoothness with the Hazen-Williams C-factor:

  • New galvanized pipe: C ≈ 120.
  • Heavily tuberculated galvanized: C < 60.

Flow capacity at a given pressure is roughly proportional to C. Cutting C in half cuts flow in half. A ¾-inch galvanized supply that started at full capacity can effectively become ½-inch or worse over 50 years — and feel like it.

Symptoms:

  • Whole-house low pressure, progressive over decades, no PRV change.
  • Shower flow halves over time, especially in upstairs fixtures.
  • Slow toilet refill at distant fixtures.
  • Reduced GPM at every fixture despite no recent supply changes.
  • Pinhole-leak risk co-occurs — thinned pipe walls under tubercles eventually fail.
  • Brown water at first draw after vacancy.

Diagnosis:

  1. Identify pipe material. Silver-gray, magnetic (refrigerator magnet sticks), threaded fittings, often shows rust at unions. That’s galvanized.
  2. Pressure test at meter vs. at fixtures. Large drop confirms internal restriction.
  3. Cut a section during repair. Cross-section shows tubercles directly.
  4. Confirm pre-1960 era.

The fix is repipe — galvanized → PEX or copper. Cleaning tubercles internally is impractical for residential supply.

Why it matters to a homeowner

Tuberculation is the slow plumbing failure that homeowners often don’t notice until they see the difference at a friend’s house or after a remodel. By the time the pressure drop is obvious, the pipe interior is severely restricted and structural failures (pinholes, fitting leaks) are imminent or already happening.

When a plumber recommends repipe, they’re acknowledging that fixing one pinhole on a tuberculated run buys weeks-to-months, not years. A typical Seattle whole-house repipe (galvanized → PEX) runs $5,000-$15,000+ depending on access and finish work. Partial repipes — kitchen or bathroom only — create galvanic corrosion zones at copper-to-galvanized transitions that fail faster than either metal alone.

Common failure modes

  • Whole-house low pressure (chronic, progressive).
  • Shower flow halving and toilet refill slowing.
  • Pinhole-leak risk co-occurs from thinned walls under tubercles.
  • Brown water at first draw after vacancy.
  • Repipe is the proper fix. Cleaning tubercles internally is impractical.

Common variants

  • Tuberculation (rust nodules in galvanized) vs. limescale (calcium-carbonate scale). Different chemistry; can occur in any pipe.
  • Tuberculation (interior, flow-loss) vs. external rust (exterior, structural).
  • Tuberculation vs. pitting. Tuberculation is buildup; pitting is a localized hole.

Washington note

Pre-1960 Seattle, Tacoma, Everett, and Bellingham housing stock is the dominant WA tuberculated population. Modern construction (post-1965) almost universally used copper or, more recently, PEX, so newer subdivisions don’t share this problem.

For pre-1960 WA homeowners with progressive whole-house pressure loss:

  • A magnet on a basement supply line tells you in seconds if it’s galvanized.
  • The pressure loss is not because anything changed — it’s because tubercles have been growing for 50+ years.
  • Whole-house repipe is durable; partial repipes shift the failure to the copper-galvanized transition.
  • Pre-purchase scope by a contractor often prices both options before closing.

Pre-1960 stock also overlaps with pre-1986 lead solder era. A first-draw lead test in any pre-1986 home with kids is cheap and answers the most important question.