Short definition
Iron pipe is umbrella vocabulary covering several distinct ferrous (iron-based) pipe materials: cast iron (CI) for drainage, galvanized iron (GI) for legacy water supply, black iron (BI) for gas, and ductile iron (DI) for utility mains. They differ by carbon content, surface coating, and intended service. If a report or older tradesperson just says “iron pipe,” ask which one.
What it is
The four materials homeowners actually encounter:
- Galvanized iron pipe — steel dipped in molten zinc. Pre-1970s residential water supply standard. Silver-gray finish that often weeps rust at threaded joints.
- Cast iron pipe — iron alloyed with 3 to 4 percent carbon, used for drains and waste. Heavy, dark gray, brittle.
- Black iron pipe — uncoated steel, dark mill-scale finish. Used for natural gas inside the building.
- Ductile iron pipe — magnesium added to cast iron for fracture resistance. Modern utility water mains.
In Washington homes, “iron pipe” most often means either galvanized supply or cast iron drain depending on what the speaker is looking at. Older-generation tradespeople use the umbrella term frequently; younger plumbers tend toward the specific names.
Why it matters to a homeowner
If your inspection report or contractor’s quote uses “iron pipe” without specifying, ask. The implications are very different:
- Galvanized supply — repipe candidate; tuberculation is the failure mode.
- Cast iron drain — long service life, scope-and-evaluate; section replacement is common.
- Black iron gas — code-compliant for gas; only modify with a licensed gas fitter.
- Ductile iron utility main — outside your boundary; not your concern unless the utility’s main breaks.