Short definition
Rigid copper tubing is hard-drawn copper sold in straight 10- and 20-foot sticks. It’s approved for residential potable water in all major US codes and comes in three wall thicknesses: Type K (thick, green lettering, buried service), Type L (medium, blue lettering, general supply), and Type M (thin, red lettering, residential indoor only). Joined primarily by sweat (solder) or press fittings.
What it is
The lettering color stamped along each pipe length tells you what you’ve got. Type M is the cheapest and thinnest, Type L is the residential workhorse, Type K is overspecified for above-ground (and required underground). All three are the same alloy and the same outside diameter at any given nominal size — just different wall thicknesses.
Why it matters to a homeowner
In Washington’s pre-2000 housing stock, copper was the dominant supply material before PEX took over the new-construction market. The grade you have affects expected service life, especially in soft acidic Cedar/Tolt water:
- Type M in 1960s–80s Seattle homes is the typical pinhole-leak target. Multiple pinholes within a year usually mean the system is approaching end of life; whole-house repipe to PEX is the standard remediation.
- Type L lasts longer than Type M on aggressive water; 30 to 60 years is typical, sometimes shorter on soft-acidic systems.
- Type K is overkill for most above-ground residential but specified for buried service because of its thicker wall.
When a real-estate inspection identifies copper supply, the question is which type — the lettering on the pipe gives the answer.
Common variants and what rigid copper is not
- Rigid (hard-drawn) vs. flexible (soft / annealed). Rigid is straight stick, less bendable; flexible is coiled, hand-bendable.
- Type K (green, thick, buried), Type L (blue, medium, general), Type M (red, thin, indoor only).
Common failure modes
- Pinhole leaks in old copper, particularly Type M in soft-acidic Cedar/Tolt water after 20 to 40 years.
- Erosion-corrosion at high-velocity sections (long, tight elbow pairs at high flow) — reaches Type M faster than Type L.
- Cold sweat joints — drips on a new install.
- External corrosion at points of contact with concrete, treated lumber, or dissimilar metals.