Plumbing installs pipes and fixtures for water and waste. It brings clean water into a building and takes waste out. It includes supply lines, drain lines, vents, traps, water heaters, and fixtures like sinks, toilets, and showers. Plumbing is one of the most regulated systems in construction. Bad plumbing can contaminate water, cause floods, and spread disease. The plumber is a skilled tradesperson. The work is hidden in walls and ceilings, but it shapes the building.

Plumbing has two main systems. The supply system carries water under pressure from the main to the fixtures. The drain system relies on gravity, so pipes must slope correctly. Vent pipes allow air into the system so water flows smoothly and sewer gases do not enter the building. Traps hold a small amount of water to seal out odors. The pipes must be sized correctly. Too small, and the water pressure drops. Too large, and the waste does not flow. The plumbing code specifies the requirements.

Plumbing materials vary. Copper is traditional and durable. PEX is flexible and easy to install. CPVC is used for hot and cold water. PVC and ABS are used for drains. Cast iron is used for drains in commercial buildings because it is quiet. The choice depends on the cost, the code, and the application. Plumbing is usually installed before the walls are closed. It must be tested for leaks and inspected. A plumbing leak can cause thousands of dollars in damage. The plumber's work is invisible when it is right. It is catastrophic when it is wrong.