How Water Moves Through a Building
Supply up, waste down: understanding the two systems makes every plumbing decision easier.
The supply side
Mains pressure brings cold water in and either feeds everything directly (combi and unvented systems) or lifts to a loft tank that gravity-feeds the house. Which system you have decides your shower pressure, your fixes and your upgrade options.
The waste side
Everything drains by gravity to the soil stack, and the stack vents above the roof so falling water does not suck the traps dry. Every fitting keeps a water seal between your rooms and the sewer, which is why dry traps announce themselves by smell.
Why this matters to you
Half of plumbing misdiagnosis is treating a supply symptom with a waste fix or vice versa. Knowing which side of the wall the problem lives on is most of the way to the right trade, the right part and the right price.