A building's plumbing is split into two separate systems: supply plumbing, which delivers clean pressurized water to taps, toilets, and appliances, and drainage (or waste) plumbing, which carries used water and waste away by gravity to a sewer or septic system; the two should never be cross-connected, as this risks contaminating the clean water supply. Common pipe materials include PVC (polyvinyl chloride), widely used for drainage and increasingly for supply lines because it is corrosion-resistant and low-cost, and copper or galvanized steel, traditionally used for supply lines where higher heat or pressure tolerance is needed. Because drainage pipes rely on gravity rather than pressure to move waste, they must be laid with a consistent downward slope, commonly around 1-2% (roughly 1-2 cm of drop per meter of pipe), or waste will pool and block the line instead of flowing away. Getting pipe sizing, material choice, and slope right during installation is far cheaper than fixing a blocked or leaking line after walls and floors are finished.