A joist is a horizontal structural beam that supports floors or ceilings in buildings. Joists run parallel to each other, typically spaced 30 to 40 centimetres apart, transferring the weight of floors to the building's walls or foundation. They can be made from wood, engineered lumber, steel, or concrete depending on the building type and load requirements. Building codes specify joist sizes, spacing, and installation methods to ensure structural safety. Building inspectors check joist installation during construction to verify compliance with codes. Understanding basic structural elements like joists helps homeowners communicate with contractors and comprehend building permit requirements.