void

A void is a cavity or gap deliberately left empty during construction, sitting between solid structural elements like floors, walls, or between a building envelope and its outer skin. Unlike accidental voids from poor workmanship, these are planned features that serve specific functions: routing services, accommodating movement, reducing weight, or managing thermal performance. Voids range from small gaps a few centimeters wide to massive multi-story cavities.

The most common type is the floor void, a horizontal gap between the structural slab and the finished floor surface, typically 150 mm to 300 mm deep. This space holds electrical conduit, water pipes, gas lines, and ventilation ducts, making them accessible without tearing up the finished floor. Similarly, cavity walls use a void between an outer brick or stone leaf and an inner concrete block or timber frame, usually 50 mm to 100 mm, to provide thermal insulation and a drainage path for water penetration. The gap between a building's structural frame and its curtain wall or cladding acts as a service void for seismic movement and thermal expansion.

Voids must be designed and detailed carefully because they create hidden spaces where problems accumulate unseen. Water can pool in horizontal voids if falls or drainage are inadequate. Dead air pockets in poorly sealed voids reduce insulation effectiveness. Service congestion can block maintenance access. Vertical voids also present fire risk if they act as chimneys for flame and smoke spread unless properly fire-stopped at each floor.

The term sometimes extends to larger intentional gaps in high-rise structures, such as expansion joints or seismic separation voids, which allow parts of a building to move independently without structural collision. In precast concrete work, voids are sometimes cast into the members themselves, either to reduce weight or to form conduit routes. These must be protected during transport and carefully inspected for concrete defects before erection.