Last edited 02 Oct 2019

Floor void

In construction, a floor void is typically a vertical opening or hole that is left in the floor construction either to accommodate services, or allow the installation of a staircase or, as an aesthetic feature.

When services such as pipework, ducts and cables are required to penetrate vertically through a floor construction, a void must be created at the time of construction otherwise costly and disruptive remedial work must be undertaken. To avoid this, architects and engineers usually delineate on their drawings where floor voids are to be located.

With in-situ-poured concrete floors, voids need to be trimmed (with appropriate reinforcement within the floor thickness) around the opening and the necessary formwork installed to create the size and shape of void required. When the concrete is poured and the formwork eventually removed, a void remains.

Small vertical holes can be formed not only by timber boxes, which are usually removed after casting, but also by other materials such as polystyrene and stiff cardboard, which may be left in position.

Sometimes, when no provision for a void has been made, and the concrete has been poured, it is possible (for small service voids), to cut the holes after the concrete is cast. However, this is uneconomical and disruptive.

With precast concrete planks and beams, depending on the size of the void, special factory-made units can incorporate a hole in the beam or plank; alternatively, for larger voids, a system of shorter planks and mild steel supporting straps or trimming beams can be used to ‘trim’ around the edge of the void.

It is generally regarded as good practice not to locate voids too close to columns and beams.

Larger voids within floors can be left for architectural reasons, eg to allow a visual link between two floors, as might be used in a reception area or atrium to open-up the space.

The term floor void can also be used to refer to the horizontal space between a ceiling and the floor above, which may accommodate the floor structure, services and so on. It may also refer to the void between the bottom floor of a building and the ground below, sometimes referred to as a crawl space.

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