An anti-void is a floor section specifically designed and installed to close an existing void — an intentional opening — between two floors in a building.
Voids are common in buildings that feature atriums, stairwells, elevator shafts or mezzanine cut-outs. When the use of a building changes and the void is no longer needed, an anti-void fills the gap to create a continuous floor surface.
Why are anti-voids needed?
Building renovations frequently alter the internal layout. A void that once served as a light well, an open staircase or a service shaft may become an obstacle when the floor plan is reorganised. Rather than demolishing and rebuilding entire floors, an anti-void provides a targeted solution: it closes the opening while meeting the structural, fire-safety and acoustic requirements of the surrounding floor.
Common scenarios include:
- Office refurbishments — closing an atrium to gain additional floor area.
- Change of use — converting a commercial space into residential units where open voids are not permitted.
- Removal of escalators or lifts — filling the shaft opening after the equipment has been taken out.
Technical considerations
An anti-void must match the performance of the existing floor in several respects:
- Load-bearing capacity — it must carry the same imposed loads as the adjacent floor, whether for office, retail or industrial use.
- Fire resistance — the anti-void must achieve the same fire-resistance rating (for example 60 or 120 minutes) as the surrounding floor slab.
- Sound insulation — in residential or mixed-use buildings, the anti-void must meet acoustic separation requirements between dwellings.
- Deflection and vibration — excessive movement can cause discomfort and damage to finishes.
Construction methods
Anti-voids can be built using in-situ reinforced concrete, precast concrete planks, steel beams with composite decking, or lightweight steel-and-timber platforms, depending on span, load and access conditions. The edges are typically connected to the existing slab with anchors or cast-in connections.
Related terms
- Floor void
- Structural opening
- Fire compartmentation
- Composite floor slab