A cassette floor, also known as a coffer floor or waffle slab, is a reinforced concrete floor system in which the underside features a grid pattern of intersecting ribs. The recesses between the ribs, called coffers or cassettes, reduce the dead weight of the slab while maintaining high load-bearing capacity over large spans.
Construction and Design
A cassette floor consists of a thin top slab supported by ribs running in two directions. These ribs form a waffle-like grid that efficiently distributes loads to the supporting columns or walls. The system is typically constructed in reinforced concrete or post-tensioned concrete.
During construction, plastic or polystyrene dome moulds are placed on the formwork to create the characteristic recesses. The moulds may be removed after curing or left in place as permanent insulation. Standard rib widths range from 100 to 150 mm, with centre-to-centre spacing of 600 to 900 mm. Total slab depths typically vary between 250 and 600 mm, depending on the span and loading requirements.
Precast cassette floor elements are also available, manufactured off-site and assembled on location to reduce construction time.
Application
Cassette floors are widely used where large column-free spans are required:
- Office buildings and public facilities
- Parking structures
- Retail and commercial spaces
- Industrial halls
Their favourable strength-to-weight ratio makes them ideal for structures where foundation capacity is limited. The coffered pattern on the underside is often left exposed as an architectural feature, lending visual depth and rhythm to ceilings.
Related terms
- Ribbed slab
- Post-tensioned concrete floor
- Hollow-core slab
Want to learn more about construction terms? Visit our knowledge base at fredsdiyplans.com.