plancher

Plancher is a French noun with several related meanings centered on the notion of a flat surface used to walk on or to support a structure. In architecture and construction, the plancher denotes the horizontal element that separates levels of a building; it can be the structural floor (slab or joists) or the finished surface laid over it, such as a wooden floor or a concrete deck. The term is also used to describe the visible floor finish inside a room, independent of the underlying structure.

In practice, plans for a plancher may specify materials and systems: plancher en bois (solid wood or engineered wood), plancher en béton (concrete slab), or other coverings such as parquet, tiles, or vinyl over a subfloor. Special types include plancher flottant (floating floor) and plancher chauffant (underfloor heating). Exterior surfaces may be referred to as decking, while interior floors are the subject of insulation, moisture protection, and acoustic considerations.

In mathematics, plancher denotes the floor function, the greatest integer less than or equal to a given number, equivalent to the mathematical floor operation.

Etymology: plancher comes from planche, meaning a flat board, reflecting the wooden-board origin of many traditional floors.