Subfloor

The structural layer beneath the finished floor covering - typically a concrete slab (ground floor) or a chipboard or plywood deck fixed to joists (upper floor). The subfloor must be level, dry, and structurally sound before any floor covering is laid. A damp subfloor will cause timber coverings to cup or bow; an uneven subfloor will cause tiles to crack. Levelling compound can be applied to smooth minor irregularities.