Timber and engineered timber are hygroscopic — they exchange moisture with the surrounding air. As relative humidity rises, boards absorb moisture and expand across the grain. As it falls, they release moisture and contract. Movement along the grain is negligible; movement across the grain is significant and cumulative across a floor.
The magnitude depends on species, construction and lamination method. Solid timber moves the most, engineered constructions less, and multi-ply substrates less again. Vinyl and hybrid systems respond thermally rather than hygroscopically and are managed differently.
Design accommodates movement, not eliminates it. Perimeter gaps, intermediate expansion joints and correct board orientation give the floor room to breathe. Where movement is constrained — locked at the perimeter, pinched at doorways, blocked by heavy fixings — the floor releases the stress by cupping, peaking or joint failure.