It is vital before laying any wood flooring be it solid or engineered that the sub floor is in a sound condition.
Solid wood floors can be nailed into existing wood subfloors such as floorboards or plywood but the original floorboards must be in good condition and level.
Alternatively and more preferable you can nail down a solid wood floor over a concrete subfloor with the use of battens. A damp proof membrane sheet must first be laid over the concrete subfloor before the battens are laid.
There are various ways of fitting an engineered floor. Produced with a tongue and groove on four sides it simply slots together. It can be secret nailed to a wood sub-floor or glued down if laying onto a concrete base.
NAILING (SECRET NAILING)
A more traditional method of laying a floor. Nails are put through the tongue of the boards and then covered when the groove is butted up to the secured board. Nailed floors can be laid over battens, directly on to the timber floor joists or straight onto a concrete sub floor.
Floating is a term describing a specific way of fitting a floor. It is installed over a suitable existing underlay such as concrete, plywood or vinyl which itself is also laid over an appropriate sub floor. A floating floor is not fixed down to the sub floor so the only part that is glued is the tongue to the groove. The whole floor relies on the weight of the floor keeping it in position. It is a popular and quick way of laying an engineered floor.