Detailed Explanation
Nail-down (or staple-down)
The traditional method for solid hardwood over plywood sub-floor. Fasteners drive through the tongue at a precise angle and spacing. Done well, it's quiet, tight, and lasts generations. Done with the wrong fastener length, spacing, or schedule, you get squeaks and movement.
Glue-down
The default for engineered hardwood over concrete and almost always correct over radiant heat. Modern moisture-cured urethane adhesives also act as a vapour retarder. Sub-floor flatness matters enormously here.
Floating
Boards lock to each other but not to the sub-floor, riding on a foam underlayment. Quickest to install. Best suited to specific engineered products with engineered locking systems. Sounds and feels different underfoot than nailed or glued.
What we obsess over before installation day
Sub-floor flatness to manufacturer spec (often 3/16″ over 10′). Moisture readings documented in writing. Correct expansion gap at every vertical surface. Fastener and adhesive selection matched to product and sub-floor. Mock-up of the first three rows for grain, length, and colour variation.