Floors in Kettering are another place where the grade of house decides the sensible answer.
In the larger Victorian houses the original boards are usually good quality softwood over generous joists, and a great many are still there under whatever was laid on top later. Lifting, cleaning and relaying them frequently produces a better floor than anything that could be bought to replace them and costs less than a good engineered board, so we look before we specify.
In the artisan housing the boards were plainer and the joists lighter, and they have often been cut about by generations of plumbers and electricians. There the honest answer is more often a new floor properly laid over a structure that has been checked and strengthened where needed.
Sub-floor ventilation is the routine check on either. The air bricks are doing a job and the number rendered over, planted in front of or built up against by a later extension is remarkable. Block them and the joist ends begin to go, and a beautiful floor over a rotting one is money wasted.
On the later estate housing the floors are solid and the questions become flatness and moisture instead. Our flooring page covers the coverings and the preparation that goes under them.