Ensure that the floor surface is even, fill all indentations level and grind or sand down all high spots to level. And where laying over a timber or particle board floor take the trouble to ensure there are no creaks or loose boards.

Timber T&G flooring may be laid over existing T&G or sheet floors (plywood or particleboard). Where the existing floor is structurally sound, either overlay flooring (generally 10 mm to 14 mm thick) or structural flooring (generally 19 mm to 21 mm thick) can be laid. Floors may be fixed into the joists or with shorter fixings at reduced centres into the existing floor only. In instances where there is doubt over the structural adequacy of the existing floor, defective boards or sheets should be replaced to make the existing floor structurally sound, or structural flooring fixed through to the joists can be used. To provide a level surface, top (face) nails in existing flooring should be re-punched and the existing floor rough sanded. Adhesives require a clean, structurally sound floor that is free from moisture, loose particles and contaminants.

It is also necessary to check that the existing floor moisture content is appropriate to accept the new floor. The cause of any excess moisture (wetting during construction, leaks, inadequate sub-floor ventilation, etc) needs to be addressed prior to installation. Moisture meters are unpredictable in sheet flooring and this may necessitate oven dry testing. Prior to laying, the existing floor should be of similar moisture content (within a few per cent) to the new floor.