When you make the choice of laying a new hardwood floor you need a tradesman or professional floor layer to examine your sub floor and determine what the new floor should be layed over.
Concrete Slab– The moisture content needs to be below 5.5% or the in slab RH below 85% and the level needs to be no more than 3mm variance over a 1.5m area using a straight edge. We recommend that slabs be sealed with an apoxy vapour barrier to prevent the timber from absorbing the moisture.
Installation over battens- We recommend that slabs be sealed with a polythene film (roll this out and finish it up the walls about 8cm) or paint on an apoxy vapour barrier to prevent the timber from absorbing the moisture. After the hardwood batten is fixed over a moisture barrier the flooring is then glued and hand nailed through the top of the board to the battens.
Plywood- Plywood remains the best substrate. Plywood is strong enough to provide the solid base you need, but will expand and contract along with the hardwood flooring, making it less likely that gaps will develop. Plywood also holds nails and flooring staples better than just about any other material. Whether it is 12mm or 15mm ply it needs to be staggered, flat, clean & dry with 6mm gaps in between & 10mm gaps to the walls.
Over existing floors-Ensure that the existing subfloor is dry. Do not install timber flooring over wet or damp surfaces. Screw or nail down any loose spots or places that squeak. Sand and level any high joints. The subfloor must be clean and sound; remove any dirt or debris that may create an uneven surface. Sweep and vacuum to remove all dust.