Either solid wood flooring or engineered wood flooring can be fitted onto a plywood subfloor. Before installing either type of hardwood floor, ensure that the existing wooden floor boards are flat, level and dry.
Ensuring the plywood is flat, level and dry
Check the level and flatness of the plywood subfloor by using a straight edge or spirit level. Underlay should not be used to level a subfloor. Ensure the plywood subfloor is dry by checking the moisture content using a moisture meter. The moisture content should be 16% MC (Moisture Content) or below before installing any hardwood flooring.
Different ways of installing a hardwood floor onto plywood
With a plywood subfloor there are different ways that a hardwood floor can be fitted, depending on the type of hardwood floor and personal requirements. The main installation methods are:
Secret nailing
Secret nailing can be used with either solid wood flooring or engineered wood flooring. If using engineered wood flooring the planks should be at least 18mm thick. For both types of hardwood flooring, a toothed nail should be used (approx. 50mm in length) to secure the flooring planks directly onto the plywood. A brad nailer or a finish nailer should not be used, as they have smooth nails which could easily come loose. The nails should be spread approximately 300mm apart to offer maximum stability.
Secret screwing
Either solid wood flooring or engineered wood flooring can be secret screwed directly onto the plywood subfloor. Engineered wood flooring must be at least 18mm thick to be secret screwed down. Screws designed especially for securing down flooring should be used (such as Tongue-Tites) and they should be placed approximately 300mm apart to provide firmness and strength. 
Gluing
Both solid wood flooring and engineered wood flooring can be glued directly onto the plywood subfloor using a flexible flooring adhesive.
Floating over an underlay
Floating a hardwood floor over an underlay is only suitable if engineered wood flooring is being installed. The planks of flooring can be glued or clicked together. When secret nailing or secret screwing any hardwood flooring down, please ensure that there are no service pipes (gas, electricity or water) underneath.
