Plywood: the new installed hardwood floor is as good as its sub-floor is. Guided on this basic rule, we give a special attention to sub-floor witch must be clean, dry and flat before wood flooring goes down. Plywood sheets are mainly used for repairing, strengthening and / or levelling the sub-floors resting on joists. At times all the original pine boards may have to be replaced to achieve a suitable base layer.