Hardwood flooring should be delivered to site kiln dried at between 6 and 8% moisture content. Your subfloor should be within 4% moisture content of the hardwood flooring. Plywood is usually at 12% when purchased from a warehouse that protects its inventory from the elements (Always check that the plywood you are buying is dry). If for example you install the Hardwood Flooring at 8% moisture content over a plywood subfloor that is 14% it is likely that the hardwood will cup (see diagram below).
The diagram shows moisture migrating through plywood and into hardwood. The hardwood swells at the bottom first which changes the shape of the boards (cupping). Moisture can be removed by drying the surrounding environment and allowing the flooring a chance to flatten out. Only when the plywood, hardwood and surrounding spaces test dry should sanding be considered.
It does not take much moisture to make the floor cup significantly, for example: If you hold the prongs of a moisture meter to the skin of your hand the meter will register approximately 28% (Remember, with hardwood at 8% and plywood at 14% the floor will cup).
Sleepers are subject to the same rules. Concrete must also be tested to confirm the slab’s moisture levels are within 4% of the Hardwood Flooring. Concrete dries, under good drying conditions, at an approximate rate of 1″ per month (no radiant heat). If the slab is 4″ , or more, thick the dry time will slow down.
Numbers mentioned are approximate and may vary from site to site. This information is a guideline only.