Grade

• Wood floors can be either solid hardwood or engineered wood. In general, it is not a good idea to install hardwood below grade, or at the basement level. Solid hardwoods, such as oak and maple, react to changes in moisture and humidity levels by expanding and contracting. Too much moisture causes wood planks to warp and bend. Additionally, installing hardwood requires nailing it to a subfloor. Engineered wood has a hard or softwood inner core with multiple, plywood-type layers on top. This type of wood is more resistant to high moisture and humidity levels, and its installation can involve either gluing it directly to concrete or using a floating-type installation.
  
  

Moisture

• New concrete must have enough time to cure, or set, before installing a hardwood floor. Curing is necessary to prevent concrete from drying too quickly, making it weaker and less resistant to cracks. Depending on your area, delay installing a hardwood floor over new concrete for at least six to eight weeks.
  
  Wood floor manufacturers have recommendations for maximum concrete moisture levels that you should not exceed. Always test concrete for moisture content, even with an older concrete floor, before installing hardwood. One method you can use to accomplish this task involves a pinless moisture meter. Pinless moisture meters use a radio signal to determine the moisture content in the top 1 inch of concrete. Another method involves taping 16-inch squares of plastic wrap at various spots on the concrete floor then waiting 24 to 48 hours to see if condensation appears.
  
  Even when concrete moisture levels are within an acceptable range, it is important to install a vapor barrier over the concrete prior to installing the hardwood floor. This will prevent normal moisture levels from eventually damaging the floor. A vapor barrier can be a brush-on type or a layer of polyethylene sheeting.
  
  Additional steps you can take to help control moisture levels include running a dehumidifier and opening windows (when possible) to allow for better air circulation.
  
  

Surface Flatness

• The concrete surface must be flat or within 3/16 inch in a 10-foot radius. To test your floor, use a 10-foot straight edge and lay it out at various spots on the floor. The best way to correct and level an uneven concrete floor is with a self-leveling compound. Most compounds mix with water to create a mortar-type paste that you apply using a trowel.