Standard Clear Greenhouse Film, 6mil - 6 Mil Polyethylene Film . I have a home-made greenhouse with a hoop roof, 20'x32'. The primary covering is your "standard clear 6 mil" plastic, stretched over the hoops. Are any of your shade coverings NOT suitable to place directly on top of the existing plastic roof? A. All coverings are suitable for this, but make sure that the cloth is stretched tight..