Q. How is "green board" different from regular drywall? Where is it supposed to be used, and what are its limitations?

A.Mike Lynn of United States Gypsum Company's Research and Technology Center responds: Green board, so called because of its green facing paper, is a water-resistant gypsum panel intended as a base for the application of ceramic tile and plastic-faced wall panels. Its face and back paper are chemically treated to combat moisture penetration, while the gypsum core utilizes a special moisture-resistant composition. Panels are available in 1/2-inch and 5/8-inch thicknesses, as well as in fire-resistant formulations. You can finish green board just as you do traditional wallboard, but when you use it on ceilings, space the framing no farther apart than 12 inches o.c. for 1/2-inch board or 16 inches o.c. for 5/8-inch board.

Green board is ideal for humid areas such as bathrooms, powder rooms, kitchens, and utility rooms. It is not intended for use in areas subject to frequent water exposure, such as shower and tub surrounds, indoor pools, and gang showers. For those wetter areas, choose a cement board product such as Durock, whose aggregated Portland cement core will not deteriorate when wet. Cement board is an excellent substrate for ceramic tile, slate, and quarry tile and can be used on both walls and floors.