DEAR TIM: My church group is restoring a house and all of the walls and ceilings in the bathroom and laundry area have been stripped down to the wall studs. How much water-resistant drywall should we install? Is it best to use it just behind the plumbing fixtures and appliances or should all of the walls and ceiling be covered? John G., Detroit, MI

DEAR JOHN: There seems to be a significant amount of confusion about drywall, water-resistant drywall and wall board that is 100-percent waterproof. Since you are in the driver's seat at this time and have a choice of what material to use, you can produce a finished result that should last for many years with little or no damage if an occasional splash or drip occur or even if lots of water floods a wall surface.

Drywall that most people are familiar with is made with a gypsum-based core that is very strong when it is dry. Typically the drywall panels have a thick-paper wrapping that protects the gypsum core from impact and abrasion damage. But the paper readily absorbs water and can transmit it to the gypsum inside the panel. When this happens, the panel loses its rigidity and either falls apart or it becomes very mushy.

The drywall used around this utility sink should have been the special water-resistant drywall. Instead, the homeowner has had to initiate damage control by sealing the peeling-paint spots on the wall with clear urethane. PHOTO BY: Tim Carter

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Water-resistant drywall has been around for over 25 years. Many people recognize it from the green-colored paper facing. In fact, it is affectionately called green board by builders and drywall hangers. It is a great product when it is used exactly as it was designed to be used.

For example, I have had great success using this green board drywall behind sinks, toilets and on walls with ceramic tile back splashes. But I have had horrific failures when I have used it behind ceramic tile in tub and shower areas. Water and water vapor can readily pass through ceramic tile grout and cause the paper facing of the drywall to disintegrate. Water-resistant drywall is simply not recommended for areas that are subject to constant moisture.

The water-resistant green board drywall also is sensitive to stud spacing. If you decide to use the green board drywall for a ceiling, the joists must be spaced 12 inches on center for 1/2 inch thick drywall. If you have 16 inch on center spacing, then you need to increase the thickness of the drywall to 5/8 inch for ceilings.

Some drywall manufacturers also state that a vapor retarder should not be used under the water-resistant drywall if the visible face of the drywall is covered by a product such as ceramic tile or other water-vapor impervious finish. I believe they are very afraid that water might get trapped within the drywall and cause damage.

In areas of these rooms that will get heavy concentrations of water, you need to use a waterproof wall material. Remember, the green board drywall is water-resistant, not waterproof. You can purchase interior panels that match the thickness of regular and water-resistant drywall that are made with cement and sand or a combination of cement, sand and other ingredients.

Furthermore, there are other innovative gypsum core products that are 100 percent waterproof. Use the waterproof wall panels behind tile and in any location where you expect water to be regularly splashed onto a wall surface.

Be sure to use hot-dipped galvanized fasteners or even stainless steel nails and screws to fasten the wall panels to the wall studs and ceiling joists. If you use raw steel fasteners, they will rust in time and the wall panels can fall off the walls or ceilings. At the very least, you will find yourself dealing with unsightly rust stains that appear through the painted surfaces should you cut corners and use the wrong fasteners.

The gypsum core of the water-resistant panels is often treated with a silicone chemical or wax-like substance. You can readily see water bead up if you wet the exposed gypsum core. The problem for years was the failure of the green-colored paper, not the actual gypsum inner core.

Because mold is in the forefront of the news, wall and ceiling panel manufacturers are constantly rolling out products that are both water-resistant and even inhibit the growth of mold on the surface of the panel. But as with any new product, always be sure to read the technical literature produced by the manufacturer.

Make sure you use the product as it was designed to be used. Always pay attention to the use of the words: water-resistant and waterproof. Waterproof means liquid water or water vapor will not harm the product. Water-resistant means limited amounts of water will not cause harm.