Wednesday, Apr. 13, 2005

 

Different drywall suits varying levels of water exposure

By TIM CARTER
Tribune Media Services

Q. 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? — J.G., Detroit

A. 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 and have a choice of which material to use, you can produce a finished result that should last for years with little or no damage if an occasional splash or drip occurs, or even if a great deal 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 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 becomes very mushy.

Thus, I don’t like to use regular drywall in areas where I think water might come into contact with the wall surface on a regular or even somewhat regular basis.

Some people think several coats of high-quality paint will protect drywall from moisture, but this is not always the best strategy. If water gets behind the paint where the paint stops and a sink top or cabinet edge begins, you have an Achilles’ heel; damage to the drywall may start if water finds this entry point from time to time.

Water-resistant drywall has been around for over 25 years. Many people recognize it from the green-colored paper facing. It is a great product when used exactly as it was designed to be used. For example, I have had great success using this green board drywall behind sinks and toilets and on walls with ceramic-tile backsplashes.

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

In areas of 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 and 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, which can cause the wall panels to 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.