Before the arrival of prefabricated wallboard or "drywall" in the 1940s and '50s, plaster was the material of choice for interior walls and ceilings in American homes. Portland cement plaster was painstakingly hand-troweled onto wood slats (lathing) nailed to the wall and ceiling framing. The excess plaster that dries behind and around the lath is referred to as the "key." The key prevents the installed plaster from pulling away from the lath. Ample fastening prevents the lath from pulling away from the framing.

Plaster is a fabulous interior finish. It is strong, has a uniform finish and can last forever. So, why isn't it used as it once was? The answer is "money." Interior lath and plaster are expensive. Drywall is cheaper to produce and install.

As have most things, plaster has its drawbacks, price aside. Anyone with plaster can tell you how it cracks. And just try hanging a picture in plaster. Also, plaster finish is far more difficult to patch than drywall is.

One of the most common problems associated with plaster is a sagging ceiling. This occurs when the key breaks and positive connection to the lath is lost. However, a sagging ceiling isn't always caused by a broken key. Often it is caused when someone missteps in the attic—setting foot on the lath rather than on ceiling joist. Such was the case in the holiday movie, "National Lampoon's Christmas Vacation." A bumbling Chevy Chase went crashing through the ceiling onto a bunk bed below in that classic film.

Years ago a caller to our nationally syndicated radio program made us aware of a little-known plaster repair device called a plaster washer—a round metal disk about the size of a quarter (about an inch in diameter). It is used to hold up a sagging ceiling. The disk has a countersunk hole in the center through which a drywall screw is inserted. The disk also has many smaller holes scattered throughout its body into which spackle or drywall joint compound attaches itself to conceal the repair.

You can take the sag out of your plaster ceiling using these little gadgets. First, using an old chisel or putty knife, remove any loose or crumbling plaster in the area that is to be repaired. Slightly undercut the edges of the existing plaster to create a solid bond with the new plaster. Use a vacuum with an upholstery attachment and an old paint brush to remove dust and surface debris.

Next, insert a galvanized drywall screw (1-5/8 inches to 2 inches) through the center hole of the plaster washer, and place the screw head into a number-2 bit on a screw gun. Drive the screw through the plaster and into the wood lath a couple inches back from the edge of the hole or crack that is to be repaired. Tighten down the drywall screw just enough to pull the sagging plaster up against the lath, and flatten out the convex-shaped washer. If the plaster does not pull up and-or the screw does not tighten up, you have likely missed the lath and should back out the screw and move it slightly in one direction or the other.

Keep in mind that there is a slight space between each strip of lath. Install several plaster washers around the area to be repaired. If there is evidence that the lath is not securely fastened to the framing, use longer drywall screws in combination with the plaster washer and drive the screws through the plaster and lath and into the framing.

Once the plaster has been resecured to the lath, using a 6-inch taping knife, apply a plaster patching compound to fill in the hole or crack. Before applying the patch material, spray the lath and existing plaster with water. This will prevent moisture from being sucked out of the new patch material, which could result in cracking and a poor bond. Small cracks can be filled with a paintable caulk. Allow the patch to dry overnight and then sand it level, using 100-grit sandpaper and a sanding block.

Conceal the plaster patch and the plaster washers by covering them with a self-adhesive fiberglass mesh joint tape. Use the 6-inch taping knife to apply a thin coat of drywall joint compound over the patch area. Feather the joint compound at the edges of the patch. Allow the joint compound to dry overnight and apply a second coat overlapping the first coat by a couple of inches in all directions. Allow the material to again dry overnight, and lightly sand using 100-grit sandpaper or sanding mesh along with a sanding pad or block. Additional coats of joint compound and sanding might be required to achieve the desired finish.

After the final sanding, use a damp rag and-or a tack cloth to clean the patch area and prepare it for painting. Prime the patch with a high-quality interior acrylic latex primer and when it's dry apply one or two coats of paint to match the existing finish.

Since you will be working overhead—and stirring up quite a mess—be sure to wear safety glasses, a dust mask and a ball cap. Use drop cloths to cover furniture or other valued items that could be dirtied or damaged.

Since plaster washers are not easy to find, we offer the following sources that will ship: Charles Street Supply in Boston (617) 367-9046; Wm. A. Kilian Hardware Co.; and Modern Way Lumber.