Plaster is both a blessing and a curse. It can be a blessing because in many cases it represents a level of craftsmanship that is either rare or nonexistent these days. However, plaster is often a curse because it tends to react unfavorably to stress and age, and the supply of people who can repair it, despite efforts of the building trades to reverse the trend, has dwindled.

The shortage is greatest in areas where new housing is dominant, since drywall is the plaster substitute for today's builders. Yet, there are many houses with plaster walls, and real estate agents must have some knowledge of them when explaining all those cracks to their buyers.

While some owners of new houses like the look of plaster, most of the work done these days involves restoration.

Some of the plaster is fancy stuff. Instead of wood, plaster was used to make cornices in older houses.

Plaster cornices are also known as "run plaster," because a template of the profile is pushed or "run" across a glob of plaster to create them. A drawing of a cornice to be reproduced is typically made on site, and the profile is transferred to a piece of sheet metal to make the template. The template is mounted to a wooden tool called a "horse," which is designed so that an edge hangs over the side of an aluminum channel or a marble table to give the template stability for the run.

A batch of plaster is poured on strips of starched cheesecloth that strengthen the cornice. The template is run along the plaster, and the cornice begins to form. Several batches of plaster and several runs later, the cornice is completed and left to dry, usually for two or three days.

Often, smaller runs of cornice, called "sinkage" ornaments, are attached or "sunk" with plaster into the larger cornices to create a more ornate look. Cornices, some of which weigh 50 pounds or more, are attached to a wall with adhesive and fasteners resembling drywall screws. What can make installation tough is that little in an old house is square, so cutting and fitting a piece of cornice can be tricky.

The primary problem is failing plaster, usually caused by problems with the material underneath. In one place the plaster can be on top of brick, and on wood lath adjacent to the brick. Masonry expands and contracts differently than wood, and as each shifts, cracks appear in the plaster. Plaster was pushed between the pieces of wood lath for added strength, and to help the top coats adhere better. That plaster, too, deteriorates over time, especially when moisture finds its way into walls and ceilings.

Nails holding the lath to the framing loosen over time, and the weight of the plaster on the weakened lath causes sagging. You can use metal plaster washers to reinforce plaster, unless it is so bad it has to be replaced. The washers, available for a few cents each at many hardware stores, are about as big as a man's thumbnail, have a hole in the center, and are perforated on the edges to hold a coat of plaster or drywall compound that also can be used.

Hold the washer to plaster and carefully drive the screw through the hole and into the ceiling or wall. If you are lucky, when it catches the loose lath, it will also make it into the framing and re-support everything.

For new plaster work, plasters use metal lath, which comes in sheets instead of strips and is nailed to the framing.

The traditional first coat also is called a "brown" coat. It is a coarse, perlited gypsum also known as Structolite. This is applied underneath the finish coat, adding Portland cement to the mix to increase hardness and making certain the coat, actually several coats, are absolutely level.

Everything on top depends on the strength and surface condition and how well the brown coat can ride out expansion and contraction of it and every other material.

The finish coat contains bonding agents that act as an adhesive.

Then there are hairline cracks. Some people take joint compound and cover them, which does not work in the long term. Watch a crack to see if it gets worse, which may be evidence of a serious structural problem. If it does not, then open up the crack, clean it out, dampen it, and fill it several times until the final cured layer is even with the surface. Cover the filled crack with a piece of self-adhering fiberglass tape cut to fit, and then use drywall compound to finish it.

Published: October 16, 2003

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Al Heavens writes about real estate and home repair and improvement. He is the author of What No One Ever Tells You About Renovating Your Home: Real-Life Advice For Hassle-free, Cost-Effective Remodeling.