Rebuilding a Wall with Three-Coat Plaster

On the Manhattan Brownstone project, Bob, Chris Vila, and architect Brian O’Keefe discovered an intact ceiling cornice that made them rethink plans to completely gut the interior space during demolition.  Instead, they decided to keep the cornice and make it a focal point for the large living area in the center of the floor-through apartment.

Facing the challenge of rebuilding the wall without changing the profile or having drywall that extended beyond the face of the cornice, the team turned to Boro Plastering of Hyde Park, N.Y., to create an original plaster wall that would dovetail with the existing cornice.

The Manhattan remodel is not a strict restoration project, but the craftsmen involved recognize the value of high-quality workmanship as evidenced in the original structure.  A true plaster job is the best way to complete a wall in a period home. There’s no doubt that “three-coat plaster is unmatched in strength and durability,” according to the National Parks Service website. “It resists fire and reduces sound transmission.” It also most closely replicates the original finishes in this Upper West Side Brownstone. 

Three-Coat Plaster

Plaster must be applied in such a way that it adheres to the support surface, bears its weight load without cracking, and exhibits a smooth, hard, finished surface suitable for painting or papering. A quality plaster job begins with the lath on which the plaster will be applied. Traditional lath and plaster jobs used wood lath, but the moisture level in wood makes it expand and contract, which can make plaster crack. Metal lath is a more reliable surface for plastering since it does not retain moisture and provides more “keys” or spaces in its surface for proper adhesion.  Interestingly, metal lath was just coming into popularity in the United States at the time this building was constructed.

The three-coat plaster system begins with two coarse or rough coats. These coats form the base of the wall and are mixed of lime or gypsum, aggregate, fiber, and water. Sand is the most common aggregate used in rough-coat plaster. The lime is typically derived from limestone or ground oyster shells. Many firms now work with gypsum since it eliminates the need for added fiber and has a much quicker set up and drying time. Boro works with a ratio of three-to-one, bags of sand to bags of gypsum, for their rough coats.

The first coat is called the scratch coat and is applied at 3/8 inch thick then scratched or scored with a comb to give it a rough texture. The second rough coat is made of the same mixture and is called the “brown coat.” The brown coat is applied directly to the scratch coat, also at a 3/8-inch thickness, but left unscored. The sand provides a rough texture that gives the light, 1/8-inch finish coat a surface to grip onto.