Ornamental plaster ceilings are a prominent feature of many historic interior spaces, but they can present unique maintenance and repair challenges. In a recent project at a historic theater, deteriorated gypsum material was observed to filter down from the ceiling into the auditorium during performances, described on-site as “snowing.” After inspection, it was found that deteriorated paint and powdery gypsum plaster can detach from the substrate and pass through cracks and other voids. Determining the source of this deterioration raised questions about how to repair and preserve the ornamental ceiling.

In the subject building, a historic theater located in a moderate climate in the United States, the coffered ceiling is composed of individual castings of fiber-reinforced gypsum plaster. The molded fiber-reinforced elements were created by pressing a mixture of plaster and plant fibers or animal hair into molds. The cast plaster elements were tied to a metal framework support system within the attic using tie wires and wadded plaster. The finish paint on the auditorium side features various colors of decorative glaze and metallic painted finishes.

During a previous repair, the entire attic-facing surface of the ceiling was coated with a sprayed acrylic. This acrylic was reportedly applied to reinforce the connection between the plaster ceiling and its support system. Although intended to strengthen the plaster as a consolidant, the material did not penetrate the plaster but instead formed a thick surface coating.

The acrylic coating on the attic side was found to have low vapor permeability, and the glazed ornamental finishes on the auditorium side also have low permeability. As originally designed, the ceiling allowed moisture to escape by drying toward the attic. However, adding a low-permeability coating on the attic side severely retarded this evaporation, trapping moisture within the plaster matrix. Vapor from high interior humidity and bulk water infiltration can degrade the gypsum, causing the plaster to powder and the surface finish paint to delaminate. Additionally, as is common in older theaters, air conditioning was installed in the auditorium, which can cause a reverse vapor drive (toward the interior) during summer months, as the plaster dries toward the conditioned space.

When a historic assembly, such as the plaster ceiling in this theater, is comprehensively modified by applying a new material, such as an acrylic coating, it is important to consider all potential impacts of the change. In this project, the coating was primarily applied for structural purposes—to hold the plaster in place—but its effects on moisture management were overlooked. A more thorough review of system performance, including aspects like thermal and moisture control, fire safety, structural integrity, and material compatibility, is advisable.

Authors

Kenneth Itle, AIA, is an architect and associate principal with Wiss, Janney, Elstner Associates, Inc. (WJE) in Northbrook, Ill., specializing in historic preservation. He can be reached at kitle@wje.com.

Matthew Haberling, RA, is an architect and principal with Wiss, Janney, Elstner Associates, Inc. (WJE) in New York, N.Y., specializing in historic preservation. He can be reached at mhaberling@wje.com.

The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect that of The Construction Specifier or CSI.