Failed Treatment Methods
Over the years, various methods have been used in an attempt to repair or restore failing or unstable plaster. The following is an overview of the three most common methods we encounter and why these methods are not recommended:
Repair Like-with-Like
In the old days (and possibly still offered by some contractors), a popular method of re-attaching plaster was based on the philosophical idea that only materials similar in nature to the original materials should be used in the repair process. This was referred to as the “repair like-with-like” method.
In this case, wire screen was spread over the upper face of the wood lath and fastened to the nearby wood joists.
A thin slurry of plaster of paris was mixed and poured over the upper side with the hope and intent that it would find its way into the space under the wood lath and connect to the coarse plaster below.
The major drawback of this method is that there can be no certainty of connection to the heavy body of plaster, which means that the ceiling may still be at risk. There are many examples of failure of this treatment method within a few years of application. One famous example of a failure of this treatment method is in the Great Hall of Drayton Hall, a National Historic Site in Charleston, South Carolina.
Top image: All the lugs are scraped off the upper side of the lath and a wire screen is fastened to the upper side of the wood.
Middle image: Plaster of paris is mixed and poured over the wire and down between the laths to attempt to bond with the old lime plaster below.
Bottom image: No real bond is established and before long, separation appears between the new plaster of paris and the original lime plaster. Eventually, the bulk of the ceiling collapses.
Wood Screws and Washers
Another simplistic , almost “do-it-yourself” approach to ceiling and wall plaster repair still offered today is the use of wood screws and flat washers to re-secure the plaster.
This involves innumerable screws with large washers installed across the ceiling surface.
In particularly distressed areas, the solution is simply more screws. In areas that seem secure, fewer screws are applied. The screw heads are usually covered with a skim of plaster and the ceiling has to be repainted. Any important decoration on the ceiling is heavily impacted by this process.
This method, while alluringly simple, does not actually solve the problem. The plaster continues to gradually deteriorate around the points of contact and eventually collapses into a heap, leaving only remnants of the ceiling attached with the screws and washers. Moreover, the screws are often over-tightened, causing additional damage.
This method requires access to the surface of the ceiling and mandates extensive redecoration.
Acrylic Resin
In this contemporary age, the use of acrylic resins has been investigated for the re-attachment of loose plaster to its wood lath substrate. In one early version of this approach, holes are drilled through the wood lath from above.
A pre-wetting agent of acrylic resin is then sprayed into these holes. This material soaks into the plaster and prepares it for a follow-up application of an acrylic paste that is pumped into the holes. This paste bonds to and strengthens the pre-wetted plaster below.
As good as the contact is at the immediate area of the drilled hole, the ceiling may still collapse because the bulk of the plaster remains untreated. The acrylic plugs are much like the screws in the previous method – that is, an isolated point of contact. The plaster fails or continues to fail around the point of contact, and the ceiling is left with a series of islands of well-attached plaster, with the bulk of it having fallen away.
John Tiedemann Inc. dealt with this very problem at the Church of Our Lady of the Scapular – St. Stephen in New York City. In this case, an entire ceiling was treated for a second time after the above treatment method failed, after only seven years. The ceilings and walls of this beautiful church were originally painted by Constantine Brumidi, who also painted much of the Capitol Building in Washington.
The Recommended Method
In our work with Historic Plaster Conservation Services (HPCS), John Tiedemann Inc. is now able to provide vastly improved treatment for unsafe ceiling plaster.