Developments in Plaster Conservation
Most ceilings in churches and public buildings from the 19th century are comprised of wood lath covered in coarse plaster and finished with a fine lime plaster putty coat. The surface is often painted with elaborate decorative art work. Canvas paintings have been applied to some ceilings while others have the painting incorporated into the finished plaster – a technique called Fresco.
In all cases, the security of the ceiling system depends on the integrity of the narrow keys and lugs of coarse plaster that were forced up through the gaps in the lath when the ceiling was constructed.
When these vital structural members begin to break, the adjacent keys and lugs take on the additional burden of holding up the ceiling. This problem usually emanates from a central point of damage or weakness and rapidly expands as the nearby keys and lugs become overloaded with weight.
Without maintenance, the ultimate result of this problem is always ceiling collapse. Fortunately, however, deterioration of plaster ceilings can be observed through timely inspection, and collapse can be prevented by appropriate treatment.
Over the years, several different approaches and methods have been developed to address structural problems in historic plaster ceilings.
This series of illustrations describes these failed treatment methods and their inherent drawbacks.
Apprehended Collapse
This illustration shows a cross-section of cracking caused by the weight of unsupported plaster. The lugs have clearly been damaged and a significant space has developed between the plaster and the lath. We call this a state of “apprehended collapse” – a very unsafe situation.
Without treatment, collapse is inevitable.
Figure 1: The keys that extend through the lath spaces are designed to carry the weight of the plaster below. When they are broken, the weight has to be carried by adjacent keys. Ultimately cracks appear and collapse is not far off.
Figure 2: When the keys break, the weight of the unsupported plaster causes a space to develop between the lath and the plaster.
Figure 3: Cracks appear in the surface and some discoloration can be observed.
Thankfully, there is a preventive maintenance solution that strengthens the plaster matrix and bonds it to the wood lath substrate. For more information about our proven methods contact JTI.