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.

“Apprehended Collapse”

Typical cross-section showing some 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 condition.

 

Without treatment, collapse is inevitable.

In all cases, the security of the ceiling system depends on the integrity of the narrow lugs and keys 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 lugs and keys 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 lugs and keys 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 treatment methods and their inherent drawbacks.

© John Tiedemann, Inc.