Failures: Understanding plaster ceilings in older buildings

by arslan_ahmed | March 11, 2024 2:36 pm

[1]

Plaster ceilings are often found during renovations and assessments of older buildings. During the 18^th to the mid-20^th centuries, lath and plaster construction was widely used to finish interior walls and ceilings. While less prevalent in modern construction, it is crucial to understand common mechanisms of deterioration and damage. This understanding aids in accurately evaluating these systems and mitigating the risk of future ceiling failure (or collapse) due to unaddressed conditions or renovation work that may compromise the existing installation’s integrity.

Plaster ceilings from this era were typically constructed by attaching lath—usually strips of wood—approximately 6 mm (0.25 in.) apart, or metal mesh, to structural framing or other supports using nails, staples, or other attachment methods. Multiple coats of gypsum-based plaster were then applied. A common method involved a three-coat system: a scratch coat, a brown coat, and a finish coat. To create structural engagement between the plaster and lath, the system relied on “keying” the scratch coat to the lath by forcing plaster through the spaces between wood lath strips or openings in the metal mesh. The surface of the scratch coat was roughened before it hardened to allow mechanical engagement of the brown coat, which was applied and smoothed to build up the thickness of the surface and form square corners of adjoining surfaces. Finally, a thinner finish coat, often a mix of gypsum and lime, provided a smooth or textured surface.

Plaster ceilings are generally more prone to larger area failures compared to wall applications for several reasons. One primary factor is the sustained tensile forces experienced by the layers of plaster, lath, and lath fasteners due to their suspended self-weight from the point of installation onwards. Over time, plaster keys may fracture, separation can occur at the bond line between coats, or lath fasteners may withdraw from supporting structural elements. Initially, failures may begin in small, localized areas without immediate collapse or significant signs of distress. However, detachment of any component within a small area increases stress on perimeter components, leading to potential enlargement of the failure area over time until collapse occurs.

[2]

Another cause of failure is the weakening of components due to several factors, which reduce the ceiling’s ability to resist sustained tensile forces. These factors can include roof or plumbing leaks, vibrations from external or internal sources such as occupant movements, particularly within unfloored attics. Additionally, damage or decay of supporting structural members, improper previous repairs, or disturbances during renovation work can contribute to weakening. Renovation work often requires penetrations for new mechanical/electrical equipment, such as ductwork or lighting fixtures, where vibrations from tools used to cut openings may cause unseen damage to surrounding ceiling areas. Further, adding weight to existing plaster ceilings through the installation of supplemental materials or equipment may overstress the ceiling if not properly supported, leading to failure.

Considering the multitude of potential issues, qualified assessment of existing plaster ceilings is crucial during building renovation or repair projects, especially when they will be disturbed as part of the work or when new suspended ceilings will be added.

Alexandar J. Mlynarczyk, PE, is a senior associate with the Princeton, N.J., office of Wiss, Janney, Elstner (WJE) Associates Inc., specializing in investigation and repair of the building structures. He can be reached at amlynarczyk@wje.com[3].

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

Source URL: https://www.constructionspecifier.com/failures-understanding-plaster-ceilings-in-older-buildings/

---