The Building Conservation Directory 2021

PROFESS IONAL SERV I CES 1 39 C AT H E D R A L C O MM U N I C AT I O N S T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 1 STRUCTURAL ASSESSMENT OF HISTORIC PLASTER CEILINGS KEVIN CLARK M UCH WELCOME guidance has been published in recent years in connection with the inspection and assessment of historic plaster ceilings, however there is limited equivalent published information concerning assessment of their supporting structures. These often overlooked elements perform an essential role in ensuring the continued integrity of historic plaster ceilings and this article will explore their common forms and the importance of a thorough assessment of their structural integrity. Obtaining a detailed understanding of the configuration and condition of the structure to which ceiling plasterwork is attached is usually the primary objective, and although the plasterwork itself is typically formed with great skill and care using proven traditional materials and techniques, the structure to which it is fixed is almost always hidden from view, difficult or impossible to access and occasionally assembled in an ad hoc manner with little thought of how it might be inspected or maintained in years to come. It is therefore essential that appropriate advice is obtained from a plaster conservation specialist acting in cooperation with a structural engineer, both of whom should have a comprehensive knowledge of the various construction materials, as well as of the challenges and risks that are encountered during the appraisal and repair of historic plaster ceilings. These requirements are not new, of course, but they have received greater attention since 2013 when part of the ceiling in the auditorium of London’s Apollo Theatre collapsed during a packed evening performance. The subsequent investigation by the Health & Safety Executive and plaster specialists culminated in the release in 2015 of guidance by the Association of British Theatre Technicians (ABTT) for the owners and operators of properties containing similar ceilings [1] which recommends a thorough survey by a competent plaster specialist and structural engineer detailing the composition, arrangement and condition of the plaster and the structure to ensure the provision of appropriate advice to building owners and operators. HISTORIC CEILING TYPES Comprehensive accounts of the historic development of plasterwork are available (see Recommended Reading) but in simple terms historic plaster ceilings are essentially of two types; solid plaster or fibrous plaster. Solid plaster may be composed of either lime plaster (calcium hydroxide from calcined limestone mixed with sand and other additives), or gypsum plaster (calcium sulphate) and often reinforced with animal hair. Fibrous plaster is composed of relatively thin panels of gypsum reinforced with a woven hessian fabric and timber battens. Wet lime plaster has been in widespread use since ancient times and can be applied in situ directly to masonry but also to thin timber strips of oak or pine (laths) interspersed with narrow gaps through which the plaster squeezes as it is trowelled on. When cured, the plaster that has squeezed-through forms resilient nibs that grip the laths and hold the thin plaster membrane in place. Lime plaster was sometimes gauged with gypsum to give it a faster set, and plaster of Paris, a relatively pure form of gypsum, was also used for decorative plasterwork in much the same way as lime, particularly from the late 18th century. In simple ceilings of the modern era consisting mostly of monoplanar flatwork, the laths are nailed directly to the underside of the floor or roof structure, but in ceilings incorporating more ambitious geometries and decorative schemes the laths are secured to a secondary structure suspended from the primary structure, often comprising a bespoke and complex framework of timber members (see Figure 1). In later 19th and early 20th century ceilings the timber laths were replaced by expanded metal lath, a multipurpose steel mesh performing the same function and secured to the supporting framework in a similar way. By the mid-19th century solid plaster had been replaced by fibrous plaster for most large-scale ceilings due to the faster assembly and lower cost arising from the repetitive pre-fabrication methods used in its creation. For its attachment fibrous plaster dispenses with laboriously fixed individual timber laths and relies instead on wads of gypsum-soaked hessian, often reinforced with steel wire ties, twisted around The highly decorative fibrous plaster ceiling of Wyndham’s Theatre in London, which is regularly inspected and defect-free (Photo: Conisbee, 2008)