The Building Conservation Directory 2021

42 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 C AT H E D R A L C O MM U N I C AT I O N S is not possible or where falls from height pose a significant risk, temporary rope access systems will be required, developed in collaboration with specialist access providers having extensive experience of working in and around historic buildings. Where access to the topside of the ceiling is not possible, the use of more intrusive inspection techniques should be considered, such as digital borescopes, following negotiation and agreement with the client, conservation officer and other stakeholders (see Figure 7). INSPECTION AND ASSESSMENT Once cleaned and safely accessible, the inspection of all relevant areas can begin. Each ceiling is unique so a site-specific strategy should be developed to suit particular considerations. Nevertheless the assessment of the ceiling support structure generally requires: • Understanding the structural system (construction, load transfer, integrity, stability) • Performing a condition assessment of structural members and investigating the causes of any defects • Making a detailed investigation of structure directly supporting defective areas of plaster as identified in the underside inspections • Carrying out a quantitative and qualitative investigation of structure supporting areas where considerable deviation/deflection of plaster has been recorded during underside inspections. It is vital to appreciate that the assessment of any historic plaster ceiling should be approached holistically taking proper account of the geometry, composition and condition of both the plasterwork and its supporting structure. This process is fundamentally collaborative and requires close cooperation between the plaster conservator and the structural engineer if it is to result in meaningful and appropriate advice to the building owners and operator. Structural analysis is typically undertaken through hand calculations to understand the existing stress state of the structural elements and to gauge the overall response to imposed loading. More often than not primary structural members and their connections are found to be of good quality and mostly of adequate capacity to perform as intended. However, it is often discovered that modern serviceability limit states for deflection are not satisfied, so access restrictions have to be introduced to minimise further distress to the plasterwork from bowing, sagging and similar movement of the structure. In special cases load testing of unusual materials or potentially compromised structural configurations may be necessary to assess their adequacy. This is necessary where the only evidence that the structure will be able to continue to satisfactorily support the dead and imposed loads to which they are subjected is wholly based on their historic performance. This is especially important when enhanced access resulting from a routine inspection and maintenance regime is to be introduced. REPAIR AND CERTIFICATION Structural repairs should be designed in accordance with accepted conservation engineering principles and practice, and commonly comprise relatively simple but reversible and distinct bolts, straps and other traditional repairs where needed to consolidate failing hangers or connections. With a view to ensuring the retention of as much original fabric as possible all structural intervention should be minimal and limited to areas where repairs are essential. Implementing a considered regime of monitoring and management are preferable in lieu of more invasive interventions. It is imperative to provide detailed reports and documentation of the construction, condition and safety of each ceiling and its supporting structure, plus further documentation outlining material condition, performance, management, access, cleaning requirements and asbestos status. The Association of British Theatre Technicians’ guidance note 20 requires the production of certificates outlining the condition and status of both the plasterwork and structure, and this should be a collaborative document produced by the structural engineer and plaster specialist. This certification may take the form of comprehensive condition reports and one-page summary documents for use as quick reference guides outlining the current condition and safety of the ceiling, maintenance requirements and recommended actions. MONITORING AND MANAGEMENT Access constraints can sometimes be so limiting that it is practically impossible to inspect the ceiling and its supporting structure from above, and whilst it is possible to draw a reasonable degree of confidence from underside inspections a programme of remote underside monitoring can be conducted to inform initial recommendations and clarify any ambiguities. This is particularly relevant where the geometry and construction of the ceiling is concerning or complex. Appropriate control protocols may also be established to protect the ceiling, including regulating access to safe access walkways and other areas above especially sensitive ceilings in order to reduce movement and vibration related damage. This guidance should be included together with any ongoing monitoring information within the operation and maintenance manual under the aegis of the building owner and operator. It can be appreciated that the strategic inspection and assessment of historic plaster ceilings and their corresponding supporting structures requires multiple skillsets and substantial experience drawing on several areas of expertise. This process should implement the recommendations contained within the latest ABTT and other relevant guidance and demonstrate the many practical challenges encountered when attempting to satisfy certification requirements. To safely access often restricted, confined, obscured and contaminated spaces requires considerable planning, design and execution, particularly where they are located above fragile plasterwork ceilings suspended over rooms in regular use. Some of these challenges and limitations are site-specific owing to the special and unique genesis and usage of each building, but many are generally and usefully applicable to a wide range of common building types from all periods. Recommended Reading 1 Guidance Note 20 – Suspended Fibrous Plaster Ceilings: Survey and Inspections , Association of British Theatre Technicians (2015) Practical Building Conservation: Mortars, Renders and Plaster , Historic England (2012) ‘Decorative lime plaster – conservation and repair’, The Journal of the Building Limes Forum pp 24–39, Richard Ireland (2014) KEVIN CLARK is a conservation accredited engineer (CARE) and Head of Heritage at Conisbee (see page 43). His heritage engineering team specialises in the inspection, appraisal, repair and conservation of all manner of historic buildings, structures and sites throughout the UK. Figure 6. Rope access is necessary where there is a risk that the ceiling might not support conventional access methods (Photo: Conisbee) Figure 7. Using a boroscope to inspect the ceiling supports where access from above is impossible (Photo: Conisbee)