The Building Conservation Directory 2021

40 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 even failure of the supporting structure. The structural integrity of the roof and its ability to safely support the attached plasterwork may also be compromised by ill-considered alteration of the ceiling support structure for pipes and cables. Access may also be restricted by the extensive presence of asbestos, debris, dust and detritus, particularly in confined spaces. An alternative might be to make new openings, but in a listed building where minimal intervention is required, the widespread removal of floor finishes or plasterwork is unlikely to be permitted. Therefore a carefully considered strategy should be undertaken entailing a comprehensive process of enabling works preceding detailed structural and plaster inspections and further steps which generally comprise: 1) desk study, 2) digital survey, 3) access and cleaning 4) inspection and assessment, 6) repair and certification and 8) monitoring and management. DESK STUDY Original drawings or other information detailing the construction of historic plaster ceilings are extremely rare, and any surviving contemporary information that may be available in accounts or similar sources is typically of limited relevance from a structural perspective. Usually to gain sufficient understanding of the actual ceiling construction, including any alterations that have been made, it is necessary to undertake a detailed methodical inspection from above and below using a combination of digital survey techniques and hands-on inspection. DIGITAL SURVEY In most cases photogrammetric reflected ceiling plans should be produced by combining point cloud laser scans with conventional digital photographs, augmented by a gridded overlay for cross-referencing specific areas during close up visual inspection. Figure 1. Solid plaster fixed by lath across a heavy timber framework (Photo: Conisbee, 2019) Figure 2. A fibrous plaster ceiling attached to a timber framework by wire ties under wads of hessian soaked in plaster (Photo: Conisbee, 2017) a supporting structure at discrete locations in a grid-like layout. As with traditional lime plaster, the fibrous plaster panels forming the ceiling may be secured directly to the primary structure or, more commonly, to a secondary timber or steel framed structure arranged to suit the ceiling’s geometrical and decorative scheme (see Figure 2). COMMON PROBLEMS AND CHALLENGES Structures built before the middle part of the 19th century were not designed using empirically derived structural analysis techniques such as those routinely used today, but were instead constructed using contemporary rules of thumb and a general reliance on tried and tested forms that had previously proved successful. This approach is perfectly adequate, albeit somewhat inefficient, when applied to traditional structures but is less successful when applied to bespoke lightweight framed structures like those built to support plaster ceilings, particularly when constructed under time pressure near the end of a project while funds were short and an unscrupulous builder would have been aware that any shoddy workmanship or materials would eventually be hidden from view by the enclosing plasterwork. Consequently, structural problems can arise, and where problems lead to the dislocation and movement of poorly assembled members, plasterwork can suffer from the associated distress and damage. Poor connections between frame members are a common concern, particularly where timber frames are nailed to the supporting structures from below (see Figure 3). This connection would doubtless have been quick and effective when installing the ceiling structure, but their widespread use raises concern regarding the structural integrity of many ceilings. The issue is assessing the pull out resistance of historic square cross section wrought iron nails from the parent timber in which they are embedded, and whether this might be sufficient to withstand tensile loads, especially those applied dynamically. One of our key challenges in conserving historic timber structure involves justifying the retention of areas affected by insect attack, particularly where it is advanced and severe. Generally, only the edible sapwood is affected by common furniture beetle or powderpost beetle, but this is not always the case. Careful assessment must be made in areas of advanced live attack to establish the interior condition of timbers and critical connections in order to ensure their load carrying capability is not compromised. Micro-drilling can be undertaken in appropriate locations to quantify the extent of section loss, so that the adequacy of the structure can then be assessed by structural calculation. Similar issues arise where the decay of timber and the corrosion of metalwork is due to water ingress. This is a frequently encountered problem where timbers are built into exterior walls or near gutters, and remains an ever present threat to the macro-integrity of the ceiling structure. Water ingress presents a particular risk to the integrity of the wads and any embedded wirework that secures fibrous plaster panels to the supporting structure. The discrete arrangement of these fixing points means that fibrous plaster ceilings possess far less structural redundancy than lime plaster ceilings, which are effectively secured in a continuous manner, so special attention must be paid to their construction and condition. All of these considerations are amplified where the supporting structure is hidden behind the ceiling and access is limited or non-existent, preventing routine inspection and maintenance. In these conditions the degradation of timber, metalwork and other elements can progress undetected and unhindered, sometimes over a period of many years, until structural failure occurs. Conversely, easy access to a roof space often leads to overloading from ill-considered storage, causing excessive deflection or

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