The Building Conservation Directory 2021

PROFESS IONAL SERV I CES 1 41 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 Undertaking a laser scan enables collation of a 3D point-cloud database for future use including the formation of 3D models, 2D reflected ceiling plans and deformation mapping, which allows targeting of key areas in complicated structural arrangements with challenging access constraints. It also provides the opportunity to incorporate movement or vibration monitoring if subsequently found to be necessary, such as to investigate the influence of nearby construction works on the ceiling’s geometry and condition. To enable deformations to be graphically identified from this baseline, scan contoured distortion maps of the ceiling may be produced which illustrate deviations from an agreed datum plane (see Figure 4). If deemed necessary, repeated periodic scans may be undertaken (usually annually) at a relatively low cost to allow clear and rapid identification of new or ongoing distortions which may require targeted hands-on investigation and further attention. This approach, combined with remote digital monitoring, can identify local distortions which are not easily detectable by other means and positively addresses re-inspection requirements and problematic access into constrained voids that may exist above the ceiling. ACCESS AND CLEANING Close up access to the underside and topside of ceilings is advisable to permit hands on inspection of the plasterwork and its supporting structure, as it is important to physically examine both sides as much as practicably possible. This requires access to any voids that may be present above the ceiling. Close up inspection of the ceiling’s visible surface requires the use of a scaffold tower or in loftier spaces a MEWP – a mobile elevated working platform (see Figure 5). The spatial and logistical challenges associated with manoeuvring and installing essential access equipment in and out of historic buildings should not be underestimated, and structural assessment of floors and stairs is often necessary to justify their ability to safely withstand the additional weight of temporary access equipment, which may be substantial. Once access to the ceiling void is obtained, cleaning is frequently found to be necessary due to the accumulation of dust and debris on the ceiling surfaces over the decades, obscuring the structural connections and projecting plaster nibs securing the ceiling to the laths. Asbestos may also be present within and around ceiling voids in residues from services encasement or firestopping so all spaces must be tested by a specialist asbestos consultant before access is gained by others. Close collaboration with the client, plaster conservator and asbestos contractor is necessary to assess and manage the health and safety implications of any contaminated environment and to ensure cleaning of the plaster nibs can be achieved without causing their damage or loss. In some cases personal air monitoring equipment may be mandatory to maintain exposure to asbestos below acceptable threshold levels. It should not be assumed that any existing walkways or crawlboards are structurally sound, appropriately positioned or securely fixed, and a structural assessment should be undertaken before access is permitted. In some cases the construction of new access walkways may be required to facilitate inspection and assessment of the ceiling and its supporting structure, and where this Figure 4 A 2D representation of a ceiling showing deviations from an agreed datum plane. Red, green and yellow represent areas close to the datum plane, while blue indicates significant localised downward movement of the ceiling, necessitating targeted detailed inspection and assessment (Photo: Conisbee, 2017) Figure 5. A MEWP in use to inspect the condition of a fibrous plaster ceiling (Photo: Conisbee) Figure 3. A failing connection where a secondary frame had been fixed to a structural timber from below using wrought iron nails (Photo: Conisbee, 2019)