The Building Conservation Directory 2024

29 CATHEDRAL COMMUNICATIONS THE BUILDING CONSERVATION DIRECTORY 2024 PROFESSIONAL SERVICES 1 The tower is constructed in Kentish Ragstone with Bath Stone dressings, whereas the spire is exclusively of Bath Stone. Kentish Ragstone is a sandy, greyblue or grey-green coloured limestone with a characteristic ragged surface when hand cut and dressed. Bath Stone is a Wiltshire limestone used in many prestigious buildings, including the Royal Crescent in Bath. It is a freestone which means it lends itself to being carved in all directions and to delivering mouldings of high intricacy. The drone survey of the tower and spire revealed areas of further weakness and so it was followed by a full close-up inspection from a MEWP. This was carried out with the assistance of a specialist stonemason and sought to establish areas of immediate risk, if any. Both the drone photography and MEWP inspection established that the Bath Stone dressings and decorative elements throughout displayed typical signs of limestone defects resulting from air pollution. This is when sulphuric (H2SO4) and sulphurous (H2SO3) acids contained in acid rain react with the limestone (CaCO3) and eventually convert it into gypsum (CaSO4). In sheltered locations the resulting gypsum simply absorbs the atmospheric dirt and turns black. In exposed locations, being water soluble, the gypsum is washed away by the rainwater thus causing loss of surface finishes and obliteration of decorative details. Even before these inspections took place it was evident that the Kentish Ragstone surfaces were affected to some extent by this mechanism. This is seen in the blackened and darker areas in the lower stages of the tower and loss elsewhere of original surfaces. However, the drone photography and MEWP inspection revealed an unexpectedly advanced state of decay to the Bath Stone which may also be attributed to the quality of the stone in particular locations. There is significant loss of detail in the crocketed gables and missing finials throughout both the tower and the spire. There are also several instances of exposed metal dowels testifying to water ingress. This has resulted in their corrosion and expansion which has caused pieces of stone to shear off and collapse to the ground. Missing finials and detached stone fragments identified stone collapses which have taken place in the past and were unknown until now. Significantly, areas of diminished structural integrity were also identified, such as the four projecting gabled niches of the lower tier of the spire. The decision was therefore taken to provide intermediate protection through the installation of supportive netting. This would contain any further collapses of the stone in the most suspect areas. TECHNICAL SPECIFICATION The netting specification, agreed and implemented in consultation with the specialist structural engineer was, as follows: Netting: Black nylon 15 mm netting or 4 mm steel netting. Fixings: Excalibur 6 mm self-tapping eyebolts which can be wound into predrilled holes; occasionally the use of resin may be required depending on the quality of the substrate. Method of securing the netting: Steel coated 4 mm cables run through the bolts, tensioned with the netting and then attached to the cables using steel C-rings. Eyebolt locations: These depend on safe netting installation requirements while leaving stone ornamentation unaffected; the number and frequency of fixings are to be dictated by site conditions and be appropriate for conditions and security of the netting. Removal of the bolts: Holes left on the masonry are to be made good using matching stone plugs without mortar. Netting condition inspection: The installation is to be subject to yearly inspection of the condition of the netting. INSTALLING THE NETTING The protective netting installation was also carried out from a MEWP which provided the opportunity for further assessment through close up inspections and consultations. The aim was to enable specialist structural engineer input in preparation for full repairs in the future. The church secured a grant for the netting installation, having agreed a budget price with a specialist mason and specialist netting subcontractor. The netting will remain in place until the funds for full repair of the ailing stonework elements become available and such work is able Advanced limestone decay on the spire had compromised the stability of the south-west gabled niche, shown here with metal fixings installed, ready for its protective netting. (Photo: Eleni Makri) Completed netting installation over the top of the gable of one of the offset buttresses of the clock stage of the tower (Photo: Eleni Makri)