C O N T E X T 1 3 4 : M A Y 2 0 1 4 45 ornamented carvings within the Great Hall. Decay was also apparent in the timber lintels of the two balconies at first-floor gallery level. Our inspection that revealed that some of the joints within the trusses, particularly the apex joint within the lantern light, showed evidence of opening up associated with settlement of the truss bearing due to the decay and crushing.We immediately suggested that all seven trusses should be propped from an internal scaffold structure while design options and solutions could be developed to replace the structural integrity of their bearings. At this point the design team, the contractor and the client agreed that the initial contact should be modified to become a contract involving the opening up of more of the adjacent flat roof and the concealed gutters behind the stone parapet and the bottom of the main pitch of the Great Hall roof, so the full extent of decay to the roof timbers could be determined.The temporary roof was retained and a number of rows of Yorkstone roof flags removed so that more of the roof timbers could be inspected. Further surveys were commissioned, such as those from specialist timber preservation surveyors so that the extent of the decay within the heart of the truss rafter bearing ends could be plotted. This effectively split the work into two phases, two consent applications and two grant applications, the second one being for emergency repairs. The decay to each truss bearing end was found to vary in extent from 200mm to 700mm. It was decided that the most effective method of repair to protect the ornate timbers below was to introduce steel flitch plates to each truss bearing. The plates were designed to be contained above the ceiling line so that they were not visible from below. The same length of plate for each truss was adopted to aid with fabrication.The variation was the amount of original decayed timber which was removed from the end of each truss. It was agreed within the design team that the opportunity should be taken to remove the timber wall plate that would remain vulnerable in the completed work if left embedded within the brick wall. At the truss bearings this was replaced with concrete to spread the load, and under the roof joists it was replaced with new English oak. A small number of the roof joist ends were also decayed. New lengths were spliced in using a traditional lapped scarf joint reinforced with bolts. Once all the roof trusses had been sequentially repaired, the load was transferred in planned stages off the temporary scaffold propping back into the truss and on to the wall of the Great Hall. Open joints previously identified in the trusses were again viewed for any change in dimension. None was found. Areas of removed timber panelling and carved features, removed from the wall because of decay and the general high moisture content of the brickwork, were placed in storage on site. Trickwork was left exposed internally to dry out naturally. Ultimately these elements will be replaced and replicated under a future separate contract. IanWeir is a senior engineer with Blackett-Ord Conservation Engineering. The end of a truss prepared for a new flitch Extensive decay to a truss bearing
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