For the first scheme, at Boldon in 1997, HBG did not carry out any acoustic testing but simply opted for a heavy roof construction with sufficient mass to provide a barrier to sound transmission. However, this solution had important implications for the cinema’s structure and foundations.
HBG had used an insitu concrete roof for the plant well, which meant that until the operator provided information on the size and position of the air-conditioning plant, the location and size of the plinths and the holes for ductwork and pipework through the roof could not be finalised. This delayed the roof construction and the design of the steelwork.
Richard Fielder, HBG’s construction director for the North-east, knew that if a similar acoustic performance could be obtained from a lighter roof, the steel frame and the size of the foundations could be reduced, giving considerable cost savings. The shell would also be weathertight earlier as this type of roof can be installed quickly.
For the Hull scheme in 1999, HBG, in partnership with novated acoustic consultant SRL, steelwork contractor Barrett Steel Buildings and cladding and roofing contractor European Roofing Systems, developed more than 50 different lightweight roof constructions that the design team knew to be durable. The contractor then had these different configurations tested to assess their acoustic performance.
Understanding of the acoustic performance of roofs is “lacking across the industry”, says HBG project co-ordinator Steve Wheller. “And different operators have different needs.” The tests for the Hull scheme allowed the main contractor, steelwork contractor and cladding specialist to combine their knowledge of roof construction, performance and durability with the specialist knowledge of the acoustic consultant.
The use of plasterboard to provide acoustic mass for the roof was one cause of concern for HBG. Fielder says the acoustic engineer was happy to use plasterboard but Fielder was unhappy about its long-term durability, particularly if it were to get wet during installation. “It just didn’t feel right using plasterboard as part of a roof construction,” says Fielder. “We wanted the repeat work, so we were aiming for zero defects.” The team came up with a series of alternatives, “some of which had better acoustic performance than plasterboard in any case”, says Fielder.
The tests also resulted in a reorientation of the roof steelwork to cut sound transmission between auditoria.
The research put Fielder and his team in an enviable position for a design-and-build contractor: it allowed them to use the cheapest form of roof to meet the operator’s acoustic criteria for a specific site safe in the knowledge that the solution would work. “Our roof designs passed the operators’ acoustic tests with flying colours,” says Fielder proudly.