"The brief was to design a leading facility for musical performances," says Pringle, "so the building's form has been driven by the need to get the acoustics right."
Indeed, acoustics have dictated the building's unusual elliptical shape, with its ground floor sunk into a sloping site and a tent-shaped pitched roof towering above it. And the key feature of the building, a 14 m high central performance venue, has to be capacious enough to achieve the sound reverberation necessary for concerts.
Circling this performance space are two floors of practice rooms and teaching spaces separated from the main auditorium by a corridor. Pringle describes the corridor as "an economic solution for providing an acoustic buffer between the areas". The dividing walls of the practice rooms are set perpendicular to the building's elliptical perimeter, so that none of the walls are parallel – again, to improve the acoustics.
This unconventional interior arrangement is reflected outside, where the pitch of the roof differentiates between the performance space and the practice rooms. The lofty, steeply pitched and faceted roof covers the auditorium, with a surrounding ring of gently sloping roof for the perimeter rooms. The inner roof is far from a straightforward elliptical shape, however. "An ellipse could have been potentially disastrous for the performance room," says Pringle, "because its curves would have focused the sound."
The faceted version of the ellipse was arrived at following advice from Arup Acoustics, and was a boon for the project's structural engineer, Buro Happold. "The faceted shape gives the roof sufficient structural strength to resist wind loads," says Buro Happold's senior associate, Steve Gregson. However, even this roof structure will have to be lined with curved timber deflection panels to ensure that sound reverberates freely around the space. "It's a simple roofing solution, but it took a lot of work to arrive at. It's a synthesis of acoustics, structural engineering and aesthetics," says Gregson.
To achieve optimum performance from the building's roof, the architect has chosen a timber roofing system called Dickholz, manufactured in Germany by Merk Holzbau. "Dickholz means thick wood," says Gregson. Shrewsbury School will be the company's first roofing application in the UK.
The solid timber roof panels are strong enough not to need additional structural support and heavy enough to provide enough mass to meet the building's acoustic requirements.
They are manufactured from strips of spruce glued together to form panels 4.2 m long and 2 m wide. For the performance space, 119 mm thick panels have been used to form the roof, while for the perimeter rooms, 85 mm thick panels were sufficient.
Pringle – who refers to the panels as "thick plywood" – came across the system when he was working on a project in Berlin. He decided the improved acoustic performance and narrow cross-section the product offered were a more appropriate solution for the school's roof than the stressed skin panels originally specified. The roof panels were delivered already cut to size.
"The manufacturers took our MicroStation drawing file and used the dimensional information to program their computerised cutting machines," says Pringle. The panels are nailed to angle brackets sprung from the loadbearing structure and spliced together using wooden strips by a team of itinerant German carpenters. The weight of the roof is transferred through dense loadbearing blockwork partition walls to the ground. The whole roof took just two weeks to erect – one week each for the perimeter section and for the main hall roof.
Once in position, the panels were covered with a vapour barrier and overlaid with insulation before being covered with lining paper. Timber battens nailed over the paper will allow the western red cedar shingles that will form the roof's outer skin to be attached. Timber will also be used to provide the ceiling finishes inside the building.
In the performance space, the shaped acoustic panels will cover the underside of the roof, while in the perimeter rooms the exposed timber of the roof panels will form a natural ceiling.
The timber roof also plays its part in conserving energy, as the amount of energy trapped in the building's fabric is minimised. Internally, the building is flooded with daylight to reduce energy consumption.
Pupils will enter the building beneath a glazed entrance canopy before passing along the corridor with its glazed roof to reach the individual practice rooms. Natural light will enter the ensemble room through glazing in the ventilation housing on the roof.
Energy consumption will also be minimised by the building's ventilation strategy. "We were anxious that natural ventilation be used as much as possible," says Pringle. The engineers have exploited the central hall's height to form a buoyancy-driven natural ventilation system. This also exploits the fact that the building is built on a slope, and that some storage areas and plant rooms are below ground. Cool air will be drawn from these spaces into the main auditorium, where it will pick up heat from the audience and rise to the roof vents. As it rises, it will draw more cool air in. Nevertheless, an air-handing unit has been installed as a precaution, to boost the ventilation system when the cooling loads are particularly high.
The external walls at ground-floor level are part-glazed and partly clad in timber boarding, while the walls at the lower-ground level will be formed from a brick/block cavity construction packed with recycled newspaper insulation.
The practice rooms are also naturally ventilated using the full-height perimeter windows. These will work in much the same way as sash windows by allowing cooler air to enter the room through the lower section of the window, while the warmed air will leave the room through the window's upper section.
So much for the environment-friendly ventilation and lighting. How will the music school tackle noise pollution? To prevent musicians annoying the school's neighbours, the architect has located practice rooms for louder instruments to the centre of the site, away from nearby housing. And any rooms devoted to rock music and percussion have been banished to a basement bunker with sealed windows.
On site, the roof structure is complete. Work on installing the timber shingles is under way. At this rate, it looks as though the school's carol singers can look forward to a harmonious festive season.
client Shrewsbury School architect Pringle Richards Sharratt quantity surveyor Davis Langdon & Everest structural and services engineer Buro Happold acoustics consultant Arup Acoustics main contractor Bowmer & Kirkland timber roof subcontractor Merk Holzbau