The 200-seat, £750 000 café, for which Tilbury Douglas is currently negotiating a contract, will serve the neighbouring Cellular Operations call centre, itself a space-age structure completed last year. But, because the café, designed by Richard Hywel Evans Architecture & Design is sited in parkland beside a lake on the outskirts of Swindon, the public will be able to dine there, too.
The beauty spot location is part of the reason for the café's bizarre appearance. The idea was to minimise its impact by making the structure as transparent as possible. The clear, floating roof and north window-wall help it blend into the background when viewed from across the river, and a bank of earth on the other side hides it from passers-by to the south.
Of all the strange elements in the building, it is the roof that has thrown up the most challenges. The 450 m sq, lightweight topping is constructed from foil pillows of ethyltetrafluoroethelene (more snappily termed ETFE), which are kept permanently inflated by small fans.
Because it weighs so little, the problem for the structural engineer, Mike Hadi of Hadi Associates, was how to stop it floating off. He solved this by attaching it to a lightweight frame set on thin aluminium channels that run parallel to the seams between roof sections. The frame rests on thin, hollow columns that also supply air to keep the pillows inflated.
The choice of materials for these columns is still being discussed. Hywel Evans favours carbon fibre or aluminium posts constructed by a yacht-mast manufacturer. However, cost and time constraints will probably result in the columns being made from thin-walled steel tube.
The air used to inflate the roof will also help combat solar gain. Inside the pillows is a flap of ETFE foil with a series of opaque dots, or "fritting", printed on it. This can be manoeuvred by changing the direction of air flow. There is a complementary pattern of dots on the top surface of the pillow. So, when the flap is raised, the dots mesh to form a translucent surface that cuts out glare. When it is lowered, sunlight can pass through to the interior. In winter, to stop snow forming and to melt any snow that collects on the roof, heaters will be incorporated into the air supply system.
Another bizarre feature of the café is a concave, sculptural wall at the rear, which will be cast in concrete by using an inflated vinyl sausage as a mould. This area of the building will house the kitchen and WCs.
Using a technique more common in the manufacture of grain silos, the contractor will enter the sausage through an air-lock. Once inside, insulation foam will be sprayed to a depth of 25 mm over the inside surface. This will set to form a rigid shell. After the mould is deflated and the surplus membrane cut away, steel reinforcing mesh will be attached to the inside face. This will be sprayed with concrete to a depth of 90 mm to form the retaining wall.
Hywel Evans says the big advantage of constructing the wall this way is that "the vinyl membrane will form the outside surface of the retaining wall, protecting the concrete from the weather and acting as a damp-proof membrane".
Outside, earth will be heaped against the wall to form a berm bank at the rear of the building. This will provide some thermal mass to help keep the café cool in summer. Inside the berm, the architect has hidden a series of rainwater storage tanks that will be used to irrigate the carpet of grass covering it. Above it is a row of high-level glazing that separates the earth berm and concrete retaining wall at the rear from the inflatable roof, and creates the illusion that the entire roof is floating like a cloud above the café.
On the north facade, Hywel Evans has exploited the building's spectacular lakeside location for the benefit of diners. Rather than isolate customers inside a sterile, artificial space, the architect has designed the café's walls as "invisible" fabric elements, to create the "ultimate outside experience inside". The glazed window wall can be peeled back to allow alfresco dining and access to a wooden deck beside the lake.
The three main construction elements – the roof, the full-height glazed wall and the earth berm – are treated as separate elements by the architect; this eliminates the problem of dimensional tolerance between each of them.
The roof is separated from the glazing by a flexible membrane; the glazing has a braced structural frame tied back to the structure supporting the roof; and any dimensional inconsistency in the earth berm is taken up by a specially designed glazing channel, which supports the glass slotted between the berm and the roof.
When the café is complete in August, the blow-up roof will not be on its own for long. A larger version is planned to crown a £1.2m, 3200 m sq phone distribution centre just behind the café. Then, the floating roof will have really taken off.