Moonbase Walsall

Walsall's new bus station is going to have the craziest roof in the West Midlands. The station's concrete canopy is a curving moonscape dotted with enough ventilation craters to support a sizeable population of Clangers. And if any did stumble across it, they would find it life-supporting – it will be covered with a carpet of flowering cactus-like plants.

The roof is the centrepiece of architect Allford Hall Monaghan Morris' £3.7m competition-winning station. Client Centro's brief was for a structure that raised passengers' expectations of bus travel, and the architect responded by forsaking pedestrian islands with separate roofs in favour of a single structure. "It's a building, not just a series of bus shelters. We wanted the feeling of entering a terminus; a public building," explains partner Peter Morris.

The living, breathing green roof puts Walsall bus station among the rapidly expanding ranks of eco-friendly mainstream buildings in the UK. As a supplier of oxygen to the atmosphere, the green topping's role may be largely symbolic, but it helps to counteract pollution and protect the roof's surface from solar damage, all for a similar price to a conventional membrane.

Now virtually complete except for the final two craters and the plants, the insitu concrete roof sits on 12 steel "tree" columns, each with three supporting branches. The roof's elliptical form is 8.5 m above the ground in the centre and 6 m at the edges and spans an area 80 × 45 m. "We loved the idea of taking that mass of concrete and floating it up in the air," says Aran Chadwick, director of structural engineer Atelier One.

The positioning of the craters on the surface appears random, but they correspond to the layout of the station below. Ten 5 m diameter curved ventilation cowls are sited above the bus lanes so engine fumes can escape, and six smaller 2 m diameter cowls are aligned with the passenger waiting areas. The cowls add stiffness to the structure, as does the 3 m wide concrete ring-beam.

The airy space below will appear to be lit by eerie shafts of light from the planet's surface. The smaller cowls can be glazed with a single sheet without glazing bars, which Morris says will give the effect of looking straight up at the sky. To hide any natural discolourations, the soffit will be painted with a light grey concrete paint that reflects light from uplighters fitted in the steel tree supports. "The soffit elevation, with the curved form and massive openings, is just as important as the roof elevation," says Morris.

The station will also have a shop, café, toilets and staff offices, contained in a concrete box with rounded glazed walls at either end to echo the elliptical shape of the roof. The sheer concrete walls have the effect of restraining lateral forces on the roof.

Contractor Shepherd constructed the roof using extensive temporary works supporting plywood formwork and steel reinforcement rods. Triangular frames linking the tips of each steel tree's branches were positioned so that they poked above the plywood base, becoming embedded in the concrete once it was poured. The slab is a consistent 325 mm thick.

AHMM and Atelier One considered several roofing options, including steel and preformed concrete, before deciding on insitu concrete. "The plastic nature of concrete as a material served us better," says Morris. "It gave us continuity across the surface. And the cowls were relatively simple. As they're all the same shape, we could reuse the same formwork," adds Chadwick.

However, the jury is still out on the safety of the concrete canopy following claims that the roof deflected more than expected when some temporary props were removed. As Building went to press, further removal of the propping had been halted and a design team meeting had been called to assess the way the roof was behaving.

Once the issue has been resolved, the final step will be to install the living roof, supplied by the UK subsidiary of German specialist Erisco-Bauder. It will be covered with a waterproof membrane overlain with a substrate of moisture-retaining granules. The plants will root themselves into the granules, then grow to a maximum height of a few centimetres. "They never spill over the edge and can survive a hot summer with just a few sprays of water," says Morris.

The green system naturally adds weight to the roof – Chadwick estimates that the loading was increased by 15-20%. "The saturated load is quite high – about 1 kN/m². We had to design the roof with more reinforcement than otherwise."

The extra loading was incorporated into Atelier One's "finite element" analysis of the stress distribution across the roof's surface. "It's a method for examining large surfaces where you don't have a rectilinear slab with predictable deflection. It gave us an understanding of the forces and how we should tweak the position of the columns."

However, the green roof apparently did not involve extra expense. Erisco-Bauder quotes £44/m2 for the bulk of the roof, and £65/m2 for the insulated area above the shop and offices. AHMM's Morris agrees that going green was perfectly affordable. "It's a good price compared with other standard membranes. If it wasn't, the client wouldn't have gone for it."

The station is due to be completed by the end of the year, when the people of Walsall will be able to catch their buses from an uplifting and calm terminal and any stray Clangers will find a welcome resting place. Unfortunately, the two groups will never meet – safety restrictions mean that AHMM will not even allow guests at the station's opening party to visit its incredible roof.