Jonathan Adams of Percy Thomas Architects has been accused of spending £100m on a giant, shell-less mollusc. Here he takes us around his Wales Millennium Centre and explains why the critics don't know their gastropods from their elbows.
To fully appreciate the Wales Millennium Centre, you have to see it in the landscape. Although the huge, tea-coloured, stainless-steel hump can be enjoyed from a the other side of Cardiff Bay, the full in-yer-face texture of the lower reaches hits you only as you get closer. Cliffs of rough-hewn, dry slate walls sweep out from either side of the metal-clad hump, then just around the corner, the style turns to oversized garden shed as the cladding changes to tip-to-toe strips of overlapping softwood.

Reaction has been mixed. Rowan Moore, a critic with the Evening Standard, stirred up howls of Welsh protest when he dismissed the building as a "giant, shell-less mollusc crawling over a pile of slate". It is perhaps understandable, therefore, that the building's designer, Jonathan Adams of Percy Thomas Architects, is a little defensive of the project he began in 1998. Adams says he has tried to make the building appeal to its users: "It hasn't been designed for other architects," he says.

"The biggest challenge was coming up with a design that was capable of generating support from a wide range of different constituencies, which have a tendency to go against each other. The most important of those was the general public, then the funding agencies, the government and the planners. We had to get all those people behind the project and also produce something different that goes against all their instincts. That is not easy to do." He pauses, then adds: "The odds were stacked against the project going ahead at all."

Nobody could accuse Adams of exaggerating. The Wales Millennium Centre owes its existence to a previous project being scrapped in the face of opposition from local politicians and business people. Architect Zaha Hadid had designed an opera house at the Cardiff Bay site, but despite her fair win in an architectural competition, the Millennium Commission declined to build the design.

So, having taken on the role of housing the Welsh National Opera, the Wales Millennium Centre is really an opera house by another name. It will also host visiting musicals and house a Welsh youth organisation called Urdd Gobaith Cymru, whose members will periodically stay in the building.

To accommodate all these users, Adams had to get the building to function as a world-class opera house with state-of-the-art acoustics, while finding space for shops, bars and restaurant facilities – all the while incorporating his personal style of architecture.

The slate walls are a key part of Adams' architectural language. The walls enclose two large concourses on either side of the stainless steel-clad auditorium. "An important theme was to ensure the building had a civic quality," says Adams. "One way of doing this is to use stone, but we couldn't afford ashlar. Slate was the only option we could afford." Fortunately, the slate was cheap because the blocks are offcuts from roofing slate production and come from quarries in north Wales.

But why use the slate in this incredibly textured way? "I was very taken by the strata on the Glamorgan Heritage coast, as the cliffs have a very architectural quality," says Adams. "This is a theme I have used throughout the building." Five different colours of slate have been used. The slate blocks are either sawn or pillared – (split at 90º to the more usual cleavage plane) to create a rougher face. These 10 combinations are arranged in bands, some of which project over others to give a powerfully stratified effect.

The idea of using slate in this way came right at the beginning of the project, and was not well received. "When I first proposed the slate walls, the big argument against it was that there weren't the skills to do it," says Adams. Undeterred, he rang a council in north Wales who had just had a road boundary built using slate walling. They referred him to a specialist contractor called GH James Cyf. "It was a real watershed for me when I met Gwilym James, showed him the drawings and asked him whether he could do it," says Adams. "He shrugged and said 'yes'."

Finding the right specialist contractors didn't end there. "We went through three main contractors before Sir Robert McAlpine finally won the contract," says James. "We had to show them what was possible with slate: the style, the specification and the dos and don'ts of slate walling, reassuring them it could be done."

The main problem with the slate walls is that they look as if they are trying to be overly "Welsh". Adams insists this had nothing to do with the reasons for using the material. "We used it because there is a lot of it lying around and it has more meaning here than in other places – and its sustainable." One of the slate walls continues beyond the concourse to form a boundary wall to the yard where lorries arrive to unload stage sets.

The sides of the auditorium facing the yard are clad with deep strips of Norwegian spruce, to continue the stratified materials theme, and contrast strongly with the slate. The overlapping strips have "waney" – rough, unplaned edges – that give the cladding a more rustic appearance. Again, expense was a factor. "The timber cladding was originally cheaper than crinkly tin. It's incredibly good value," says Adams. The cladding was going to be installed as prefabricated panels but McAlpine decided to do the work on site as there was concern about how the edges of the panels would weather.

Carry on to the back of the building and the cladding changes again – this time to brick. Adams was concerned about the building relationship with two buildings opposite – the brick-built Pierhead Building and Crickhowell House, which houses the Welsh assembly. However, he didn't want the wall to look like brick. "We didn't want people to take the brick wall for granted and dismiss it as just another brick wall," he says. "The parapet is curved, the wall slopes backward and the windows punch through to emphasise the lean." Despite this, the 2º lean is almost imperceptible.

Which is more than can be said for the architectural feature that has set tempers rising: the curving hump of the stainless steel-clad auditorium. This has to accommodate the fly-tower, a large box that must be twice the height of the visible stage so scenery can be winched up completely out of sight. "The fly-tower is normally a box that sticks out of the roof. You don't get any sense of what it is and what it does," says Adams. "The idea was to make the fly-tower and the auditorium into one logical profile which becomes part of the identity of the building."

Above the main entrance, windows in the shape of giant letters are set into the stainless steel. The Welsh language inscription translates as "Created in truth as clear as glass in the furnace of inspiration", while the English inscription reads "In these stones the horizons sing". Adams explains the reasoning behind the inscriptions: "The letters were a crucial part of the architectural concept. They make the building seem less big. They can relate to the lettering as something you read." The letters are intended to represent the importance of literature to Wales and, slightly more prosaically, indicate where the main entrance is.

Adams wanted the visitors' initial experience of the building to be sustained once they were inside. For this reason, the front-of-house area has a stainless steel-clad soffit. On each side of the main entrance are concourses with shops and restaurants. From here people ascend on a series of stairs and balconies that overlook the concourse. The stratified theme is continued here, with horizontal strips of hardwood cladding this area. White paint is used for the interior and colour comes only from the materials.

The 1900-seater auditorium presented its own problems, primarily with the acoustics. Opera is not amplified whereas musicals need to be. So, while natural sound demands hard surfaces to keep it sharp and distinct, amplification requires an acoustically absorbent surface that deadens reverberation. The solution is called variable acoustics: sound-absorbing mats are dropped down into the auditorium before a musical to absorb sound but are left hidden for an opera. The walls have been clad with gypsum tiles that have a multifaceted surface to break up the reflections of high frequency notes for a cleaner, more natural sound. Acoustic considerations have also affected the design of the services and there are acoustically isolated rehearsal and smaller performance rooms.

The building has already hosted its first performance – when Welsh singer Ria Jones performed for a hard-hat-wearing audience. The rest of Wales will have to wait until 26 November, when the building opens to the public, to hear a performance. Although opinions may be divided on the building, Welsh music lovers will be less concerned about its appearance. Regardless of the controversy, the fact that Wales will be able to attract a variety of world-class music should ensure the Wales Millennium Centre wins a place in the hearts of Cardiff's people.

Sir Robert McAlpine on site

“Everything is bespoke on this building,” says Mark Williamson, Sir Robert McAlpine’s construction manager. “The letterwork, the timber cladding, the slate walls and the variable acoustics – there are so many things on this building that have not been done before.”

The roof was one of the more difficult aspects of the project. “A lot of time was spent getting it right,“ Williamson says. This included ensuring the geometry of the curves was correct and carrying out wind tunnel and corrosion testing to check it would stand up to the strong, salt-laden winds blowing off Cardiff Bay. The stainless-steel cladding was chemically patinated to give its brown, weathered look. Beneath the cladding is a rainscreen with a waterproof layer of Kalzip standing-seam roofing. This was chosen because it was easy to attach to the cladding panels.

Building the letters posed problems of its own. “The question was: how do you support the centre of the O?” asks Williamson. Steel box-beams span the horizontal gaps between the letters and thin steel bars criss-cross between the beams to hold the centre of the letters in place. The three-dimensional sides of the letters were made from grp.

For the cladding, the slate walls had to be built using a technique borrowed from road building. This involved creating a cavity between the slate block and the main, retaining wall which is then filled with insulation and concrete.

The brick walls at the back of the building were also awkward to build because of the 2° lean. ”That gave us problems,” says Williamson. “The brickwork becomes unstable as you build it, so we could only put up 500 mm a day instead of 900 mm. We also had to mark each brick so it reflected the same way.”

Most of this tricky work is now behind McAlpine. The cladding is virtually finished and work is progressing well on the complex building services. Some parts of the building have even been finished and are locked up ready for the grand opening later in the year.