Oscar Niemeyer is 100 on Saturday and he looks every minute of it. He shuffles, cloaked in cigar smoke, from his tiny studio, helped by his second wife and long-term aide, Vera, 38 years his junior, whom he married last year. This titan of architecture, who once battled with Le Corbusier, who designed the city of Brasilia and defined the look of postwar Brazil just as João Gilberto defined its sound, who prefigured PostModernism in architecture, who began his career before the Wall Street Crash and who, in his youth, was the brooding image of Marlon Brando, is now as shrunken and ancient as a Ming vase.

It’s like meeting a legend, a name from the history books – Rodin, say, Picasso or Jesse James.

For his 100th birthday, it’ll be business as usual, he says. He’ll probably come here to his office, on the 11th floor of an Art Deco block with bosomy bay windows – “the Mae West building” – on the beach at Copacabana, Rio de Janeiro. He might sketch, smoke his cigars, see his huge extended family, have a long lunch.

“The date is not important. The age is not important. Time is not important,” he mutters in Portuguese. It’s not just turning 100 – Niemeyer seems always to have talked like the Delphic Oracle. His eyes squint into the middle distance for something, “searching for the ghosts of old friends”, he grins. He doesn’t particularly like talking about architecture (“it’s not important,” he utters, with a dismissive wave of a hand).

Instead, on the agenda this morning is his theory of the cosmos, Brazilian politics and general gnomic philosophising. “Life is very fleeting. It’s important to be gentle and optimistic. We look behind and think what we’ve done in this life has been good. It was simple; it was modest. Everyone creates their own story and moves on. That’s it. I don’t feel particularly important. What we create is not important. We’re very insignificant.”

So, don’t send him flowers this weekend. Instead, Niemeyer’s got a present for us: a new building. Just opened across Guanabara Bay from Rio, in the city of Niterói, is the Teatro Popular, the latest in a huge, eight-building cultural complex: Caminho Niemeyer – Niemeyer Way. In 1991 the city’s mayor, Jorge Roberto Silveira, commissioned Niemeyer to build the first one, the Museum of Contemporary Art (MAC). It was six years after the end of Brazil’s 21-year, right-wing military dictatorship, much of which Niemeyer, a communist, spent in self-imposed exile. It was “like a celebration for me, for Brazil’s renewal”.

He built a giant, white flower on a stalk, he says, though many have likened its retro, Kubrick-style looks to a flying saucer. Some call it his greatest building. It’s certainly striking, perched on the cliffs, though it’s perhaps too instantly digestible to be truly great, and the art inside is easily outshone by the glorious landscape framed in its wraparound windows. But it was a return to form and the first of a new late flowering of work, much of it focused on the Caminho Niemeyer.

The Teatro Popular is a mile farther along the coast, right in the heart of the city, poised on one of those vast, bleak Modernist squares Niemeyer is fond of, but which turn all human visitors into ants. Again, it’s striking: the slim line of a back-of-the-envelope sketch turned 3-D and vast – in this case, he adds, the line of a lady’s body sunbathing on her side. There’s the T and there’s the A. (Niemeyer is comfortably unreconstructed prefeminism. On his desk there’s a photograph of a landscape composed of female breasts and buttocks, with pubic hair for trees. That pretty much sums up his architecture.)

The theatre’s egg-yellow flanks can open to the elements, so that performances can seat 400 inside, or tens of thousands on the plaza outside. A trademark curly-wurly pigtail ramp rises up and into the A. What would Freud make of that?

Silveira’s plan was transparent: the second-largest concentration of Niemeyer buildings in the world, after the capital, Brasilia, though born of a somewhat less idealistic philosophy than that socialist idyll.

It’s the Bilbao effect, only using the man who practically invented show-stopping “iconic” architecture when the septuagenarian Frank Gehry was in short trousers.

The theatre is No 3 after the MAC and a new ferry terminal. Nearing completion are the Museum of Brazilian Cinema and the Memorial Roberto Silveira. Two cathedrals – Catholic and Baptist – are in the offing, plus the new building for the Oscar Niemeyer Foundation. And that’s just the half of it.

Niemeyer brings out drawing after drawing – a theatre in Spain, not just a school but a whole new educational concept, a new city for Algeria – some being built, others speculative. Imagine still dreaming at 100? “Every day I work,” he says.

“And still it is difficult. The theatre has been one of the most troublesome of my career.” It opened eight years late and radically altered, its budget cut. He may not do the nitty-gritty any more – that’s outsourced to a project architect – but why put himself through such heartache? He can’t help it, he says. People come to him, he draws something, though, he confesses in English, he’s “finding it harder and harder to sketch these days”.

Yet if any architect conforms to the stereotype of the back-of-the-envelope, willed-to-form genius, it’s Niemeyer. The languid, sensuous curves of his buildings are defined by the effortless lines that poured from his felt pen. When he emerged in the 1930s, from a typical Carioca (Rio) bourgeois family (his father was a typographer; his grandfather a Supreme Court judge), it was just what Brazil needed to conjure up the shape of the young republic.

Its Roosevelt-like President Vargas imported Le Corbusier in 1936 to design the new Ministry of Education in Rio, but he made sure that six of Brazil’s finest Young Turks – Niemeyer included – were shadowing, learning. Niemeyer emerged as the greatest of this new generation, the first in the postcolonial architectural world to take the culture issuing forth from the Old World – in this case the International Style – pull it apart and reconstruct it. The Old World Modernists such as Le Corbusier and Gropius were dismissive and envious of this new architecture – definitely modern with its exposed concrete and abstraction, but not “their” modern: this was tropical Modernism. Niemeyer and his generation were the first really to understand reinforced concrete – unlike steel and glass, easily and cheaply manipulable by unskilled Brazilian labour, and able to create larger, more expressive liquid forms.

“We hated Bauhaus,” Niemeyer recalls. “It was a bad time in architecture. They just didn’t have any talent. All they had were rules. Even for knives and forks they created rules. Picasso would never have accepted rules. The house is like a ma-chine? No! The mechanical is ugly. The rule is the worst thing. You just want to break it.” And so he did. This was the heroic period, until the generals took over in 1964, when he became a household name in Brazil, up there with Pelé. From his pen poured astonishing factories, schools, houses, offices, capital cities – Brasilia, “far too quickly made”, he regrets. A World Heritage Site it may be, but its concrete is shabby, its monuments scabbed with favelas, its idealism soured – Utopia gone bad.

In exile he simply exported his tropical Modernism round the world, increasingly hankering after iconic forms. But he never matched the vigour of his youth: at his absolute best, from the Forties to the Sixties, form, function, ego and social purpose melt into each other. Since then, at his halfway best, form gets the upper hand, but what a form. At his worst, though, form runs riot, the ego unchallenged.

What gets him up every morning is the same as ever, he says: “The struggle, simple communism.” The world, South America, Brazil, Rio, they’re all in a better state now, he believes, George Bush and Rio’s favelas notwithstanding. “Fidel, Chávez, they represent where the struggle is today. Capitalism dominates. But capitalism will fail. Of that I have faith. The revolution cannot stop.”

His reading of communism isn’t dogmatic. “I was from a Catholic family, full of prejudices. Life was so difficult, so unjust. Architecture was my way of expressing my ideals: to be simple, to create a world equal to everyone, to look at people with optimism, that everyone has a gift. I don’t want anything but general happiness. Why is that bad?”

The master at work: Oscar Niemeyer’s tropical icons, from pasta to piety

Brasilia Cathedral (1958-70)

This was the first building to switch me on to architecture as a boy, even though the closest I got to seeing it was in a book. At Niemeyer’s worst, grandiose form dominates and belittles the visitor. But sometimes when the form is as breathtaking as this, that’s what you want.

The UN Headquarters, New York (1947-50)

By 1947 Niemeyer was world-famous and was invited to join a team, along with Le Corbusier, to design the prestigious world HQ of the newly founded organisation. Niemeyer’s design won over the jury but, perhaps still nervous of the great master, he agreed to collaborate with Le Corbusier. The result, many argue, is mostly Niemeyer’s – though Le Corbusier got most of the glory.

The Duchen factory, São Paulo (1950-51)

Little-known and, alas, demolished in 1980 during Brazil’s military dictatorship, this stunning design allowed Niemeyer to go hell for leather for the first time with the plastic qualities of reinforced concrete, still, at that time, stuck in right angles and straight lines. The factory made pasta shapes, and what Niemeyer delivered, with Hélio Uchôa and Joaquim Cardoso, is a giant fusilli of concrete arches as elegant as a gymnast’s legs.

House, Canoas, Brazil (1952-53)

Niemeyer’s take on the Modernist glass house perplexed the strait-laced European Modernists. Niemeyer wasn’t interested in making a “machine for living in”. A home, he said – his own home – “must be close to your heart”. This is a sensuous house, curvaceous, and able to be opened to the elements. It nestles in the rainforest in the hills high above Rio, the landscape, in the shape of the rocks beneath, literally bursting into the living room.

Church of São Francisco de Assis, Pampulha, Brazil (1940-43)

Niemeyer finally came out from Le Corbusier’s shadow when asked by the mayor of Belo Horizonte, Juscelino Kubitschek, to create the whole civic centre to the city’s new suburb. It was this church, seemingly created from a single ribbon of concrete and decorated with Portuguese tiles, that was Niemeyer’s calling card. Sixteen years later, Kubitschek, now the President of Brazil, would ask Niemeyer to design the nation’s new capital, Brasilia.

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