The rain in Spain falls mainly on the artworks in new museum wing

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The spectacular extension to Madrid's Reina Sofia modern art museum, hailed as a breathtaking achievement when it opened last September, had a serious design fault: the roof leaked.

Days before the new wing, designed by France's leading avant-garde architect Jean Nouvel, was inaugurated by Queen Sofia to international acclaim, drops falling from the ceiling left marks on an important painting by the Spanish master of cubism Juan Gris, Frutero y periodico (fruitbowl and newspaper).

Nouvel's tinted glass and reflective metal structure cost €92m (£63m) and took six years to build. It extends the 18th-century pile which houses works by Dali, Miró and Picasso. The Reina Sofia rivals the Prado museum as a top attraction for visitors to the Spanish capital.

"There were certain construction deficiencies in the design and building of the roof and the drainage system," the director of the museum, Ana Martinez de Aguilar, told MPs.

Nouvel's futuristic buildings, which include the iridescent Agbar tower in Barcelona, are renowned for their innovative use of light and water effects, but this was not what the architect had in mind.

Water dripped into a room in the museum containing an exhibition of Gris works assembled to mark the inauguration of the new annexe. The drips were put down to the carelessness of a cleaner who had left a tap on, with a hose attached that trailed across a roof terrace. But closer examination revealed that the design and construction of the roof just didn't do the job of keeping the rain out.

Builders had no sooner walked off the site than the museum's technical team had to draw up and carry out a completely new set of roof designs "to substitute and repair the defects of the original project", Ms Martinez said.

Those works, including new gutters and carpentry to the façade, were now complete and museum was completely watertight, she insisted. Which is just as well, since the gallery is preparing, jointly with the Prado, a blockbuster Picasso exhibition in June to celebrate the 25th anniversary of the return of Guernica to Spain.

The artist painted his passionate anti-war protest in horror at the German bombing of the Basque town in 1937, and exhibited it in France the following year. The canvas never returned to Franco's Spain, but was shipped to New York's Museum of Modern Art, where it stayed until it returned to Madridamid national rejoicing in 1981. Picasso refused to let the painting enter Spain while the dictator lived and its emotional homecoming was seen as endorsement of the fledgling democracy.

Ms Martinez also confirmed that the 38-ton steel structure Equal-Parallel/ Guernica-Bengsai, by the American sculptor Richard Serra, which the musuem acquired in 1987 for €218,000, was missing, without trace. When the storage company contracted to guard the work refused a request to deliver it last October, the museum launched a police investigation into the sculpture's whereabouts.