THE iconic extension to the National Library of Scotland is suffering from structural problems that require a £12 million facelift in the run-up to its 25th birthday.

The seven-storey building in Causewayside, to the south of Edinburgh city centre, includes the map library and nearly two-thirds of the NLS's whole collection.

It will have its entire exterior replaced in a three-year project starting next year. The library only has one-quarter of the funding and is preparing to request the rest from the Scottish Government.

Structural experts have declared that the £50m building – completed in 1989 and extended in 1995 – is not windproof and watertight.

Linda MacMillan, NLS estates development manager, pointed to problems with the original choice of Newbigging sandstone. She said: "The stone that was chosen probably wasn't the right one. It is very soft and starting to fail, and so are the glazing units and the roof."

MacMillan said: "The fact that it is not wind and watertight creates a risk to the collection - there are bits of the building that are always leaking. It will become more of a problem as the building deteriorates."

She described the building as "very iconic and innovative at the time that it was built" and "might one day be listed". However, the exterior work will not be able to replace the existing design brick by brick for structural reasons, so instead the aim will be to mirror it as closely as possible.

Andrew Merrylees, 80, the distinguished Scots architect who designed the building, said he was "shocked" by the news, which he learned from the Sunday Herald.

He said: "When you design and build buildings they become like your children, and it's a shock to hear that it is ill. The materials were properly tested and approved by the Property Services Agency, the government body at the time."

The work comes on top of a £1.8m refurbishment project which is due to start at the library's headquarters on George IV Bridge in September.