Gliding up the south side of the Philadelphia Museum of Art on platform lifts, past the tap-tap-tap of masons repointing mortar around brightly colored terra cotta and near the oxidized-green griffins keeping watch over the city, Gail Harrity points to the museum's four acres of cobalt and sea-green roof tiles with awe.

"Isn't that fantastic? It's a work of art," she says.

The building that houses the museum's collection may be a work of art in itself, but the main job of the building envelope is protecting the collection, and lately it hasn't been up to the task. The roof leaks. Water has poured in. Harrity, the museum's chief operating officer, says no art has been damaged, but even as it takes on an ambitious expansion, the museum is spending $31 million - more than some arts groups would spend on an entirely new home - to seal and restore its enormous limestone-clad 1920s temple on Fairmount.

It seems the entire museum is a busy if well-ordered construction zone these days. Excavation has begun on the site of a $34 million 440-car underground parking garage and sculpture garden on the west side of the main building. The $90 million Perelman annex is nearing completion, with an opening planned just after Labor Day.

And in the fall work began on the exterior of the main building, well before a Frank O. Gehry reimagination of the interior has begun. The current price tag for all this work: $590 million. The museum's city-side face is shrouded, Christo-like, in scaffolding. From the safety of the structure, workers are repairing and replacing stone with stock from perhaps the same quarries that supplied the original stone; cleaning terra cotta with lasers and dry ice; and replacing gutters with a new system featuring a second gutter under the first.

Preservation architects are working with art conservators from the museum's own staff to make many of the decisions.

"I think perhaps the museum views the building as the biggest object in its collection," said Michael Holleman, principal in charge from the Vitetta Group, which is overseeing the project.

After eight decades of exposure to the elements, it's time for action. The gutters were redone last in 1956, and the last repointing job was in 1976, architects say. The entire project should be done by spring 2009, Harrity said.

"It'll be in pretty good shape once we stop the water pouring in . . .," she said Monday during a tour of the project.

Viewed from ground level, it's hard to appreciate the building's brilliantly colored and sometimes riotous level of detail - lions' faces lining the roof perimeter every few feet, for instance. It's equally difficult to see the decay, but a close look reveals lots to do.

A few of the enormous glazed tiles on the roof are cracked. Rub your hand along some of the limestone that encases the unseen brick structure and a bit comes off on your hand. Gilt that once covered the griffins and other rooftop sculptures has worn away, though a fleck or two can still be seen. And of course, there's water to be kept away from the priceless collection of delicate art below. After all, what good is climate control if it's raining inside?

"What this project is about is stopping water infiltration into the exterior envelope of the building," said Holleman, whose Philadelphia architecture firm has also worked on the restoration of the Academy of Music and City Hall.

What's surprising, actually, is how thoroughly the building's original architects understood the way materials would hold up over time, Holleman said.

The glaze on the trapezoid roof tiles, greenish on top and a deep blue on the sides, shows beautiful spider-webs of surface cracking, but very few must be replaced - perhaps a few dozen out of thousands - even if the structure beneath them has been letting in water.

"Our belief is that they are in about as good condition now as the day they went down," Holleman said.

Tiffany-designed wrought iron and bronze grillwork pieces in front of the windows were removed this week, and will be shipped to Utah for restoration.

Stone has arrived from quarries in Minnesota, one of which was the source for the building's original limestone cladding. Nan Gutterman, another Vitetta architect leading the project, says only three large pieces and about 20 smaller ones have to be replaced.

Strange to think, but even after the two-year restoration is done, the Philadelphia Museum of Art will still be incomplete. Never finished were the sculptural scenes in all the pediments. One was done, a triangular space containing bright terra-cotta figures of Zeus and other Olympians. But seven are blank, still only brick walls.

The visual vacuum caught the attention of at least one Art Museum donor. A retired secretary from Glenside, Ethel Elizabeth Greenwood, died in 1992 and left about $1.04 million to the museum with the stipulation that it be used to "add the sculptures in any or all of the uncompleted pediments around the Philadelphia Museum of Art."

A decade and a half later, the museum does not have a current cost estimate for fabricating the remaining sculpture, Harrity said. In 2002, the museum obtained an estimate of $1.8 million to complete one pediment, which exceeded the balance of the gift at that time. (One pediment, depicting wisdom, was designed for the south side, but was never built.)

"Seven to go," Harrity said jokingly of the apparently fallow project.

The principal of Greenwood's gift is held in a restricted account, and the principal and interest are currently valued at $2.04 million, the museum says.

The terra cotta that is in place, both in the one pediment and elsewhere near the top of the building, will be restored as necessary. A few lion heads and other ornaments lining the roof perimeter are missing, Harrity said.

Not every cracked piece of terra cotta will be replaced. If a crack is small enough that the piece is not in danger of falling or of absorbing water, it might be left alone.

Sometimes the wisest preservation decision is akin to medicine.

Says Holleman: "It's like the line 'First, do no harm.' "

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