Changing Skyline: Glass box is a tweak for the better at Kimmel Center

It can take time for a new building to work out all the kinks, even when the architecture is very good. In the case of Rafael Viñoly's Kimmel Center, which falls well short of that mark, the tweaking has been going on for more than a decade.

In the last year, the Broad Street performing arts center has finally begun to set things right, starting with the acoustics in its Verizon Hall. The Kimmel hopes to cross another big headache off its list Tuesday, when it reopens the dramatic, but brutally hot, rooftop terrace on top of its Perelman Theater. Once people no longer broil there like Labor Day hot dogs on a grill, the Kimmel will be a step closer to becoming the great public space that Philadelphians were promised.

I've already wandered up to the terrace, so I can report that visitors will not break into the slightest sweat. The Dorrance H. Hamilton Rooftop Garden, as it is officially known, has been fully encased in a sheer, climate-controlled glass box that will make it usable no matter what the weather outside. This elegant new room was designed by BLT Architects - yes, you heard me right - a firm that typically specializes in lowest-common-denominator architecture, most notoriously Symphony House.

Enclosing a small rooftop garden may not sound like much of an accomplishment, but the $6 million project is the first in a series of carefully calibrated architectural moves that are intended to win back the public's affection for the luckless Kimmel and ultimately help it to thrive financially. Roofing over the terrace was a key recommendation in the 2011 master plan developed to help the Kimmel overcome design mistakes that made it feel so unwelcoming.

While the old rooftop garden was probably Viñoly's most thrilling space, offering the public panoramic views of the multilevel arts center and the surrounding city, it was nearly useless for revenue-generating events. During the summer, when the sun beat down on the Kimmel's vaulted glass roof, temperatures could rise as high as 120 degrees. Even when it was cool enough to use, noise from revelers would leach into the Kimmel's two main concert halls. In the end, the center's management calculates, the garden could be rented out no more than nine days a year.

The glass veil changes everything. No longer will the public be denied access because of dangerous temperatures. The enclosure also enables the Kimmel to book the light-filled rooftop terrace summer and winter, day and night, 365 days a year, and earn a regular income from party fees. The first wedding is set for Sept. 15.

No doubt there will be many more. BLT, led by founding partner John A. Bower Jr., has given the Kimmel a crisp, thoughtful design that promises to be very functional. It is easily the best work the firm has done in decades, and is further evidence that the partnership between client and designer is crucial to good design.

The idea was to create a glass room inside the glass bubble of the Kimmel's soaring roof. Admittedly, the terrace space is now less of a garden and more of an ordinary room than it was before. The southern views have also been obstructed by a new kitchen (helpfully blocking out Symphony House, on Pine Street). All that means you don't quite get the same heart-racing feeling when you peer down into the Kimmel's magical snowglobe world. The trade-off is that the terrace will have a column-free central space that can seat 200 people at round tables, while still accommodating a decent dance floor.

Part of what makes the room so pleasant are the high-end finishes and precise detailing. BLT removed the ungainly forest of air-conditioning towers and grouped them into one discreet unit. The new ducts and other intrusions are camouflaged in warm anigre wood panels, notched with brass fittings. It's a long way from the ungainly thick-limbed glass entry canopies BLT designed for SEPTA at 15th and Market Streets.

Like those headhouses, the terrace required an elaborate truss system to hold up the glass roof. While the metal structure is not totally invisible, it is light and unobtrusive, yet still capable of supporting lighting and sound equipment.

On the room's west side, the roof cantilevers out beyond the supporting columns, so you get a taste of the expansiveness that used to exist before the glass veil came down. There are still excellent views of Viñoly's ribbed glass vault and the Arco tower (just purchased by the Pestronk brothers for an apartment conversion), as well as panoramic views of the Delaware River and New Jersey. From the Kimmel lobby, you can glimpse the blue-tinted enclosure capping the Perelman roof.

Chef Jose Garces' company was hired to cater the terrace parties. As part of the Kimmel's ongoing effort to lure a more diverse crowd into its building, this winter he will also open a new restaurant on the ground floor in what used to be the bookshop along Spruce Street.

KieranTimberlake, which prepared the Kimmel's master plan, is doing the architecture of the restaurant. It intends to break open Spruce Street's long, dull brick wall to create a street entrance. The architects will build a sidewalk terrace deep enough for a few tables, and give it its own glass enclosure.

Bit by bit, the Kimmel hopes to carry out the other big-ticket items in the master plan, like replacing the overly grand lobby bar with something more quirky.

The Kimmel is following the same path taken by Lincoln Center, another flawed performing arts center, in trying to turn itself into a hip, welcoming hub, as part of a stealth effort to generate new audiences for its programs.

At Lincoln Center, playful architectural designs by Diller Scofidio + Renfro are places to hang out, as well as revenue-generators. People bring their laptops to sit at a small outdoor amphitheater and lounge on an elevated wedge of grass lawn built atop the Hypar Pavilion, among the sexiest restaurants in Manhattan.

Nothing yet at the Kimmel comes close to matching Lincoln Center's enticing, theatrical public spaces, but there is still opportunity. Philadelphia's performing arts center remains a work in progress.