Unless you live in a sod house or one with a green roof, you probably don't want a tree growing on top of your home.

But when Colorado Preservation Inc. and, later, developer Larry Nelson acquired Hangar 61 a few years ago, that's what was sprouting from the cracked and deteriorating diamond-shaped concrete roof of this idiosyncratic structure at Stapleton.

The buttresses were crumbling. Scores of birds lived in the place, whose windows were broken and boarded up. It was, in short, a mess. For a period, Hangar 61 was threatened by the continued growth at the mega-development in northeast Denver that for decades served as the site of the region's major airport.

It took months for this one-of-a- kind emblem of the golden age of aviation to pass from one owner to the next, be rezoned for office space and eventually be designated a Denver landmark and placed on the Colorado State Register of Historic Properties.

Hangar 61 also needed a lot of work, complicated by its unusual construction. After CPI purchased it, the organization helped win a $200,000 grant to stabilize the building. Later, Nelson, a preservation-minded developer if ever there were one, acquired the property.

Since then, his 620 Corp Inc. has spent about $1.3 million on the project, including constructing a parking lot and adding the frame for a new entryway. He estimates another $300,000 to $400,000 will be necessary to bring the hangar up to the point at which someone can lease or buy it to use as office space.

And what office space, with a heritage that places the unusual building in the category of "icon." That word gets thrown like birdseed, but in the case of Hangar 61, it's valid.

The structure was designed by noted Denver architects Fisher and Fisher and Davis, with engineer Milo Ketchum, the founder of what is now the firm of Martin/Martin Inc. It is a rare surviving example of thin-shell concrete construction and engineering: a diamond- shaped domed roof balanced and supported by two immense concrete buttresses.

Add to that the fact that Ideal Basic Cement Co. housed its corporate airplane there, invoking the name of Colorado's important Boettcher family.

After plowing out the interior, the major work has been repairing the buttresses, or bulwarks, without adding any bulk to their graceful lines. These massive swaths of concrete literally hold down the roof; to fortify them, workers added interior columns, drilling down 40 feet into bedrock and adding caissons to anchor the building in place and stabilize the roof.

The cracks and crumbling material were "upsetting the tension between the roof and the buttresses," Nelson said on a recent tour.

Sprung Construction used huge amounts of epoxy to fill some cracks before smoothing out the exterior surfaces.

Workers hammered loose concrete off the roof - removing the burgeoning cottonwood - and applied a cement-based coating. The electrical system has been upgraded, new pipes installed and the broken windows have been replaced.

The massive hangar doors, which slide in on tracks, were still in perfect shape, Nelson said. A special opening to accommodate an airplane's tail is still operational, as is an unusual scaffolding setup to support those who worked on that part of the craft.

"Structurally, it's in good shape," Nelson said.

It's a rare preservation success story, and a reminder that saving a building takes not just money, but time and perseverance.

After all, several entities were involved in the fate of Hangar 61: the city of Denver, through its Department of Aviation (landowner); the Stapleton Development Corp. (manager of the assets of the old Stapleton site); and Forest City (buyer of that land and developer of the new Stapleton community).

The fight to save Hangar 61 began in 2004, when artist David Walter, a co-founder of Ironton Studios and Gallery, saw the building and fell in love with its lines - cracks and all. He began a campaign to designate it a landmark, stressing its architectural heritage and place in state history.

People responded, including officials of Colorado Preservation Inc. and developer Nelson. With a lot of effort, Hangar 61 began the slow march to recovery.

Can this be more tangled than saving equally notable historic structures at the old CU Health Sciences Center, which sit on land still owned by the University of Colorado but being eyed for development by Shea Properties?

Not really. In both cases, the structures are a reminder of what came before, part of the evolution of a site.

Now, Hangar 61 just needs someone to turn it into an office unlike any other. It's an instant logo. It also is a reminder that when a building is gone, it's gone. But this one is not.

Hangar 61

* What: Thin-shell concrete structure at Stapleton, built in 1959 to house the Ideal Basic Cement Co.'s Fairchild F-27 turboprop

* Where: 8695 Montview Blvd.

* Status: Stabilization and repairs nearing completion

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