There’s More Than One Way to Build a Roof Deck

The amenities race in New York City co-ops and condos has gotten crazy, but for many, a rooftop deck still takes the prize. Even if the view isn’t spectacular, being able to relax al fresco in a secluded setting is a top-tier luxury. So it’s no wonder older buildings have been installing wooden decks or pavers up on their roofs, provided the building is strong enough to carry the extra weight. 

For buildings without that kind of muscle, however, decks have been an impossible dream, since strengthening the underlying structure usually involves tearing up the existing roof and putting in new beams and joists – a huge undertaking that would cost a small fortune. But there is a more affordable alternative: a steel overbuild. Simply put, it’s an elevated platform supported by the parapet walls, or by short columns set directly on top of existing support columns, thereby taking the load off the roof. If your co-op or condo board has been dreaming of getting all decked out, here’s what you need to know. 

“Think of an overbuild as a table that supports the deck flooring itself, but hovers on top of your roof without actually touching it,” says Eric Cowley, president of Cowley Engineering, who supervised the recent installation of a steel overbuild at the Hudson Gables co-op at 125 Cabrini Boulevard in Washington Heights. The board had something relatively modest in mind – a freestanding, elevated deck, roughly 20 by 40 feet, on the west side of the building, facing the Hudson River and the George Washington Bridge. Problem was, the roof joists at the six-story, 39-unit property, which was built in 1927, couldn’t handle the load. 

“In many older steel- and wood-frame buildings like this one, there are flat ceilings on the top-floor apartments, but the roof is sloped to allow for rainwater drainage,” Cowley explains. That creates an empty space called a cockloft – similar to an attic – and the angled rafters above it are rarely strong enough to carry the load of a recreational roofing system as well people, chairs, tables, and plants. 

In narrow buildings, an overbuild – basically, a steel frame made of joists supported by girders – can be constructed in several ways. It can rest on the parapets, or sections of the parapet walls can be dug out to create pockets, where the girders are mortared in. The larger roof at Hudson Gables made those configurations impossible. But overbuilds can also be supported by “stub” columns, created by extending internal steel columns upward through the roof membrane, which then serve as the table legs. 

The structural steel columns at Hudson Gables were located in the right place and were also strong enough to make this scenario possible. Completed in 2016, the handsome 790-square-foot elevated deck, which has a composite-wood plank floor and a 42-inch railing, offers breathtaking views. The price tag was $69,000. It’s so popular with residents that the board had to install a walkway made of rubber pavers between the bulkhead door and the deck’s stairs, Cowley says, “to protect the roof from spilled cocktails and high heels.”