One of the more interesting new bits of construction in downtown Charleston is also the least visible.

And it’s something most people probably hope they’ll never see.

Still, the story behind the design and construction of the new $1.8 million helipad on the roof of Roper Hospital is intriguing, even if the structure itself will only be fully appreciated by pilots, paramedics and their patients.

It’s just not every day that architects and engineers are asked to retrofit the roof of a seven-story building to handle a 6-ton helicopter — and do it next in a historic district with the extra scrutiny that comes with it.

Peter DiNicola, Roper St. Francis’ director of plant and biomedical engineering, says simply, “To do it right was not easy.”

The first challenge was finding the best spot on Roper’s complex, considering prevailing winds as well as nearby obstructions. The best location was the South Tower, built in the late 1970s.

Daniel Scheaffer, an architect with LS3P, says handling the historic district part may have been one of the easier elements. Instead of a vertical fence, the pad is surrounded by more of a horizontal metal mesh —one that can’t be noticed from those looking at the hospital from a distance.

“Charleston is known for its skyline,” he says. “We wanted to be as unobtrusive as possible. We want to be able to keep the pad low.”

The greater challenges were more engineering than aesthetic in nature. These included determining how to distribute the structural load; getting pipes up to the roof for firefighting purposes, ensuring the penetrations that tie the pad to the building don’t leak; creating two exits off the roof; and complying with codes for the city, State Fire Marshall and Federal Aviation Authority.

Brian Bates, an engineer with DWG Consulting Engineering, says another challenge was rerouting existing roof vents.

“You don’t want to blow a lot of air straight up when you’re landing a helicopter,” he says.

Also, the design had to include an oil and water separator so any leakage from the helicopter while on the pad doesn’t ultimately end up in the Ashley River.

The pad is meant only for copters to touch down, drop off and take off, so they were spared the extra complication of a refueling station.

The pad is connected to the hospital by a short walkway that leads to a set of doors that swing in, not out. An LED lighting system on the roof can be activated by the helicopter’s radio.

An elevator is nearby to take patients directly to the emergency room, and Roper officials estimate the pad can shave 15 minutes off the time it would take to transport a patient from the helicopter landing pad at the Bee Street garage a few blocks away.

The pad also includes a foam fire suppression system capable of dousing the entire roof.

“God forbid we ever have to use it,” DiNicola says.

NBM Construction built the pad, which was funded through philanthropic donations. It received its first patients earlier this year.

It’s too bad those using it will be either too busy or too ill to notice the views, but, hopefully, they may get a chance at some later date.

Reach Robert Behre at 937-5771.