Use Caution When Decking the Roof

Remodeling magazine

Writer: Jim Cory

North Carolina remodeler David Tyson sees them all the time down at beachfront properties on the coast. “Somebody gets a bright idea: Why not put a deck up there? But with no thought to loads, weight, or waterproofing.”

He’s talking about decks that are built over existing living space. And one of the big problems with them, he points out, is that because the deck covers a roof, any problems that develop with the roof will mean that the deck has to be removed in order to accomplish repairs.

Waterproof, waterproof, waterproof

Decks over living space are usually desirable because of the view they afford. They seem simple, but actually are fraught with potential for problems, say contractors, unless you’re used to doing that kind of job and are familiar with the requirements.

Chief among the requirements is protecting them from moisture. “Waterproof, waterproof, waterproof,” Tyson reiterates. “Anytime you build something you walk on over living space, you’ve got to make sure no water comes in.”

Water leakage into the space below can result from several improper installation procedures. First off, a wood deck that isn’t properly flashed where it’s bolted to the house will sooner or later leak. A second problem is load. Deck posts placed on flat roofs, even those resting on 2-by block to distribute weight, risk puncturing the roof membrane, causing leakage.

Decks without wood

Some contractors get around that by treating the deck like a roof, instead of the other way round. Tom’s River, N.J., remodeler Eric Borden builds decks over living space on a fairly regular basis, mostly as part of a larger remodel. On a recent project that involved interior alterations and construction of a family room, the client wanted a deck on top of the addition to capture river views. Borden gave the roof a 1#142;4-inch pitch for drainage and made the roof itself the deck by applying a fiberglass coating to the roof surface, which creates a seamless membrane similar in texture and density to a boat bottom.

The fiberglass coating system he uses goes down in one layer and takes a day to dry. Posts for the railing system are attached through the deck and counter-flashed to prevent water coming through the posts. Proper drainage off the nearly flat surface is key. “We’ve done them with scuppers, outside roll-offs, you name it,” Borden says.

Call an engineer

The last time Jim LeFaivre, LeFaivre Construction, Taneytown, Md., built a deck over living space, it involved a 12-by-15-foot treated wood deck on a flat roof off a second-floor bedroom. “I was very apprehensive,” he recalls, “but we’d done a lot of work for these people.”

LeFaivre first verified that the structure could support the deck, then he coated the roof surface with rubber before installing 4-foot-square sections, so that the sections can be lifted individually in the event repairs need to be made.

Tyson suggests it’s often wise to get an engineer involved. “Make sure your roof load can support it or that you put direct support under it going down to load-bearing walls so that they support the weight,” he says. Calling in an engineer is “cheap insurance. That way, the engineer gives you specs on how to do it, the liability is off the contractors’ shoulders, and you have something to hand the inspector.”