FIRST, DO NO HARM

We're talking about a construction project on top of the thin waterproof membrane that protects the house or building structure from water penetration. A poorly designed or constructed deck can impede roof drainage, cause excessive pooling of rainwater and lead to fast rotting of any wood members. Far more serious, the roof membrane is at risk of being lacerated, perforated or over-stressed either before, during or even long after the construction of the deck. Whatever the cause of roof failure, the resulting water penetration is costly and disruptive. Identifying the location of the water intrusion can be surprisingly difficult. I've met more than one homeowner who describes their flat roof as an absolute nightmare. 

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Let's respect and protect your waterproof membrane by making the deck your roof's best friend.

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RIGHT MATERIALS 

With a wide variety of surface options to choose from, it's easy to get lost in the aesthetic considerations and forget about important physical characteristics of the materials. The paver on a bedroom balcony may not be suitable for a public plaza. Some materials and deck designs are much more difficult to install than others. Certain materials do better in one climate over another. Roof drainage and breathability is critical in Seattle's wet climate, but less so Tucson. Not all pedestal systems are created equal. Levels of environmental sustainability and impact vary widely among different material options. Budget-wise, there are cost savings to be found in design considerations, material choices, and construction methods.

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DOING THE MATH

Accurate material calculations are needed prior to ordering deck materials. Exact measurements and careful math are critical to ensuring the construction goes smoothly. Most manufacturers of decking systems assist with calculating materials, but the measurements and details they obtain seldom allow for more than 80-90% accuracy of the exact set of pedestal components needed to fit the rooftop environment. That's not adequate to avoid either work stoppages (as additional materials are purchased and shipped) or cheap alternative fixes that commonly fail over time. Ordering additional materials adds time, money and frustration to the project. Excess leftover materials are wasteful – and all-too-common. Quality decks installations also require additional accessory materials, such as protective slip sheets, that manufacturers neither account for nor supply. 

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THE DETAILS MATTER 

Even with correct materials, an initially handsome deck will quickly "unravel" if not constructed properly. The common results are aesthetic annoyances of inconsistent gap lines, pavers that rock underfoot, and perimeter edges that "migrate" outward. Fixing or tuning up such a deck is no easy task. Best to do it right the first time. The deck builder needs to understand the principles of pedestal decking before he or she begins the work, have the proper materials and tools on-hand, and pay close attention to the key details throughout the construction. 

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TAPERED SLEEPERS 

To build a simple deck over a low-slope roof, it's a common practice among carpenters to lay down tapered "sleepers" across the roof (tapered to compensate for slope). The sleepers serve as loose-laid joists to which traditional long-plank decking is screw-fastened. This is how it's done by thousands of contractors who either don't know much about pedestal decking, can't afford the time to educate themselves, or opt to avoid the additional expense and complications of pedestals.

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Me? I don't like laying sleepers over a waterproof roof membrane because they put the roof at risk. No doubt, there can be savings of both time and materials, and some sleeper decks are carefully built. But even the best of these decks don't last long. This is because a sleeper deck doesn't breathe. Both water and air are trapped beneath the surface, causing the wood and composite members to warp and rot prematurely, especially in a moist climate. Far worse, there are a half dozen ways that a sleeper system – with all the tools, screws, nails and hardware – puts the roof itself at risk both during and long after construction of the deck. 

 

I'm not saying a sleeper system can't be done well. Indeed, I visited a beautiful deck of long-plank Mahogany over sleepers in lovely old-town Kennewick, Washington (desert country with annual rainfall of just 8 inches). The owner was a true craftsman, designed it thoughtfully and built the deck himself. The decking was stable, handsome and durable – and significantly more expensive than most pedestal deck systems. The sleepers were not tapered and the deck mirrored the very low and one-directional slope of the roof. I trust this craftsman and wouldn't want to change anything about his deck . . . unless it was located in Seattle. Or Atlanta, Nashville, Boston, Denver – basically, anywhere it rains.  

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CUSTOM-BUILT PALLETS 

Some contractors build customized modular pallets, framed with pressure treated lumber as the structure beneath cedar or composite decking. The idea is that the "floating" pallets can be lifted and removed for cleaning and maintenance. Not bad in theory but, in practice, the problems are similar to those of a sleeper system. There's no easy flow of air and water. The lumber soon warps, resulting in pallets that lift irregularly, making for an uneven deck with trip hazards that rocks and exerts inconsistent pressure points on the roof membrane.

 

Both pallets and sleepers commonly obstruct drainage and rot prematurely. The decks are noticeably less attractive year-by-year. But more importantly, as the wood structure deteriorates, the sharp points of nail and screw tips may end up dangerously close to the waterproof membrane. That, along with all the construction activity itself happening on top of the membrane, makes me advise against framing a rooftop deck with either sleepers or pallets.