Installing a Roof Deck

The residents of our six-story cooperative would like to turn our roof into a recreational space suitable for small gatherings, eating, sunbathing, and other casual activities. Our existing roof has an underlying wood-deck structure underneath with a black asphalt surface on top that gets very hot in the summer. What are our options for turning our roof into a more comfortable and usable space? Are there any code issues we should be aware of?

First, a word of caution: until a protective structure is installed over the surface, your rooftop should not be used for recreational purposes. People walking or playing on the roof, lounge furniture, equipment, and broken glass and other debris, for example, can damage the roofing membranes and weaken their waterproofing ability. In addition, a large group of people or heavy objects can strain the roof's underlying structure if it is not designed to handle such loads.

Also, keep in mind that installing a recreational roofing system over a defective roof will not correct the underlying problems, only cover them up. Your first step, therefore, should be to have an engineer conduct a roof level survey to check for problem areas, such as ponding on the roof, torn or peeling roofing membranes, shifted parapet walls, cracked brickwork or coping stones, deteriorated caulking, stains, or other signs of water damage. Investigative probes may need to be taken to determine the condition of the underlying roof structure. Any damaged areas or potential trouble spots should be repaired before installing the new recreational roofing system.

The engineer should also perform a structural evaluation and load calculation to determine if the existing roof can support the additional weight of the new recreational roofing system - the so-called "dead loads" - as well as the expected "live loads" (people, chairs, tables, plants, etc.). Heavy planters, for example, or a hot tub will obviously impose greater loads than light patio furniture. Similarly, a surface intended to hold many people at the same time must have greater support than one where only a few will be on the roof simultaneously. Therefore, it's important to make sure the engineer knows the exact purposes for which your cooperative plans to use the roof. If additional structural support is necessary, it could entail the very expensive proposition of tearing up your existing roof and installing additional beams and joists to shore the wood deck underneath.

Wood Decks. One option for converting a roof into a usable outdoor space is a raised wood deck. This system (not to be confused with the underlying wood deck structure that supports the roof) consists of interlocking pallet-like sections of treated wood, typically three-to-four-feet wide by three-to-four-feet long, placed on protective pads to prevent damage to the roofing membranes. (In some configurations, the wood deck sits on top of a steel framing system supported by the parapet walls instead of the roof itself.) The top surface of the deck is raised several inches to allow water to run underneath, so it's important that support beams resting on the roof surface do not obstruct drainage.

Deck sections should be removable so the underlying roof can be easily accessed for maintenance and repairs without tearing out the deck. Some roof deck structures, however, are large, single-section platform decks. With this type of construction there should be enough crawl space underneath for roof surface access, and a guardrail around the deck for safety.

The major limitation with wood decks is that, because wood is a combustible material, it cannot cover more than 20 percent of a roof level, as stipulated by New York City Building Code. (Incidentally, the New York City Fire Code also prohibits barbecuing on the roof for the same reason.) A wood deck system therefore is probably not the best choice if you're looking for a structure that will enable you to use as much of the roof area as possible.

Paver Systems. A popular and probably more common alternative to a wood deck system is installing pavers on the roof. Pavers are durable tiles an inch to several inches thick, typically measuring two-feet-by-two-feet, and they commonly come in two types of materials: concrete and rubber. Concrete pavers are placed on small pedestals so there is a space between the paver and the roof surface to allow for drainage underneath. Rubber pavers often have built-in legs to provide drainage clearance. Because most concrete pavers weigh more than rubberized ones, the added load should be factored in when determining the extent of any additional structural support required to augment the underlying roof structure.

The advantage of a paver system is that the pavers can be easily removed to access the roof surface for maintenance and repair. They can also be replaced individually if damaged. Additionally, pavers come in a wide variety of colors, styles, and finishes for creating a customized appearance. Concrete paver systems run about $15 to $20 per square foot, and rubberized pavers about $10 per square foot. Unlike wood decks, paver systems are not limited to 20 percent of the roof level surface; they can cover the entire roof area.

For both wood decks and paver systems, all the sections should be secure and stable, and surfaces should be flush, with no raised corners or edges that could create a tripping hazard. Sections above drains should be marked so building maintenance can regularly check for blockage or other problems. Some lightweight rubberized pavers, especially those on high-rise roofs, may be susceptible to wind uplift, so they are sometimes secured to each other by caulking or interlocking grooves.

Mineral-embedded Surfaces. A third and usually less expensive option for creating a recreational roofing surface is to apply a mineral-embedded layer. With this type of system, a liquid solution is squeegeed onto the top roof surface, followed by a felt membrane that is absorbed into it. Another layer of solution is then applied on top of the new layer, and an aggregate material - such as crushed gravel, ceramic, or acrylic - is spread over the entire surface. The raised, gritty finish of the embedded minerals provides a protective surface for the underlying membranes. Aggregate finishes come in different colors and textures for creating the desired appearance and feel.

Mineral-embedded surfaces are best applied as a top layer of a new roof rather than added to an existing roof. The topmost roofing membrane has to be in good condition and compatible with the liquid solution used to create the new layer, which can make retrofitting problematic. It's best to check with the roofing manufacturer before proceeding with any such application.

Regardless of what recreational roof system is installed, the New York City Building Code requires parapet walls to rise at least three feet, six inches higher than the finished surface of roofing. Installing pavers will decrease the height difference between the topmost roof surface and the parapet wall. If the resulting height of the parapet wall is less than the minimum 42 inches, it can be remedied by installing railings into the side of the parapet so they extend high enough beyond the top of the wall. Alternatively, the parapet wall can also be built up with new masonry to make it taller, although this method tends to cost more than new railings.

Roofing Warranties. An important consideration when installing any type of recreational roofing system is how it will affect your roof warranty. The typical "no dollar limit" warranty, which pays for the full replacement cost of a new roof for the entire term of the warranty, excludes alterations or additions made to the roof without the manufacturer's authorization. If you install pavers or a wood deck and your roof begins to leak - even if the water infiltration is not caused by the new system - the warranty will be void if the installation did not conform with the manufacturer's specifications. To keep your warranty intact, it's best to consult the manufacturer in advance with installation plans and detailing to make sure the recreational system does not damage the roofing membranes or underlying structure. Above all, you want to get the manufacturer's blessing.

For buildings replacing their roofs and planning to install a roof deck or paving system on top, owners can get what is called an "overburden clause" from the roofing manufacturer. This clause specifies additional structural and waterproofing protection that a recreational roofing system requires. In any case, boards may have to modify their plans as necessary so the manufacturer doesn't find reason to cancel the warranty.

Finally, adding a recreational roofing system requires a permit from the Department of Buildings and, if your building is a landmark property or located in a historic landmark district, a permit from the Landmarks Preservation Commission is also needed.