Other Types of Roofing Systems

In addition to slabs and pedestal systems, other types of roofing systems include conventional ballasted roof systems, extensive roof gardens, and intensive green roofs.

The ballasted systems are typically made of crushed stone between 1.5 inches and 2 inches. Applicable for flat roofs with slopes of no more than 1:6, these systems are made up of just a membrane or a membrane and a substrate material that is loosely laid over a deck using ballast to hold the system in place.

Lightweight interlocking pavers with integral cementitious coating can also be used as the ballast material. Unlike crushed rock, the pavers offer a smoother walking surface and a nicer aesthetic.

While ballasted systems are generally a more cost-effective way to cover the roof, it should be noted that the areas of the roofs that are ballasted are not intended for use other than for servicing of buildings.

Similarly, extensive roof gardens themselves are not designed to support heavy foot traffic. However, walking paths built around the gardens can be traversed. Consequently, designs that incorporate both hardscape and plantings are much more optimal for supporting rooftop activities. These systems typically incorporate plants growing in low-profile planting mediums. They are self-sustaining, have shallow root systems, and do not require artificial irrigation. Grown in a prepared, shallow soil medium within modular trays, they are located on top of a waterproof roof membrane.

The other type of planted roof system is intensive green roofs. The systems support a greater soil medium depth, accommodating a wider variety of plant types and species, including trees. Designed to support human traffic, these systems often incorporate larger paved areas to best leverage the space as an attractive, frequently used amenity.

In addition to the beautiful, inviting atmosphere they create, the green roofs provide stormwater management via rainwater absorption, give insulation to the building, and contribute to a reduction in the urban heat island effect.

While developing a roof deck can be an expensive proposition, the time and cost is usually a worthwhile investment because it creates tremendous value from a sales or leasing perspective.

For example, Aventon is seeing roof decks retrofitted into community gardens, places for wildlife, and pollinators. Additionally, rooftops are used as rainwater harvesting nodes where captured water can be used to irrigate other parts of the complex.

At the same time, developers should be made aware that hardscaped and landscaped roof decks add complexity to the design process, as the roof must be designed for live loads and the structural load of wet soil.

Putting things into perspective, private outdoor living spaces typically provide between 450 to 700 square feet of space, whereas roof decks can offer 1,600 square feet or more. In urban environments, most city lots are tiny and secluded with tall fences and garages, with space for a few lawn chairs at best. The roof deck allows for more open and inviting rooms to be planned, and separations and amenities such as BBQ islands and fire places with retaining wall blocks and other easily built modular wall systems.

For Stantec’s nationwide developments, roof decks have become one of the top amenities of choice and come in all shapes and sizes. On the higher end, one recent project has an elaborate pool deck and a full-scale, professional urban farm generating produce for residents and local businesses. On the smaller side, Harvey and her colleagues are designing contemplative space and yoga gardens or simply providing another place to hang out with neighbors.

Stormwater Management

As owners and developers embrace this growing world of outdoor amenities, the building codes still dictate a certain level of stormwater management that properties must address. While regulations will vary based upon jurisdiction, generally speaking, they are growing stricter as time goes on.

For example, Toronto’s Green Standards require stormwater collection on-site, typically through irrigation, to help minimize the amount of water going into the City’s stormwater system.

Overall, projects are usually required to retain all of their rainfall for a period of time until the water can be released into the city drainage system at a controlled rate.

“Stormwater management is becoming a massive concern in older congested cites,” states Duane Draughon, an outdoor living designer with VizX Design Studios in Lisle, Illinois. “Most are scrambling to figure out the best solution to handle large masses of water runoff during heavy rains.”

While there are several ways to accomplish this, most best management practices (BMPs) involve a large footprint and expense. For example, underground stormwater retention cisterns are expensive and occupy valuable underground space that could be used for resident parking or other purposes.

“Space is limited, and all opportunities for water infiltration, capture, etc., are valuable. These amenity spaces are becoming development and regulatory assets,” says Gary Sorge, vice president, community development, discipline leader, New Haven, Connecticut.

Fortunately, there are a couple BMPs with smaller footprints that actually enhance the ground’s aesthetics: green planted roofs and permeable pavers.

Green roofs absorb rainwater, help keep the roof cool, and can be integrated with a rainwater collection system for reuse.

Regarding the latter, “permeable pavers are advantageous as they help to reduce the area of impervious site coverage and allow for infiltration, reduced runoff, and water quality enhancements by integrating filtration through engineered and natural systems,” Sorge explains. “Roof deck pavers and slabs also allow rooftop surfaces to function as amenity spaces, thus reducing the need to expand site coverage with similar amenities at ground level.”

As highly flexible, simple solutions, the pavers can easily be removed when access of the drainage and utilities below become necessary. If water collection is desired, inlets and downspouts can receive the water that flows through permeable roof deck pavers and slabs, and then divert the flow to water storage cisterns.

“Permeable pavers allow the design team to vary textures while increasing a site’s permeability, reducing a project’s impact to the local storm system,” explains Scott Roberson, AIA NCARB, partner and studio director, Architecture Demarest, Dallas. “While roof deck pavers will not increase a site’s permeability, they do allow the design team to capture impermeable square footage and use it for programmable space, and any rainfall collected from that area can also be easily controlled and directed into the storm sewer,” or to the natural groundwater system.

“Pavers create a heavy-duty drain system without having to engineer the large older-style drainage system,” Draughon adds. “Pavers also remove the need for large retention ponds that are a massive waste of space and a haven for mosquitos.”

As noted, another advantage with permeable pavers is less water pollution. When rain flows over asphalt and concrete, it picks up contaminants, whereas permeable surfaces provide natural filtration, increase evapotranspiration, and mitigate elevated water temperatures caused by contact with impervious surfaces.

Available in a wide variety of finishes, colors, shapes, styles, and levels of permeability, permeable pavers create an appealing and inviting space, in addition to their stormwater management contribution.

“These integrated systems become passive systems tied into other site features rather than dedicating valuable real estate to detention basins or other expensive subsurface treatment measures,” Sorge adds. “Though these subsurface treatment measures are often required with permeable paver/pavement systems, they can be constructed in a single profile, thus reducing surface area.”

Despite all these benefits, Draughon observes that many cities and municipalities are behind the times when it comes to permeable pavers. Still approaching stormwater management with old system and mindset, they turn to piping systems to move the stormwater.

“Permeable pavers can handle thousands of gallons of water in spaces as small as 300 square feet in under a few minutes,” states Draughon. “The water will disappear entirely back into the water table below the ground and this releases significant pressure on old failing stormwater systems. Permeable pavers will also keep the chemicals and toxins out of the riverways because of the oils and trash being washed into the streams and rivers.”

Ultimately, designers and developers must work closely together to strategically address how to best meet the site’s stormwater requirements.

Sustainable Design

In the realm of sustainable design and potentially qualifying for LEED certification and/or the Sustainable SITES program, permeable pavers can make a strong contribution toward that end.

In addition to reducing stormwater runoff, they contribute to the removal of suspended solids and phosphorus in flows, and can reduce surface albedo if specified appropriately, says Sorge. And because they are widespread in the industry, they are easily available and can be locally sourced, typically well within 500 miles of a project site. They can also contribute to reducing heat island impacts by cooling surfaces and increasing reflectivity, particularly in comparison to widely used bituminous surfaces.

As delineated on an American Society of Landscape Architects continuing-education unit titled “Trends in Urban Outdoor Amenity Spaces,” roof deck systems have the potential to contribute to Sustainable SITES criteria in the following areas:

• Site context: location in existing developed area; project connects to multimodal transit networks
• Site design soils and vegetation: reduced urban heat island effect with lighter colors on roof
• Site design materials selection: design for adaptability and disassembly; use of recycled content materials; support transparency and safer chemistry, sustainability in materials manufacturing, and sustainability in plant production
• Site design human health and well-being: support mental restoration, physical activity, and social connection

For projects seeking LEED certification, the following may apply:

• Sustainable SITES: maximize open space; heat island effect
• Energy and Optimization: optimize energy performance through reduction of energy demand
• Material and Resources: recycled content; regional material