Photo courtesy of Standard Industries

A vineyard and bee hives on the roof of City Hall in Paris.

Let’s Start with a Definition

In roofing, overburden is defined as “any manner of material, equipment or installation that is situated on top of, and covering all or a portion of, a roof or waterproofing membrane assembly.”¹
This excludes thermal insulation but includes:

• Planters and everything they contain;
• Vegetative roof assemblies in trays, mats or other similar containers;
• Loose growing media, gravel, sand or any other granular material;
• Non-structural water features, inclusive of the water like water tanks, stormwater retention/detention, etc.;
• Void fill like EPS, XPS, and/or Polyiso rigid foam
• Plaza deck materials like tiles, pavers, supporting pedestals or other similar materials
• Equipment and/or installations like PV arrays²

Photo courtesy of GAF

Adhered TPO re-roof with solar photovoltaic panels on the Atlantic City Convention Center. Atlantic City, NJ.

Introduction

Since its inception a roof’s primary purpose has been to shelter its inhabitants from the elements, but now the underutilized potential of roof surfaces is being realized. For roofs with large surface areas, the potential for large overburden installations, such as solar, vegetative roofing, or amenity decks can be exceptional. Even smaller roofs can have overburden that make a significant impact on the sustainability goals of a building including: increased energy efficiency, rainwater retention, energy generation, biohabitat restoration, food production, reduced urban heat island effect, and outdoor space.

Once thought to be a ‘burden’ for a roofing system, overburden benefits are increasingly emerging as a roofing advancement that's here to stay. In fact, according to Roofing Contractor Magazine, vegetative roofs alone are expected to grow to $14 billion per year by 2026.³ Similarly, according to the Solar Energy Industry Association (SEIA), “Double-digit growth in commercial solar volumes is expected for the next two years.” And, if there is “the passage of federal clean energy incentives, the commercial solar forecast would increase by 21 percent from 2022-2026.”⁴ Due to the increased frequency and intensity of severe rain-related events, roofs are now considered more often as part of an overall resiliency strategy to reduce flooding from rainwater runoff.

However, selection of the overburden system is only part of the design. Selection and design of the roof membrane, the waterproofing layer that protects the building, is critical for a long-lasting installation. Failure of the membrane, whether it requires repair or replacement, may necessitate removal of the overburden. The removal of the overburden can result in lost energy generation for solar installations, and loss of rainwater capture for both vegetative and blue or purple roof assemblies. Appropriate selection of the entire assembly, including proper detailing and integration of the roof assembly, as well as installation, are paramount to the overall success and longevity of the overburden system.

Overburden for Resilient Design

As defined by the Resilient Design Institute, “Resilience is the capacity to adapt to changing conditions and to maintain or regain functionality and vitality in the face of stress or disturbance. It is the capacity to bounce back after a disturbance or interruption.”⁵ Climate change related disasters are increasing in frequency, intensity, and cost. Buildings need to be prepared to manage not only the extreme weather related events but also the power outages, and health and safety risks that come along with them. Passive energy efficient design strategies, such as robust continuous insulation and airtightness, ensure that building tenants will be able to shelter in place for extended periods without power and not suffer consequences of extreme heat or extreme cold. By focusing on energy-efficiency first and then adding overburden strategies such as rainwater management, onsite renewables with battery storage, and food production to the roof, building tenants may have little interruption to normal daily activities.

While each property is unique, there are many overburden options and roof assembly considerations to meet increasingly stringent sustainability requirements. After the selection of the overburden type, the roof assembly will need to be determined, which is dependent on the type of overburden and the ultimate use of the roof space. The success of the overburden is dependent on the roof assembly beneath. Installations such as solar or vegetation, the plants and solar array will need to be removed for repairs or replacement. Considerations such as increased foot traffic, overburden movement, and leak detection should all be incorporated into the holistic overburden design. For example, a more robust membrane and integrated leak detection may be warranted for the long-term durability and serviceability of the complete roof and overburden assembly. Another approach is to look at “future-proofing” the roof with a performance-based view to the energy efficiency requirements for thermal insulation, thermal bridges, and air tightness versus simply following the prescriptive code minimum requirements to ensure a long-lasting roof assembly and to minimize the number of roof replacements over the life of the building.

Fun fact: According to Standard Industries, the rooftops of NYC’s 1 million buildings cover nearly 40,000 acres. Under the city’s landmark Climate Mobilization Act, all new buildings must be fitted with solar energy or a green roof. Think of all of the overburden possibilities just in NYC!

Photo courtesy of Standard Industries

Overburden.

Vegetative Roofs

Vegetative roofs can be installed in trays or built in-place on the roof with extensive or intensive plant configurations. Tray systems and extensive roofs are generally in depths of less than 6 inches and consist of shallow rooted plants, such as sedums. Intensive vegetative roof systems have deeper soil depths and can accommodate larger plant installations. Soil mediums are engineered for specific installations that retain moisture, and are lighter when saturated so as to not overload roof structures. The benefits of vegetative roofs (also called green roofs, living roofs, and garden roofs) are well documented and can help achieve carbon reduction goals, and building resiliency through waste diversion, rainwater management, energy efficiency, increased biodiversity, and urban agriculture.

The benefits of vegetative roofs include and are not limited to:

1. Extending the useful life of the roofing membrane by protecting it from expansion/contraction stresses due to temperature extremes, and ultraviolet light degradation.
2. Managing and retaining rainwater in the substrate for the plants to use. The retention of rainwater helps not only to reduce the volume of water flowing onto impervious surfaces and into the sewer system, but reduces the strain on aging sewer infrastructure by increasing the speed of the flow of water.
3. Increasing thermal efficiency by reducing heat loss.
4. Biodiversity restoration in urban areas, which may also reduce the urban heat island effect.
5. Reducing the ambient temperature on rooftops, thereby aiding solar arrays to function more efficiently and produce more energy.
6. Supplying locally grown agriculture in urban “food deserts.”⁶

Blue Roofs

Beyond urban agriculture and clean energy generation, rooftops are now also improving on the rainwater management capabilities of vegetative roofs with “blue roof,” “blue-green roof,” and “purple roof” advancements. A blue roof is “a roof designed for the retention of rainwater above the waterproofing element of the roof. Blue roofs are typically flat [or low slope], without any fall, with control devices to regulate drainage outlets that enable water to be retained or drained.”⁷

“Blue roofs are systems that are designed to provide rainwater detention. Rainfall onto the roof is managed using orifices, weirs, or other outlet devices that control the discharge rate of rooftop runoff. By reducing flow rates from rooftops, blue roofs are effective in reducing the size of downstream detention basins.”⁸ Blue roofs retain water through passive orifice restriction or detain water through an active mechanical valve or gate apparatus. Similar to a bathtub or sink overflow drain, blue roofs also incorporate an overflow drain to prevent spillover.

Image courtesy of NJ.gov

Figure 1: Section detail of a blue roof.

Image courtesy of NJ.gov

Figure 2: Clog prevention for blue roofs.