Overburden

Not to be overlooked as an option for going over asphaltic roof systems is overburden. While the first job of a roof is to act as the primary defense to prevent water entering into a building, due to current market trends and legislation, this previously underutilized asset is being recognized by owners for the opportunity it can offer when considering sustainability certifications or stretch code compliance. 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." ⁵There are many overburden options and roof assembly considerations when meeting sustainability goals. This can include vegetative roof assemblies, planters, plaza decks, and even equipment such as solar photovoltaic arrays.*

1. Vegetative Roofs: Vegetative roofs consist of plants that are installed in trays or assemblies that are built in-place on the roof with extensive or intensive plant configurations. Tray systems and extensive roofs are generally in depths of less than six inches and consist of shallow rooted plants, such as sedums. Intensive vegetative roof systems have deeper soil depths and can accommodate larger plant installations.
2. Blue Roofs: Blue roofs are systems that are designed to provide stormwater detention. Rainfall onto the roof is managed using orifices, weirs, or other outlet devices that control the discharge rate of rooftop runoff.
3. Blue-Green Roofs: A blue-green roof is a blue roof with a vegetative roof assembly.
4. Purple Roofs: A purple roof is a "sponge" roof that is designed to retain and detain rainwater. It incorporates a sponge-like layer made of hydrophilic mineral wool, a dense polyester fabric detention layer, and may or may not include an additional honeycomb layer to increase the volume of rainwater that can be managed.
5. Rooftop Solar: Solar PV panels come in both single sided and bifacial (double-sided) modules. Modules are traditionally supported by racking systems on the rooftop that are either mechanically attached to the roof, mechanically attached to a structural canopy that is attached to the roof, or held in place with ballast.
6. Agrivoltaics: The emerging practice of agrivoltaics, or agriculture combined with photovoltaics, is showing promise as a mutually beneficial symbiotic solution to not only increase the efficiency of solar panels, but also increase plant size and crop yields by shading and limiting soil evaporation.

Photo courtesy GAF/Siplast

A vegetative low-slope roof.

The overburden's success depends on the roof assembly beneath. For installations such as solar or vegetative, the plants and solar array will need to be accessed for service or removed for repairs or replacement. These anticipated actions require a more robust roof assembly for the long-term durability and serviceability of the complete roof. For fully covered roofs such as vegetative roofs or plaza decks, a smooth modified sheet cap sheet may be the best option. For solar arrays, a cap sheet with a more finished appearance may be selected as it will be seen and weathered throughout the life of the project. Nevertheless, asphaltic roof systems provide the redundancy and durability required to be buried under these assemblies.

Dougherty County Jail, Georgia

Photo courtesy GAF/Siplast

To meet financial and environmental goals, a hybrid system was made for the reroof. This involved a two-ply modified bitumen system, consisting of a high tensile fiberglass scrim/fiberglass mat composite impregnated and coated with high-quality SBS-modified bitumen.

Challenge
In addition to the primary concern of completing the reroof of this facility with minimal impact to daily operations, the Dougherty County Board of Commissioners was seeking a roof system that was both economical and would help advance the County's environmental objectives.

Solution

• Existing lightweight insulating concrete on one half of the building proved its worth as a sustainable insulation and was left in place, while a system combining the unique properties of lightweight insulating concrete and premium-expanded polystyrene foam insulation board allowed the other half of the facility to be reroofed without tear-off of the old tapered system. This saved money and reduced the roofing material waste from this reroofing project.
• In both cases, the use of combined lightweight insulating concrete and premium-expanded polystyrene foam insulation board created proper slope-to-drain and provided sufficient R-value. The material selection also allowed project completion without impacting prison operations.
• Finally, the entire facility was reroofed with a two-ply modified bitumen system. The system consists of a high tensile fiberglass scrim/fiberglass mat composite impregnated and coated with high-quality SBS-modified bitumen. The finish plies are granule-surfaced and protect the base from the elements and abuse. This combination of asphaltic roofing was selected in order to protect the building and maintain health and operational effectiveness for years to come.