Maximizing Water Detention Relative to Weight

One of the most important considerations for any green roof design is figuring out how to maximize water storage while controlling the amount of weight being added to a roofing assembly. Hybrid blue/green roof assemblies (featuring a solution like structured drainage media to provide extra water storage space) can offer significant advantages here compared to a conventional soil-only green roof assembly.

For example, to detain 3 inches of water:

• In a conventional assembly with only soil holding water, we can assume we will see roughly 30 percent water storage by volume. In this case, it would require 10 inches of soil to hold our 3 inches of water. Given that water-filled green roof soil weights about 7 pounds per square-foot, we end up with 70 pounds per square-foot of pressure on the roof.
• In a blue/green roof assembly featuring structural drainage media and soil, we can expect again to see roughly 30 percent water storage by volume in the soil, but nearly 100 percent storage by volume in the structural drainage media’s voids. With 4 inches of soil retaining 1.2 inches of water exerting 28 pounds per square-foot of pressure and 1.8 inches of water in the structural void exerting roughly 10 pounds per square-foot of pressure, we end up with 38 pounds per square-foot of pressure on the roof, which is a little more than half the weight we would see with a conventional assembly.

The cost between these two assemblies is similar, though the blue/green roof assembly will often prove to be the less-expensive option. More importantly, the water storage relative to weight in the blue/green assembly is clearly superior to that of a traditional green roof assembly. Given that rainwater management is the main economic driver for green roof development, this ability to store water more efficiently showcases why a blue/green roof assembly is definitely worthy of consideration.

The blue roof allows you to balance vegetation, hardscape and water-retention needs through a customized, multilayered structure.

The key benefit of a blue roof assembly is its ability to allow for the inclusion of paved or otherwise hardscaped areas on a rooftop, enabling economic benefits through more human use of the space while still promoting rainwater management. Conventional paved areas produce rainwater runoff, but hybrid hardscaped areas that promote short-term storage (detention) of rainwater below the pavement help manage the water, relieving the burden on combined sewer systems. In some cases, long-term water storage for reuse (retention) is possible, depending on local regulations, and that water can help supplement irrigation, toilet flushing, cooling tower needs and water features.

Conclusion

As long as there are regulations governing rainwater runoff, there will be a strong case for building owners to consider implementation of green roof construction. And as long as there is potential for owners of buildings (whether residential or institutional) to provide more amenities or greater utility by using the real estate they already own on their rooftop, there will always be a necessity to balance the amount of vegetation on a green roof with space that can be used by building occupants. Small vegetated areas atop a roof will not provide the water capacity necessary to significantly mitigate rainwater runoff, so the industry needs to embrace another option.

That option is the blue roof. Whether vegetation covers 90 percent of the roof or 10 percent, with the right water-retaining layers installed beneath plants, pavers and pool decks alike, it is possible to balance the need for water storage and utility in one setting. There are a number of methods to developing a blue roof, from installing cuspated or entanglement sheets to pouring in expanded shale granules or relying on heavy-duty structural media to provide a void space. Different constructions will make more sense than others depending upon the application. But no matter what you choose, be sure you are enabling an easy lateral flow of water toward drains, finding ways to detain rainwater for plant access and have some means of keeping soil moisture consistent across the rooftop—more often than not, that means using materials that enable capillary action.

It is time that we stop thinking of green roofs in terms of how deep their soil is or how they are installed. More important is how well they fulfill their main function of rainwater management, keeping water safely on the roof so that it is out of sight, out of the way of building occupants using the space, and, most importantly, out of overloaded sewer systems.

Tom Stuewe is the senior product manager - team lead at SOPREMA, Inc. His more than fifteen-year career in the building industry has provided him with dynamic experience in helping to develop industry-leading roof membranes and systems, restoration coatings, low odor and low VOC membrane adhesives, and plaza deck systems for the commercial building envelope. Stuewe graduated from The Ohio State University with a degree in construction systems management.