Now imagine a traditional vegetated roof of customary design. The vegetated roof acts like a sponge. However, when the sponge is wet, the entire system is designed to drain rapidly. As mentioned earlier, the rapid drainage is written into most common green roof guidelines, by requiring a high permeability of the growing media mix (engineered soil), and quick draining drainage layers. The ‘sponge’ (i.e. vegetated roof) is only able to absorb a predefined amount of rainfall until saturated, and then behaves exactly like a non-vegetated roof facilitating instant runoff. This is because the distance to drain, slope, and friction on the surface of the roofing membrane remain the same whether the roof is vegetated or not.
How a vegetated roof can achieve more efficient detention
Though it is possible to achieve detention via increased distance to drain and reduction of the slope, those two changes are often impractical or impermissible. However, the introduction of friction to a vegetated roof system is possible. A new technology involves replacing the traditional fast-draining drainage layer with a ‘friction’ or ‘detention’ layer specially calibrated to flow freely at low volumes but to slowdown runoff at higher volumes. Hence, the friction slows runoff enough to ensure that during large storms, runoff rates are lower than rainfall, thus causing a temporary accumulation of water within the vegetated roof. This accumulated water is detention. The water then slowly drains out, often within six to 12 hours of the large rainfall event.
Friction-detention technology offers uniformity and redundancy. Detention is activated by the friction-detention layer’s own large surface rather than relying on a single-flow restrictor at a roof drain (which can fail or clog). Uniform detention is advantageous because it means it can work on roofs sloped up to two per cent, as well as flat roofs. Blue and blue-green technology that achieve detention through ponding almost always require a dead flat surface, not always possible to construct. If placed on a sloped roof, ponding-based blue/blue-green technology does not use the surface area efficiently, as most of the water is stored on the lowest part of the roof. If there are internal roof drains, the detention area makes up less than half the roof, which is an inefficient use of space. Further, ponding blue/blue-green technology on sloped roofs would concentrate all the weight around roof drains, versus distributing evenly. This is why those options are almost always limited to dead flat roofs.
In addition to better uniformity, the friction-based blue-green solutions distribute water more evenly throughout and across the entire roof because the whole surface of the detention layer is providing resistance. This uniform resistance or ‘friction’ causes temporary accumulation of water above the said layer. This excess water is then filtered back up and throughout the entire vegetated roof profile causing all the pores in the system to fill with water. When a vegetated roof profile is temporarily sunk in water in this manner, it ensures every pore is fully saturated. In contrast, a cross-section of a vegetated roof system after a rainfall event would reveal some wet and dry areas. Rain flows through most vegetated systems by preferential flow paths that prevent 100 per cent saturation of the profile. Fully saturating a vegetated roof via friction-detention has many advantages as it increases retention (all the dry areas are now saturated), provides plants with an increased and more even distribution of water, and as a result, increases evapotranspiration, which helps to recharge the vegetated system for the next rainfall (read “Green roof performance towards management of runoff water quantity and quality: A review” by Berndtsson J. Czemiel for Ecological Engineering, 2010).
The research behind vegetated systems with friction-based detention
The images in Figure 1 show the difference in water filling and emptying pore spaces between a conventional vegetated roof using simple drainage cups to expedite fast drainage, a vegetated roof with a retention layer, such as mineral wool, and a vegetated roof with both retention as well as friction-detention layers. The images are based on data from two years of research and several hundreds of tests at the Green Roof Diagnostics (the firm of one of the authors) rain laboratory (consult B. Garner’s Green Roof Diagnostics data accessible via Purple-Roof Green Roof Modeler, 2019).The full animation can be found on www.purple-roof.com. The images/animation has been produced with the purpose to describe this phenomenon conceptually.
New, innovative technology provides engineers, architects, and building owners the option of incorporating detention into the vegetated roof design for quantifiable and reliable stormwater management. This is best suited to projects in dense urban areas with a large footprint where bioswales or permeable parking is uncommon. Ideally, it is designed at the inception of a project helping to downsize or eliminate a cistern. However, if not designed from the start, a qualified engineer may compute rainfall detention calculations for the site and allow for a vegetated roof detention system substitution to achieve the required rooftop SWM. This helps the building owners achieve ROI by saving valuable space, while meeting municipal requirements to reduce runoff volume and lessen the potential and severity of urban flooding.
Sasha Aguilera, B.Arch, GRP, is design consultant for Next Level Stormwater Management. More than a decade as a designer and technical consultant on various vegetated roofing projects across the country has established Aguilera as one of Canada’s foremost green roof professionals. She was honoured with the F. Ross Browne Award in 2017. Aguilera can be reached via e-mail at firstname.lastname@example.org.
Brad Garner is a stormwater researcher at Green Roof Diagnostics, where he blends his experience as a landscape architect with his software engineering skills. Garner’s custom software aggregates and manages data to evaluate solutions for managing urban stormwater. Garner can be reached via e-mail