Blue roofs pond rainwater on the roof and slowly release it over time. Runoff is controlled at the roof drains through a flow restrictor or a mechanical valve that opens and closes using smart technology. Water is detained on the roof and released slowly to prevent overwhelming the storm sewers during heavy rainstorms to avoid flash floods. The International Building Code (IBC) requires all water must drain off the roof within 48 hours of precipitation, although local jurisdictions may have different maximum drain time.
Blue roofs are often the most economical rooftop stormwater management tool. While effective, this technology has some inherent disadvantages that prevent it from gaining popularity in North America. First, dirt picked up by the runoff and wind-blow debris tend to collect at the control flow drains. Clogging affects the operation and effectiveness of blue roofs.
Also, blue roofs require zero percent slope to maximize water storage as even a one to two percent slope can prevent the surfaces further from the drain to be fully utilized. However, IBC requires a minimum slope of two percent to promote positive flow to the drain (with exception for recovering or replacement), which can greatly impact the storage efficiency of blue roofs (see Figure 2, page 33).
In addition, ponding water exerts hydrostatic pressure, which can force water to leak through small defects in the waterproofing. Consequently, special attention must be paid to the membrane type, installation method, and workmanship to ensure the roofing system warranties are valid. Lastly, standing water may pose a risk of disease and safety issues to maintenance personnel.
Rooftop stormwater management options
With increasing interests in using rooftops for stormwater management, green roofs and blue roofs have evolved to combine enhanced retention and advanced detention to provide greater stormwater management capabilities. We have summarized the advantages and limitations of a few options below. Selection will depend on the design intents and site constraints of the specific projects.
Enhanced retention green roofs
An enhanced retention green roof consists of highly absorbent materials to increase water storage capacity while reducing system weight. Water retention fleeces and horticultural mineral wool are lightweight and highly absorbent materials that can retain seven to 14 times their own weight in water. They have a long history in the hydroponic industry and are increasingly being incorporated in green roofs.
Water retention layers can boost the water storage capacity while keeping the weight low for green roof systems (e.g. a 25-mm [0.98-in.] thick mineral wool mat retains about 24 l/m2 [0.59 gal/sf] of water compared to 12 to 15 l/m2 [0.29 to 0.37 gal/sf] for a typical green roof growing medium of the same thickness).
Water retention fleece and horticultural mineral wool are designed under the growing medium in a green roof system (Figure 3a). Additional water retention lowers irrigation needs, increases resilience of the plants, and reduces annual runoff. Enhanced retention green roofs are particularly attractive on buildings where structural capacity is limited, such as on retrofitted buildings.
Although these systems have significantly higher water storage capacity than regular green roofs, like any retention-based systems, the enhanced retention layer will become saturated eventually and cannot retain more water. It still needs to dry out before it can retain more water, so it is not effective in managing back-to-back rainfall events or large intense storms.
A blue-green roof consists of a green roof installed on top of a blue roof basin—a reservoir formed by a geo-cellular unit (Figure 3b). The upper vegetated portion filters and retains the rainwater and provides all the benefits of a green roof. Excess water infiltrates through the green roof and ponds in the blue roof underneath, which is detained and slowly released through control flow drains.
This lower portion enables the blue-green roof to manage back-to-back rainfall events and large intense storms regardless of the antecedent weather conditions (i.e. even when the upper green roof is fully saturated). Water is released slowly, so the blue roof basin is emptied or ‘recharged’ for the next storm. Blue-green roofs offer reliable controlled release of runoff like traditional detention-based systems.
At first glance, blue-green roofs offer the best of both worlds, combining retention and detention to maximize stormwater management potentials. Unfortunately, as detention is achieved through control flow drains, it also inherits many of the blue roof’s weaknesses such as the dependence on zero-degree slope for water storage efficiency and clogging at the drains can affect the operation and effectiveness. As a result, blue-green roofs work best on dead flat roofs, preferably on large regular-shaped roofs to minimize the number of flow restrictors required for best economics.