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. All water must drain off the roof within 24 hours as per National Building Codes of Canada.
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 per cent slope to maximize water storage as even a one to two per cent slope can prevent the surfaces further from the drain to be fully utilized. However, the National Building Code of Canada requires a minimum slope of two per cent to promote positive flow to drain, which can greatly impact the storage efficiency of blue roofs (see Figure 2).
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 on the membrane type, installation method and workmanship to ensure the roofing system warranties are valid. Lastly, standing water may pose risk of disease and safety issues to maintenance personnel.
Advanced 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 weights in water. They have 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 (see Figure 3[a]). The 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 retrofits.
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.