Green roofs are not created equal: the hydrologic and thermal performance of six different extensive green roofs and reflective and non-reflective roofs in a sub-tropical climate

Abstract

Green roofs have the potential to retain stormwater on the roof surface and lower the thermal loading on buildings. Because of this, the greatest environmental benefits from green roofs might be achieved in subtropical climates characterized by high temperatures and intense rain events. There is, however, little research to support this. In a replicated study in Texas, we compared the performance of six different extensive green roof designs vegetated with native species, to non-reflective (black) roofs, and reflective (white) roofs. Preliminary hydrologic and thermal profile data indicated not only differences between green and non-vegetated roofs, but also among green roof designs. Maximum green roof temperatures were cooler than conventional roofs by 38°C at the roof membrane and 18°C inside air temperature, with little variation among green roofs. Maximum run-off retention was 88% and 44% for medium and large rain events but some green roof types showed very limited retention characteristics. These data demonstrate indicate that: 1. Green roofs can greatly affect the roof temperature profile-cooling surface layers and internal space on warm days. 2. Green roofs can retain significant amounts of rainfall, this is dependent on the size of the rain event and design and can fail if not designed correctly. We suggest that as green roofs vary so much in their design and performance, they must be designed according to specific goals rather than relying on assumed intrinsic attributes.

The Influence of Hydrologic Parameters on the Hydraulic Efficiency of an Extensive Green Roof in Mediterranean Area

“…Carter and Rasmussen [12], in Georgia, have shown that, for small rainfall events (<25.4 mm), the 88% of the stormwater volume was retained, for an average rainfall (25.4-76.2 mm) more than the 54%, and for the events higher than 76.2 mm, 48%. Similarly, Simmons et al [23], in Texas, have demonstrated that, while small rainfall events (<10 mm) were totally retained by the green roof, rainfall events with a total depth of 49 mm and a duration of 140 min produced a retention efficiency up to 44%. Villareal and Bengtsson [25] (in Lund) have observed that the retention of a green roof largely depends on the intensity of rainfall; in fact, lower rain intensity events produce greater retention.…”

“…Locatelli et al [20] have noted that the delay of the peak flow rate was up to 40 min, while Getter et al [10] and Villarreal and Bengtsson [25] have found minimal delays. However, for the majority of studies the delay of the peak flow rate was found to be up to 30 min [12,23].…”

“…As previously mentioned, literature studies have shown that green roofs have a positive effect on the reduction of stormwater runoff [12,15,16,[19][20][21][22][23]. The water retention capacity mainly depends on the physical features of the green roof (number of layers and materials, soil type and thickness, type of vegetation and percentage of roof covered, roof geometry), and the weather conditions (length of antecedent dry weather period, season/climate, characteristics of rain event like intensity and duration) [14].…”

Environmental impacts of reflective materials: Is high albedo a ‘silver bullet’ for mitigating urban heat island?

“…Compared to reflective roofs, green roofs have better performances in cold climates and in non-insulated buildings [158,159]. Under certain conditions, green roofs are more effective in reducing surface temperature [52], indoor air temperature [160], and annual building energy loads [161]. Even in terms of pavements, albedo is not the only determinant parameter that affects the thermal performances.…”

İstanbul’un Yağmur Suyu Taşkınlarına Karşı Yeşil Teras Çatıların Önleyici Rolü

“…Bu sayede şiddetli yağmurlarda yeşil çatılara gelen yağmur suları tutulur ve belli bir süre sonra şebekeye gönderilir. Böylelikle şebekelerdeki yağmur yükü hafiflemekte, debi pikleri düşmekte ve taşkın riski azalmaktadır [17][18].…”