City rooftops are designed to be sturdy and impermeable to weather. However, in areas facing severe rainfall and weather-related flooding, the high proportions of impermeable surfaces found in cities are not desireable. In the absence of permeable surfaces like dirt, water has nowhere to go and flooding is extremely frequent. Cities like Dhaka, at the northern end of the delta, are largely concrete and have very poor drainage.[1] As there are no materials capable of absorbing water, the streets frequently flood after storms. Furthermore, roofs made of metal, concrete, or similar materials amplify the Urban Heat Island effect, a phenomenon in which the air temperatures in urban areas are higher than those of the surrounding rural areas. As Bangladesh is already facing heat waves, increased urban temperatures lead to higher heat mortality.
Green roofs, or roofs partially or completely covered by vegetation, would impart many benefits for buildings in Bangladesh capable of supporting them, including cooling and flood protection. These roofs have a waterproof membrane covered in a growing medium, in which vegetation can be planted. This vegetation inherently cools the air around it as a consequence of transpiration.[2] On a hot day, the surface temperature of a green roof is actually cooler than the ambient air temperature, whereas traditional roofs can get up to 90 degrees hotter than the surrounding air.[3] This cooling effect would help keep the buildings cooler during heat waves. A Canadian study showed that buildings with green roofs used up to 75% less energy on air conditioning per day.[3] As Bangladesh is already beginning to experience severe heat waves, this would save energy and money for urban buildings that are air conditioned, and lower heat stress for buildings that are not. A study done in Bangladesh by a Malaysian Ph.D. student tracked temperatures of buildings in Dhaka with and without green roofs, and confirmed that green roofs had a significantly beneficial impact on building temperatures in the summer.[4]
Green roofs also act as flood protection by assisting with stormwater management, as they are capable of absorbing a quantity of water before water starts to drain to the streets below. While this would not help with managing flooding when it happens, large numbers of green roofs in cities would raise the threshold of storm intensity for which street flooding would become an issue. In environments like Dhaka and other cities like it, where drainage systems often are clogged and floodwater builds up easily, a greater number of green roofs would lower the burden of increased rainfall.
Green roofs cost money to install, (the Dhaka organization Green Savers installs green roofs for approximately the equivalent of $300 USD), but they are worth the investment.[5] The vegetation protects the rooftop structure from weather and UV radiation, which causes them to have longer lifespans than traditional roofs.[3] The greater expense can pay off with decreased roof maintenance and replacement costs, in addition to the flooding and temperature benefits. While the Dhaka government is currently attempting use subsidies to incentivize building owners to invest in green roofs, the government’s budget is not large enough to accommodate substantial subsidies.[4] This approach might work better if various NGOs contributed funding.
This rooftop model is also only viable for urban environments, as cities are most likely to have the sturdy buildings necessary. While they would work in cities with large, concrete or otherwise-reinforced buildings, many of the houses in rural areas are made of woven bamboo with thatched or tin roofs, which would not be able to support vegetation.[1] This is not too much of an issue, as rural houses in the delta region of Bangladesh tend to be near enough natural vegetation to negate the need for green roofs.
By Caroline Boone
References
- Lamb, Z. (2017, October 5). Personal Interview.
- How Plants Can Change Our Climate. (n.d.). NASA Earth Observatory. Retrieved from https://earthobservatory.nasa.gov/Features/LAI/LAI2.php
- Liu, K., and B. Baskaran. (2003). Thermal Performance of Green Roofs Through Field Evaluation. National Research Council Canada, Institute for Research in Construction. Retrieved from http://nparc.cisti-icist.nrc-cnrc.gc.ca/eng/view/accepted/?id=11095d5f-ac30-41f3-93402f2382ba40de
- Rashid, Rumana (2012). Thermal Performance of Green Roofs at Dhaka City in Bangladesh. Retrieved October 27, 2017 from http://eprints.utm.my/31600/
- Mridha, Limana Solaiman (2017, March 31). Green Roofs. The Independent, Bangladesh. Retrieved from http://www.theindependentbd.com/printversion/details/87734.