Green Roofs and their Carbon Footprint

Green roofs have become an increasingly popular solution to neutralize a building's carbon emissions, by mitigating thermal heat loss and gain, in addition to adding to the biodiversity of urban areas. However, designers should consider the additional structural measures needed to support the added weight of soil and vegetation and how this increase in structural material will affect the building's overall carbon footprint. This case study uses a typical steel framed structure to analyze the positive impacts green roofs have on the operational energy usage and to compare the embodied carbon footprint of the building once the additional structural material is considered. To complete this analysis, the project uses a baseline building design: a commercial building located in Oklahoma. The structural design criteria were determined using typical dead and live roof loading, which was obtained from the ASCE 7-16, and the structural framing needed to support this loading. From the baseline data, a load determination and resulting structural design was performed for four different intensities of green roofs. Steel design followed the American Institute of Steel Construction 15th Edition Manual. The structural system, including beams, girders, and columns, were increased in size to accommodate the different loading conditions. An environmental analysis was performed alongside these iterations, with the goal of accounting for increased environmental performance due to the green roofs while also taking into consideration the carbon footprint associated with an increase in structural material. Tools used in this analysis include Revit for documentation, RISA 3D to perform structural design, eQuest to evaluate the operational energy, and Tally to account for the carbon footprint of each system. Once the environmental analysis and life cycle assessment was performed for each green roof type, this allowed for a comparison between the increased embodied carbon and the improved energy efficiency of the building due to the impacts of green roofs. The paper includes a precedent analysis that surveys the implementation of green roofs, along with an overview of how green roofs provide energy savings. Findings indicate that even though green roofs increase the need for structural material and increase the overall embodied carbon at the beginning of a building's life, the aid that green roofs provide in the reduction of thermal heat loss or gain over the lifetime of the building is more beneficial.