Economic and environmental benefits of green roofs depend on their performance [41]. Consequently, operation and maintenance of vegetative roofs are extremely important to ensure its positive impacts. Acks [1] estimated that annual O&M cost of green roofs to be between $0.7/m2 ($0.06/ft2) to $13.5/m2 ($1.25/ft2). A uniform distribution was assumed to model the variability of O&M cost for each green roof type.