Design frequently involves a series of trade-offs to obtain the "optimal" solution to a design problem. Green roofs have many different characteristics based on a variety of variables. Designers typically weigh the impacts of these characteristics in an implicit process based on intuition or past experience. But since vegetated roofing is a relatively complex and comparatively new technology to many practitioners, a rational, explicit method to help organize and rank the trade-offs made during the design process is useful.

This research comprises the creation of a framework diagramming the decision process involved in the selection of vegetated roofing systems. Through a series of expert interviews and case studies, the available knowledge is captured and organized to determine the critical parameters affecting design decisions. A set of six case study projects in North America is analyzed and six critically important evaluative categories are identified: storm water management, energy consumption, acoustics, structure, compliance with regulatory guidelines and governmental incentives, and cost. These six factors are key decision-making parameters in the selection of vegetated roofing systems and they form the basis of this study. They are addressed in the context of a decision support system for green roof designers. A summation of the total importance of the advantages represented by each alternative is used to determine the most feasible green roof system for a particular project. The decision-making framework developed in this dissertation will ultimately be adaptable to digital processing and a computer-based design assistance tool.