AbstractThe North American Vegetated Roof Assembly (VRA—a.k.a. green roofs) market is dominated by two different types: modular vegetated roof assemblies (MVRAs) and built-in-place vegetated roof assemblies (BVRAs). In MVRA, the vegetation is pregrown in transportable trays and installed on the roof, whereas in the BVRA, the vegetation develops on the roof. In 2015, the National Research Council of Canada (NRC), in collaboration with the green roof industry and roofing industry, set the national standard for the wind resistance evaluation of MVRA. Practitioners and building authorities have widely used this standard. In 2017, NRC and the industry collaborators started a new research project on BVRA to expand the current standards. The wind performance of BVRA is highly dependent on the growth stage of the vegetation. This variable was investigated by testing four BVRA sources (S1, S2, S3, and S4) at three different growth stages, namely, as-built, 12 weeks, and 1 year of growth. The interaction between the approaching wind speed and the BVRA response was monitored by installing load cells underneath the test mock-up. The load cells were intended to capture the growth media loss as the wind flow develops over the BVRA mock-up. The wind speed at which the vegetated system’s (VS) weight reaches 85% of the initial weight was used to compute the BVRA wind performance. The tests conducted at the three growth stages permitted developing an empirical growth modifier factor (GMF). The GMF is a practical engineering solution to estimate the wind performance of a 1-year BVRA using the as-built performance data. The paper presents the development of this new testing methodology and the standardization process.