We examined the performance of ecoroofs with different substrate depths (75 mm, 125 mm) as well as a conventional roof using rainfall and runoff data collected between 2014 and 2017 in Portland, Oregon, USA. The two ecoroof plots exhibited similar effectiveness in retaining storm runoff across all storm size categories, and both ecoroofs retained the most stormwater during small storms. Retention ratio (RR) was highest during the dry season and the lowest in the early and middle of the wet season. RR was also positively associated with antecedent dry weather period (ADWP) with significantly higher retention when ADWP was longer than 3 days. Together, event storm size and ADWP explain approximately 50% of RR variation in middle and late-wet seasons. Overall, these findings indicate that environmental parameters such as storm frequency and size had a greater influence on greenroof performance than an additional 50 mm of substrate.