Green roofs have become increasingly common in urban areas to slow and reduce stormwater runoff from buildings. However, phosphorus has been found to leach from green roofs. Water treatment residuals (WTRs), a waste product in the water treatment process, may help reduce leaching of phosphorus from green roofs. This study evaluated a green roof retrofit that consisted of a WTR filter at the green roof drain to reduce phosphorus in runoff. Samples of rainfall, runoff from a green roof, and runoff from a regular roof were collected every time it rained and analyzed for total phosphorus (TP), phosphate (PO43−), copper (Cu) and zinc (Zn). Samples were taken January–March 2020 and September 2020–March 2021. Phosphate and TP concentrations were significantly lower (p<0.05) with the retrofit during the winter (Jan–March 2020 and 2021) but not during the fall (Sept–December 2020). The change in WTR efficacy may be due to the WTRs drying out during the summer months, although it appears the WTRs are effective once they have been resaturated during the fall wetting period. This study shows that the WTR retrofit may reduce phosphorus leaching as long as the WTRs do not dry out. The WTRs should be replaced each fall in regions that experience dry summer months to maximize efficacy.