Green roofs can help cool hot cities, and the Weather Research and Forecasting model lets us test how much. We upgraded its multi-layer green-roof scheme, adding plant shading, realistic heat flow in wet soil, evaporation from soil and wet leaves, and water uptake from deeper soil. Tested on a green roof in London, Canada, the new scheme cut errors in heat entering the roof by about three-quarters and captured bursts of evaporation after rain, improving how we study green-roof cooling.