OK - so I understand that both white and green roofs are better than standard black or tiled roofs in terms of their albedo characteristics (their reflectivity and propensity to absorb or reflect heat).

But, is a white roof or a green roof better overall? Green roofs have several advantages that I know of over normal roofs - they encourage biodiversity, alleviate rainwater run-off, provide insulation and protection for the roof itself, act as carbon sinks (the plants that make up the green roof take carbon dioxide out of the air via photosynthesis) and they bring some 'green' into our cities which I believe is good for general psychological well-being.

Does anyone know of any comparisons, tools or sources for assessing which style of roof has the edge?