Because it's the cheapest solution. Unfortunately they act as heatsinks during the day, and radiate the heat during the night, never letting the surroundings co

Because it's the cheapest solution. Unfortunately they act as heatsinks during the day, and radiate the heat during the night, never letting the surroundings cool. Montreal has made white roofs mandatory for new construction and major repairs/replacement [1]. Large swathes of the city consist of dense residential construction, two- and three-story walkups arranged in rows, all with flat roofs. If you do a flyover, it's easy to tell who'd had their roof redone in the past several years. I imagine with time, this will reduce the "heat island" effect in the urban environment. 1: https://montreal.ca/en/how-to/renovate-flat-roof-or-low-sloped-roof?arrondissement=PMR https://montreal.ca/en/how-to/renovate-flat-roof-or-low-slop...