After three consecutive summers with temperatures exceeding 50°C (122°F), Phoenix has become the first major U.S. city to mandate reflective "cool roofs" on all new residential and commercial construction. The ordinance, passed unanimously by the city council on Monday, requires roofing materials with a solar reflectance index of at least 0.65.
The science behind cool roofs is straightforward: dark surfaces absorb sunlight and convert it to heat, while light surfaces reflect it. A standard dark roof can reach 70°C on a summer afternoon; a white-coated roof under the same conditions stays below 40°C. The result is dramatically lower cooling costs and reduced strain on the electrical grid.
Thermal imaging shows a 30°C difference between adjacent dark and cool roofs."This isn't just about comfort — it's about survival," said Councilmember Diana Reyes, who sponsored the ordinance. "We had 31 heat-related deaths last summer. Cool roofs are one of the most cost-effective interventions we have."