Portland requires all new buildings with a net building area of 20,000 square feet or more in the Central City Plan District zoned CX, EX, RX, and IG1 to have green roofs (33.510.243 Ecoroofs). Up to 40% of the roof can alternatively house mechanical equipment, skylights, solar panels, wind turbines, rain capture equipment, or common areas. Green roofs are cooler than conventional roofs, help with air pollution, and keep buildings cooler.