Cities in North America are increasingly adopting policies to mandate green roofs on new buildings, following Toronto's pioneering 2009 bylaw. Over 15 years later, Toronto has installed 640 such roofs, while at least 25 cities now encourage or require them. Recent research highlights their potential to reduce energy use and combat climate change.
In 2009, Toronto, Ontario, became the first city in North America to pass a law requiring green roofs on newly constructed buildings over a certain size. More than 15 years on, the city has installed 640 new green roofs under this bylaw, contributing to a broader continental trend where the number of green roofs has grown significantly.
Green roofs, also known as living roofs, support vegetation on rooftops with systems including waterproofing, drainage, filter cloth, and lightweight growing media. Plants can range from drought-tolerant sedums to shrubs and trees. According to Project Drawdown, green roofs and cool roofs rank #73 among solutions to global warming, potentially reducing CO2 emissions by 0.77 gigatons by 2050 if they cover 30% and 60% of global roof space, respectively, totaling about 407 billion square feet.
Despite benefits, barriers persist. A University of Michigan study estimates a 21,000-square-foot green roof costs $100,000 more to install than a conventional one, with added maintenance and structural reinforcement needs for older buildings. Technical coordination across disciplines and policy gaps in permitting also hinder adoption.
Research underscores advantages. A 2024 Nature Cities study found Seoul's green roofs could cut building energy use by up to 7.7%. Another 2024 study in Nature Urban Sustainability projected up to 71.72% HVAC reduction by 2100 in six cities. A 2023 meta-analysis showed average summer rooftop temperature drops of 30°C. A U.S. General Services Administration study estimated installation costs recouped in 6.2 years through heating and cooling savings.
Policy momentum builds. San Francisco's 2017 Better Roofs Ordinance was the first U.S. mandate. Portland, Oregon, required 100% vegetation on central city roofs over 20,000 square feet in 2020. Denver mandates 20% to 60% vegetation based on building size for structures over 25,000 square feet. New York City's 2019 Climate Mobilization Act requires green roofs or solar panels on new builds and major renovations, with incentives like a $4.50 per square foot tax abatement up to $100,000 and grants covering up to 100% of costs. Chicago offers floor area ratio bonuses for developers.
Notable installations include Chicago City Hall's roof with over 20,000 plants from 150 species, the Vancouver Convention Centre's six-acre living roof, and San Francisco's California Academy of Sciences with 1.7 million plants. Green Roofs for Healthy Cities updated its 2023 policy guide to aid professionals and policymakers.