What are some considerations, if an older building is considering creating a rooftop green-space? What are the structural requirements for the roof? Are there long-term costs vs. short term costs? What are some of the benefits to a rooftop garden?
Thank you for asking Toronto Master Gardeners this very interesting question about creating a rooftop green-space on an older building.
Rooftop Bylaw: Toronto is an excellent city to consider planting a rooftop garden. In 2009, Toronto was the first city in North America to adopt a bylaw to require and govern the construction of green roofs. The Green Roof Bylaw sets out a graduated green roof requirement for new development or additions that are greater than 2,000 m² in gross floor area. The requirement ranges from 20-60% of the available roof space of a building. Read more here: https://www.toronto.ca/city-government/planning-development/official-plan-guidelines/green-roofs/green-roof-bylaw/ Unfortunately, the requirements required to convert older buildings to a green roof seem to be less accessible.
Examples of roof top gardens: However, there are some green roof gardens that you can visit and from whom you get direct advice. Five of the biggest are described here: https://locallove.ca/places/look-up-explore-torontos-green-roofs/. I asked for some advice on rooftop gardens from a friend who has a lovely rooftop garden, and he said that he thought that the structural requirements were not nearly as difficult to manage as the issues of sun, wind, access, water, getting materials up there, and safety, all of which are rather site-specific.
Cost: Some of the considerations when building a roof top garden are listed in this website: https://www.thespruce.com/rooftop-gardening-1403340. They do try to outline the major capital costs, but it totally depends upon how much are you willing to spend, and whether you can start small and add on, buying more pots and plants (and soil) as you go. The real expense comes when you want to start hardscaping and building on the roof. Laying tiles or stone, building raised beds and boxes, adding lighting and furniture can all start to add up. Plus, you may need more structural work to support them.
Benefits: The benefits of a rooftop garden have been well explained on the Toronto Botanical Gardens’ website. The TBG has a lovely rooftop garden which you can also visit to view: https://torontobotanicalgarden.ca/explore/themed-gardens/greenroof/
Other references: The Master Gardeners have been asked this question in different forms before, and I hope you will find those answers relevant and interesting:
https://www.torontomastergardeners.ca/askagardener/balcony-garden-4/
https://www.torontomastergardeners.ca/askagardener/rooftop-gardening/
I do hope you proceed with this wonderful project. Good luck!