The Pros and Cons of Green Roofs: More Complicated Than You Think
Of course I love the idea of green roofs.
As a volunteer in New York’s community gardens, I recognize the necessity of more green space in our cities.
On some level, the question of green roofs is simple. The more green space, the better.
However, my experience in environmental movements and volunteer spaces is that it’s easy to get caught up in good ideas, and hard to execute them and then manage them long-term.
Green roofs come with challenges that need honest discussion.
It comes down to the fact that stewarding land takes resources of some kind.
It’s not magic. Green roofs don’t maintain themselves. That will either mean volunteer stewards or funding from somewhere.
Understanding what successful green roof stewardship requires can help you make informed decisions whether you’re starting your own rooftop garden, advocating for green roof policies, or simply trying to understand why some green roofs flourish while others fail.
What Are Green Roofs?
There are lots of fancy definitions for it, but it comes down to plants that are on a roof. If it’s a roof, and it’s now green because of plants, it’s a green roof.
Most formal definitions require there to be soil built into the roof itself, at least 2-6 inches deep.
The concept has gained serious policy momentum in recent years. New York City’s Local Laws 92 and 94 now require that roofs of new buildings and those undergoing major renovations be partially covered with green roofs, solar panels, or a combination of both.
I live in a new building. By law, it has to have a green roof. I’m very happy we have it.
These covered construction projects include new construction, vertical and horizontal extensions, and major modifications to the roof requiring a permit.
Policies like these reflect a tide of people who view green roofs as essential infrastructure.
Cities need more green space, better stormwater management, and strategies to reduce urban heat. Green roofs check all these boxes in theory and more. And they do so without requiring extra land, since they go right on the roofs.
The Benefits of Green Roofs: A Green Urban Fantasy
All right, let’s get into the benefits. This alone could be a 10,000-word article, so I’ll keep it brief.
Solve The Land Use Challenge
Unlike parks and gardens, green roofs don’t require extra space. We don’t have to acquire more land, we just need to activate the unused rooftop space we already have.
It solves one of the biggest challenges of cities: land use.
Rooftop space in New York City covers about 40,000 acres. For context, Central Park is 843 acres. This means it’s one of the few places where, at scale, cities could genuinely transform into sustainable spaces.
Environmental Advantages
The environmental benefits of green roofs are well-documented and genuinely impressive.
Stormwater Management + Surrounding Water Quality
Green roofs absorb rainfall that would otherwise rush into storm drains, reducing flood risks and decreasing the burden on urban water infrastructure.
This is key for more than just serious storms. Many U.S. cities have combined sewer, which means rainwater and toilet water go to the same places. When it rains a lot, about an inch in New York, they have to discharge untreated sewage into the water. This is a disaster for these water ecosystems.
By absorbing rainwater, less of it goes into the sewer, reducing the amount of untreated sewage discharge.
Urban Heat Island Mitigation
Concrete and buildings re-emit heat. This makes places with fewer plants and more concrete hotter. This is called the “urban heat island effect.” It means more hostile summers and higher AC bills (if you have AC.)
Green roofs are like air conditioners. The evapotranspiration from plants creates natural air conditioning that can reduce surrounding temperatures by several degrees.
They prove both direct and ambient cooling effects.
Habitat Creation
From pollinators like bees and butterflies, to insects, to birds, green roofs can be as valuable for habitats as gardens.
My building’s rooftop has lots of pollinator plants. I love watching the bees do their thing.
This is especially important in urban environments, where native plants and pollinators have faced the destruction of most of their habitat. Here are two native bees from my roof!
Energy Efficiency
Whether it’s summer or winter, green roofs provide insulation that can reduce both heating and cooling needs for the buildings beneath them. The soil and vegetation create a thermal barrier that moderates temperature extremes.
Healthy Food
While an extreme example, Brooklyn Grange Farm does some serious vegetable gardening on a roof in Brooklyn.
It doesn’t need to be this intense, though. You could have one raised bed and grow a surprising amount of vegetables.
Mental Health
It’s clear that being around plants is good for our mental health. Why do we like being on green roofs in the first place?
These green roofs provide residents access to a green space they otherwise wouldn’t have.
An Economic Case for Green Roofs
While green roofs are more expensive than conventional roofs, they provide relative benefits over a 50-year lifecycle (e.g., reduced energy use, surrounding temperature, noise pollution, stormwater runoff).
The reduced costs of these externalities are the economic case for green roofs.
This is a great argument in theory, but in practice, developers aren’t as interested in those benefits because they don’t capture the economic benefits. Residents would, in the form of cheaper bills, and we all would, in the form of a better quality of life and better protection from storms.
This is a case for local governments to subsidize the construction of maintenance of green roofs.
The Hidden Costs and Challenges
The Ecological Challenges and Limiting Options
The environmental benefits I just described depend entirely on proper design, appropriate plant selection, adequate structural support, and ongoing stewardship.
Even well-designed green roofs face inherent challenges.
We are starting with a narrow assortment of plants that can survive in shallow soil, extreme temperature swings, high winds, and intense sun exposure.
A green roof is a bigger challenge than a typical garden in these respects. (I’ve seen small trees pulled out of their roots by strong winds. That’s what happens when the soil is only four inches deep.)
Success depends on matching the right plants to the specific conditions of each roof.
It also means green roofs often need additional irrigation during establishment and dry periods.
These aren’t one-time installation costs. Green roofs require ongoing stewardship for the benefits to materialize. And it’s harder than a typical garden.
Financial Burden on Housing
If you require a developer to build a green roof, or if a developer wants to build a green roof, that costs money. It costs upfront money during construction, and in the form of maintanence will be an ongoing cost. Developers will have to buy soil, the plants, and have people come and take care of it.
Some of these costs will either get passed on to residents or it means the housing won’t get built at all.
In the grand scheme, this is not a major requirement. But when we consider the array of regulations on housing, from parking minimums to zoning regulations to accessibility requirements, you can see why it’s so hard to build housing in many big cities.
Yes, a lot of these are good things. But we can’t pretend they don’t have a cost. One regulation may not be an issue, but when you realize the array of them, you can see why cities like New York don’t have enough housing: it’s too complicated and expensive to build.
In markets already struggling with housing affordability, green roof mandates represent another regulatory requirement that increases construction costs.
The ongoing maintenance expenses accumulate year after year, creating a permanent cost burden that residents will likely bear through higher rents, increased condo fees, or reduced building services elsewhere.
The Stewardship Gap
Green roofs face a fundamental challenge that community gardens face too: the stewardship gap. But the private, inaccessible nature of rooftops means you can’t just grab volunteers off the street like you can for a community garden.
In older buildings, maybe residents can volunteer to be the stewards. In this case, the fight is for access to the space from teh landlords, as not all rooftops are technically available. Once you have that, now it’s likely on residents to get the materials.
In any case, there are initial and ongoing “costs.” It could be financial, or it could be people’s time.
If it’s a volunteer space, does the building have enough residents interested in the space to manage it in perpetuity?
Green Roofs vs. Community Gardens: A Tale of Accessibility
In some ways, this is a false dichotomy. Why not both? We should do both. Well, as someone with limited hours, I want to know how I can multiply the benefits of those volunteer hours.
The contrast between green roofs and community gardens reveals the importance of accessibility in building sustainable stewardship.
Does requiring expensive green roofs on private buildings achieve greater environmental impact than investing the same resources in accessible community gardens, public parks, or street tree programs?
Again, maybe this is a false dichotomy: we should have both. We should invest the resources, if we can, into all green spaces.
But it’s worth considering whether mandates that create isolated green spaces provide the best return on investment compared to green infrastructure that communities can actually see, use, and help maintain.
I remain a strong supporter of green roofs as part of an urban sustainability strategy. Their environmental benefits are real and valuable, and cities desperately need more green infrastructure of all kinds.
But my experience in community gardens has taught me that good intentions without adequate resources lead to failed projects and wasted opportunities.
My honest assessment of costs and challenges isn’t meant to discourage green roofs. Rather, I hope it can encourage realistic planning that ensures long-term success.
We need policies that account for the full lifecycle of green infrastructure, funding mechanisms that support ongoing stewardship, and implementation strategies that match environmental ambitions with practical capacity.
When we plan for the real work of stewardship from the beginning, green roofs can fulfill their genuine promise of making our cities more sustainable, resilient, and livable. But only if we’re willing to do the work that makes them succeed.
3 Comments