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Rooftop gardens have expanded rapidly in dense cities as planners search for ways to reduce urban heat and manage stormwater without acquiring costly ground-level land. A layer of soil and vegetation absorbs rainfall that would otherwise overwhelm sewer systems during heavy storms, while also insulating buildings and lowering cooling costs in summer. Despite these benefits, retrofitting older buildings with rooftop gardens is not always feasible, since many structures were not engineered to bear the additional weight of soil, plants, and retained water. Structural reinforcement can be prohibitively expensive, which is why rooftop gardens are more commonly found on new construction where load-bearing capacity is planned from the outset. Even so, several cities now offer tax incentives specifically to encourage retrofitting wherever it is structurally possible.