Anatomy of a Green Roof

Also means someone has to be paid to maintain it.

Not to mention the additional weight will probably mean rebuilding the roof. If this is in the north you have to compensate for the additional weight of the soil, retained moisture and in the winter the accumulation of heavy snow.

Off hand it sounds like a costly idea.

Green roofs are becoming more common. I don't claim to know much about them, but obviously buildings can be built economically to hold the load. I have seen York University's Computer Sciences building's green roof. It's planted with wild grasses mostly, and maintenance is almost nil. Most of these roofs are for environmental and energy-reduction purposes, not for human ideals of beauty.

Here's a very readable article:

green roofs

Come to think of it, since Toronto stopped herbicide spraying public areas I've noticed a remarkable increase in butterflies ~ and no doubt the green roofs are contributing to balancing their habitat as well.

I don't doubt green roofs can be built but I don't think you can take an existing roof and make it a green one without strengthing it. That may be costly. So I don't see existing roofs in the inner city being converted into green roofs.

Are there any engineers out there who can comment on this?

Well, I'm a landscape architect, not an engineer. Green roofs like some of the series on the show on the ASLA website above appear mostly to be built anew. However, in doing roof gardens for an existing building, one has a structural engineer look at the level of loadbearing inplace structure. One designs to have soil be light weight, drainage adequate, waterproofing adequate, and to have any larger plants within a lightweight container that would be heavy with adequate water be placed directly over support columns. Perhaps a structural engineer will show up here and address the question.

Well, hopefully an engineer will show up to discuss. However, looking at the article I linked, it listed at least 5 buildings that were built long before green roofs were thought about, some over 100 years old, that have obviously been successfully retrofitted for a green roof.