To make sure it wouldn’t collapse, he hired a structural engineer who calculated how much weight the roof could support. The answer was 35 pounds per square foot, dry weight, or 60 pounds per square foot, if saturated with rain.

That calculation limited Mr. Puchkoff’s green roof to no more than eight inches of soil, so he chose seven, just to be safe, even building a little crest of a hill, over a lightweight polystyrene mound, “because I didn’t want it perfectly flat,” he said.

He sealed the roof with a combination of polyethylene and woven polyester from the Andek Corporation, whose products he had used over the years to seal custom-built bathtubs. (Cost: $1,500, including labor.)

Then he was ready to install the four-layered system he chose from American Hydrotech.

The layers consist of a five-millimeter polyethylene membrane that keeps roots from penetrating the roof; then a spongy moisture retention layer, which absorbs any water that overflows the next layer of “Floradrains,” from a German company named ZinCo. These are cup-like plastic units that look like upside-down egg cartons; when laid together, they hold water that seeps down through the layer of soil, which is laid over a filter that prevents sifting and clogging of the drains.

The multilayered system establishes not only a reservoir of water for plants, but also a backup supply, held by the moisture retention sponge, which evaporates slowly, in dry times, to moisten plant roots.

Image Talinum calycinum, an American wildflower, with Sedum sexangulare, in seven inches of soil. Credit... John Lei for The New York Times

At the final stage, drip tubes are laid down on top of the soil filter, before the soil medium is spread. These drip lines, plus some early top-watering, supplied water to the young plants — which arrived as plugs with three-inch roots, and were planted eight inches apart — until they were well-established. (The Hydrotech system cost $3,800, plus $800 labor.)