SAN FRANCISCO - On the roof of Glide Memorial Church here, gardeners use wine boxes and coconut coir. In Chicago, they plant on wooden platforms. In New York City, one homeowner installed steel beams.

As more people turn to roof gardens to grow their own food, they are coming up with all kinds of ways to keep those plots light, and avert roof sagging and cave-ins.

"Weight is a huge factor," says Josephine Quiocho, a project organizer at Graze the Roof, a community garden that grows spinach, mustard, kale, sweet peas, and other crops on Glide Memorial's roof.

Graze the Roof has perfected an ultralight "soil" that includes coconut coir (husk fiber) and perlite, a volcanic mineral that aerates plants. "The coconut coir allows more air into the soil, lightens it up, and helps trap moisture. We don't need heavy, water-clogged soil that can weigh on your roof," Quiocho says.

The Glide Memorial farmers grow their crops in bread boxes and wine boxes, in part because the boxes are light, in part to show congregants that they can grow plants in containers found in the house.

In New York, Lize Mogel took a structural approach: She had five steel beams constructed under her roof so she could begin planting a roof farm this year. Mogel, an artist who has grown roof crops for years, always dreamed of having a full roof farm.

"I had to do renovations anyway, so I spoke to the architect and it didn't cost that much more to reinforce the roof," she says. "Because the steel beams will be attached directly to the gratings on which I'll grow the crops, the gratings can take a massively heavy load. I could raise elephants up there if I want."

For urban farmers with weaker roofs, shaving off every pound of extra weight is a hobby in itself.

In Chicago, Heidi Hough, who cowrites a blog for rooftop farmers called greenroofgrowers.blogspot.com, has turned her passion for urban agriculture into a search for the perfect lightweight roof garden.

Above her home, she grows plants on wooden platforms, which helps take pressure off the roof. A local hot dog factory also gives her plastic pickle barrels which she "recycles and upcycles" to make planters for vegetables. Hough and some friends have developed an irrigation system that requires little soil and water, vastly reducing the weight of the farm.

"You can buy very good, lightweight irrigation boxes, but we just looked around and copied the system with our own material," she says.

Hough grows her plants with one 5-gallon bucket planted inside another, minimizing the amount of water used and reducing the weight on the roof. She uses lightweight peat moss and vermiculite, a mineral often used in soilless gardens.

Because roof farmers use only as much water as they need, they have to be wary of evaporation.

Hough uses a plastic covering over her peat-moss mix to stop evaporation, while Johanne Daoust, a Toronto roof farmer, uses "shade cloths" she spotted for sale while traveling in China. The cloths block the sun while allowing a free flow of air to the plants.

"Everybody you speak to has their own way of planting those roofs," says Daoust. "If you can pick your own breakfast every morning instead of having it flown 3,000 miles by cargo plane, then you are doing it right. As long as you take precautions, it can transform your life."