How to Convert a Roof Into a Green Roof

Examine your roof to make sure it's strong enough to handle a green roof. Vegetated roof covers must support these materials if they're to thrive: commercial waterproof sheets, an irrigation system, soil, plants and other materials needed to keep the grass manicured, cultivated and maintained. Your green roof must be able to support dead and live weight loads. You may wish to consult a structural engineer to help determine your roof's weight-bearing limits.

Decide between an "extensive" green roof and an "intensive" one--fancy ways of asking about your design objective. Extensive green roofs are thinner, lighter and simpler plots with a minimum soil depth between 2 1/2 and 6 inches. These roofs can support between 10 and 50 pounds of material per square foot. Intensive green roofs are more complex. They're capable of sustaining deep soil depth and plantings growing to 15 or more feet at between 80 and 100 pounds of weight per square foot.

Choose an irrigation system based on your budget and the complexity of your project. A simple rooftop lawn may require only a long hose to deliver moisture during nonrainy periods. Extensive rooftop gardens usually need a sophisticated system of pipes, pumps and sprinklers to maintain vegetation. Install the irrigation system you prefer once you've covered your roof with strong, multilayered, industrial-strength waterproofing material to protect the structure beneath.

Frame up your growing area with lumber, rocks, bricks or other materials to limit soil runoff and confine the grass. Install soil that works best in your geographic area and plant hardy turf grass. Establish the grass before adding plants. You can grow nonindigenous species on your green roof, but you'll find your gardening tasks easier if you stick with indigenous materials.

Explore an alternative green roof system developed by Toyota if you want to save time and effort. Twenty-inch-square TM9 Korean velvet grass (korai shiba) mats (think floor tiles) require little installation work. They're fitted with water tubes for easy irrigation and are impregnated with grass seed that requires cutting only once a year.