Lawns on decks on rooftops serve a number of purposes besides creating a pleasant place for you to relax. Rooftop green spaces hold rainwater, reducing runoff into sewers, and also help to reduce the urban "heat island" effect by absorbing the sun's rays. Green roofs and decks weigh a lot more than conventional ones, so be sure that your building is structurally capable of supporting the weight of soil, plants and captured rainwater.
Analyze your roof situation to determine its suitability for a rooftop lawn. Unless you have training in engineering yourself, you will need to hire a structural engineer to do this. Don't take chances when it comes to the structural integrity of your house. A wet rooftop lawn can weigh between 80 and 150 pounds per square foot, although this weight can be substantially reduced by mixing the soil with other, light-weight agents. If your home has a gable roof on it, you will have to remove it and replace it with a flat roof for this project to be feasible. If your home lacks the necessary structural strength, you will need to retrofit a steel framework around the house to support a roof-sized rooftop lawn. You could, however, install a small patch of green grass on a flat roof (less than 6 by 6 feet) with few concerns about structural strength.
Install a waterproof barrier on the flat surface where the lawn will be going. Many flat roofs feature rubber membranes, but a green roof requires a waterproof membrane of at least 20 mm thickness as well as a root guard that prevents roots from poking through the waterproof membrane. Other methods include the application of brushed-on tar waterproofing rather than a rolled-on rubber membrane.
Install 4 to 6 inches of topsoil on top of the waterproof membrane. You will need to either taper the soil off near the edges of the roof or include a low retaining wall to contain it. Retaining walls should include drainage holes at the bottom to allow excess water to escape. A green roof will absorb most of the rainwater that falls on it. The little that runs off will enter the gutters just like water off a conventional roof.
Plant the things you want to grow. A lawn can create a pleasant green space, but remember that it will need mowing on the roof just like in the yard. Appropriate plants for a rooftop lawn include mosses, sedum, bluegrass and rye grass. Other possibilities include leafy groundcover and edible plants. Keeping your rooftop grass cut very short will help to keep its roots short as well.