1. Get a structural engineering report for the live load of the structure and roof truss design. A standard roof is built to take about ten to twenty pounds of pressure per square foot. A three-foot-square garden won't add a significant amount of weight; however, a twenty-foot-square garden, complete with wet soil and plants, can weigh thousands of pounds. After investing time and effort on a beautiful garden, the last thing you want is for it to come crashing down.
2. Shore it up. Reinforcing your intended structure entails more than putting supports under the rooftoy box; likely, your structure will require lateral supports as well. Imagine holding a kite string: The wind exerts pressure not only on the kite itself, but your body. Any wind and rain will exert the same force on your rooftop plants.
3. Lay down the liner. You may want to consult a roofer to install a commercial seamless roof. If you're building on top of an uninhabited structure, lay down a standard pond liner. The liner will keep the water from seeping into the building; it will also keep the plant roots from eating into the building structure.
4. Set up the lattice. Skip this step if your roof is flat. Roofs with a slope will need a grid set up over the liner to keep the dirt from sliding off.
5. Consult a look book. How much effort are you willing to invest? Obviously, more ornate plants are going to require more work than minimalist moss. Wildflowers and their seeds will attract birds and butterflies; scattered items like logs will attract small rodents (and give you a place to sit down). Grasses will need to be mowed occasionally, and moss, while low-maintenance, is...moss. Now might also be a good time to consult your engineering report and decide how heavy your plant load can be.
6. Mix and lay down your potting soil. Depending on your choice of plant life, the soil will probably have to be custom-mixed. The separate components usually consist of mineral content, such as sand or dirt; organic matter, such as coconut husk or peat moss; and a water-hoarding material like SoilMoist. The organic matter will decompose, fertilizing your garden; SoilMoist absorbs water and releases it as the soil dries out.
7. Plant your plants! Seedlings, or plugs, are slightly less frustrating than seeds.
8. Enjoy!

This DIY project originally appeared on WIRED's How-To Wiki.