Safety FirstSafety. For those with young children who like to explore, safety is a major concern. Construct a fence with a safe railing height around the roof's perimeter to keep little ones contained. Don't want to hide a good view with a fence? Use glass or fences with openings that allow you to see, but not move, through. Mitchell also suggests keeping planter boxes and furniture away from railings because they can serve as stepladders.
Accessibility. This is a multi-step process. Parking is one thing to consider when unloading materials, especially if you live in the city. How will you get materials into the building, through narrow hallways and to the roof? If you're hauling in lots of materials, especially heavy rock or lumber, don't overlook the elevator's weight capacity. Don't have an elevator? Consider using bulk-free materials you can carry upstairs.
Be friendly to other neighbors in your building, reminds Carmen DeVito, landscape designer in Brooklyn, N.Y. If you have neighbors trying to use the elevator or stairs, step aside and let them by. Perhaps they'll offer to help you move some materials.
Once the materials have been hauled to the roof, how will the garden be accessed by visitors? If possible, make paths wide enough for two people to comfortably walk next to each other and build smooth walking surfaces for minimizing tripping.
Drainage. After a heavy rain, take a look at where the water drains. Where does water rest after it rains? Are there drainage pipes for runoff? Place outdoor furniture in a location away from drainage pipes or where puddles form. If there are major drainage issues, consult a professional who can help direct runoff to a gutter system. You can limit water runoff by using drip irrigation.
Again, don't forget about your neighbors. Avoid redirecting water runoff or using containers where they'll drain directly over a neighbor's balcony. Keep neighbors happy by directing water runoff to another part of the roof or adding saucers under containers that will capture the moisture.
--Photo courtesy of Chicago Specialty Gardens