Building a rooftop garden

Plastic wading pools, often used by children in the summer, make ideal sub-irrigated containers for a rooftop garden. Image: Lando and Associates, Landscape Architecture

You’ll savor more than fresh, sun-ripened produce when you convert your condo or apartment building roof to garden space. You’ll also enjoy growing relationships with your neighbors as you work together to raise food that can reduce the annual food budget for each family of four by $200 to $300.

Rooftop gardens usually are planted in containers. This type of garden permits design flexibility that’s ideal for a rooftop location. With start-up costs as little as $5 per sq. ft. when shared by five or six families, a rooftop community garden offers a good return on investment. You can achieve payback in three to four years.

Research zoning laws

Once your condo or apartment association is organized and ready to create a garden, the first step is to hire a structural engineer to draft a framing plan. The plan will indicate any specific structural issues that must be addressed, indicate if there are restrictions on the types of materials that may be used, and detail building codes that must be followed. Expect to pay $300 to $500 for an evaluation.

A rooftop garden must conform to local building codes for setbacks, space use, and maximum building height. Apartment buildings are considered public, and they’re often governed by country and municipal building codes. If your building is part of a historical neighborhood, you’ll face further regulations in developing a rooftop garden.

Because the roof space is considered public, you’ll have to incorporate plans for daily and emergency access, railings, parapet walls, and security. Cityscape gardens, with their support structures and potentially dry plant material, can create combustible areas that must be mitigated through site design.

Address this issue by consulting professionals, such as a landscape architect, local fire department, and the building architect. Potting benches, gazebos, or benches may also require adherence to city codes as they relate to placement, height, and flammability.

Consider water and power

Determine how you’ll supply water to the rooftop garden. Some rooftop gardeners add water faucets; some run hoses from the nearest interior faucet to the garden area. If you’re using municipal water, the garden water source will need a meter so use can be tracked and billed to your garden group. Cost to install a new faucet and dedicated meter is about $2,800.

In some situations, you may be able to avoid city water charges by diverting the existing gutter system so that it delivers rain runoff to a rooftop rain barrel. According to the Environmental Protection Agency, a single rain barrel will save about 1,300 gallons of water during the peak summer months—more than enough to water several family garden plots. Be sure to get permission from the building’s owner before installing diverting gutters to a rain barrel.

You’ll need an electrical power source during construction and also for night lighting if you plan to add that. It’s also helpful to have a secure storage area for shared garden tools—purchase a lockable, all-weather vinyl shed for $500 to $800 at a home improvement center.

Choose plot size, containers, and crops

In a traditional on-the-ground community garden, families typically tend plots that range in size from 50 sq. ft. to over 200 sq. ft. Although you’ll be using individual containers, about 200 sq. ft. of garden space will adequately provide vegetables and herbs for a family of four.

You can grow edible plants in almost any container that’s strong, waterproof, and has holes for drainage. Because your container will drain directly onto the roofing surface, you won’t have to worry about excess water or spillage. You may want to support containers on pieces of pressure-treated wood or lattice to ensure that drainage holes won’t clog.

Rooftop gardens are exposed to wind and plenty of sun, which can dry out soils quickly. For this reason, it’s important to cover soils with a heavy layer of mulch. Avoid using unglazed terra-cotta pots in your rooftop garden; the pottery dries out too quickly for raising vegetables.

The deeper the container, the more room roots have to grow. More roots mean a greater harvest. Use a container at least 10 inches deep for larger plants, such as tomato, melon, and eggplant. Eight-inch-deep containers host peppers, peas, herbs, and carrots. Plant shallow-rooted spinach, lettuce, beets, onions, and herbs in 6-inch-deep containers.

Sub-irrigated containers

Because plants grown in containers require special attention when it comes to watering and protecting them from excessive soil evaporation, sub-irrigated containers are good systems for the rooftop gardener. A sub-irrigated container can double the yields of traditional gardens with less fertilizer, less water, and less effort.

These types of containers suspend the growing medium—the soil—over a reservoir of water. The water is drawn into the soil by capillary action as needed by the plant. Sub-irrigated containers are virtually impossible to overwater. In addition, they greatly reduce the amount of time devoted to watering plants. Some units include wheels, making them easy to rearrange.

You can purchase sub-irrigation growing containers from online sources, such as Gardener’s Supply Company (look for self-watering planters) and Earthbox. Expect to pay for $50 to $110 per container. Approximately 30 containers will feed a family of four for the growing season.

Wading pool planters

You can make your own sub-irrigated container from a child’s plastic wading pool. A pool 6 to 8 feet in diameter and 12 inches deep is ideal for growing most vegetables and herbs.

Drill ½-inch holes every 12 to 18 inches around the circumference of the pool, about 3 inches up from the bottom. The holes provide drainage, yet allow a reservoir of water to stay in the bottom of the pool. This excess water can wick up through the soil as needed by plants, reducing the need for daily watering.

Pools cost about $10, and the soil and fertilizer to fill one costs an additional $20. Seven 6-foot-diameter pools create about 200 sq. ft. of garden space—enough for a family of four.

Set up the garden

Schedule all your community garden participants for a working weekend to assemble and fill containers. The hard work involves hauling soil and fertilizer to the roof using a building elevator or bucket brigade.

A more expensive (but much quicker and less back-breaking) way is to rent a crane with a lift bucket. Prices vary with location, building height, and degree of difficulty for the crane to maneuver into position, but expect to pay $300 to $750 per hour, including transit time. The method you use must be approved by the building owner and comply with city codes.

Once containers are full, you’re ready to plant seeds or seedlings. Water after planting. Stand back, and watch your garden grow.

Julie Martens is a writer with 21 years’ experience in the field of gardening. Her bylines appear in magazines such as Nature’s Garden, Country Gardens, and Garden Ideas & Outdoor Living. She recently moved into a renovated 1915 home and is busily working on a new garden