Five Crucial Considerations for Rooftop and Terrace Gardens

It sure would make it easy if every grower had a spacious, level backyard with six hours of sunlight, a water supply nearby, and a convenient pathway that leads from the back door to the organic garden. But much of the world’s population lives in multifamily housing. There are lots of places to call home that don’t have a backyard ; from two-story townhomes in tranquil neighborhoods, to urban condo and apartment buildings that stretch high into the skyline of the busiest cities, and everything in between. Sometimes these multifamily communities have a site on ground level for an organic garden that residents can share. But often, the only option is to place raised beds on the building’s actual rooftop, or on an existing, above-ground recreation area, like a pool deck or green roof park. And some multi-family building residents do have the option to have a garden on their own unit’s terrace or balcony, which is certainly more convenient than a shared rooftop garden, but is also subject to many of the same considerations described below.

Strength of the roof

Remember that raised beds filled with organic soil are heavy, and this is perhaps the very first consideration, before you embark on a rooftop garden project. One 4’x8’x18” Kitchen Garden raised beds, for example, requires about two cubic yards of soil, which tips the scale at about 2000 pounds for just the dry soil alone. Imagine the added weight that single garden bed takes on when every cubic inch of the soil is soaked through during a rainstorm, plus add in the weight of the actual cedar garden bed itself, and...well, you get the idea.

Many high-rise condo and apartment buildings already have a pool deck or similar community outdoor recreation deck, with landscaping, trees, pathways, benches, a covered area for grilling, etc. Sites like this are almost surely more than strong enough to carry the additional load of an organic garden that includes multiple raised beds, and the property will have the technical data to confirm the load capacity of the roof. For smaller rooftop gardens without this data readily available, a structural engineer should always be consulted to confirm that the roof can carry the additional load of a rooftop garden. What sad and dangerous irony to put an organic  garden up on the roof, only to have it crash through the ceiling on top of the dinner party where the first harvest of seasonal greens has just been plated and served to your guests.

Once the load capacity of the roof is confirmed, consider the actual roof material itself ; the rubber roof membrane, asphalt shingles, slate, or other roofing material that actually keeps rainwater from leaking into the building. Could raised beds realistically be installed without damaging the roof material? Can the existing roof material handle the added foot traffic of gardeners walking to, from, and around the garden? Should the existing roof be covered with a protective product before any work begins? Consider this carefully, and remember that roof repairs can get very pricey, especially on large buildings with high-tech, multi layer roof membrane material.

Drainage

Soil drainage is always important in an organic garden, and often even more so on a rooftop garden. In a backyard setting, it’s easy enough to evaluate the setting, and place a raised bed so that it doesn’t block any existing stormwater drainage swales or channels, and then fill the planter with a lightweight organic soil blend that allows water to easily pass through the grow media, and into the native soil below the raised bed. Drainage issues solved, time to start planting.

But It’s not always that simple with a rooftop or terrace garden. An existing rooftop recreation deck that already has landscaping, should already have a carefully designed system to channel rainwater safely off of the rooftop. But before you add an organic garden, you should consider how the water runoff from the garden will drain through the existing drainage system. Is the existing system adequate? Should it be modified to accommodate the addition of raised beds on the rooftop? 

For terrace and balcony plantings, will the water drain neatly away into an existing rain gutter system, or would it cascade like a waterfall onto the downstairs neighbor’s balcony?

Sunlight

Many popular edible plant varieties need a minimum of six hours of direct sunlight, and at first glance, it would seem that any rooftop would surely deliver those six hours, and probably  many more. But in many urban areas, the rooftop of one building may be shaded for some or even much of the day by taller, adjacent buildings. Often the above-ground outdoor recreation area of a high rise apartment building may only four or five stories high, meaning that the deck may be shaded by the much taller, residential portion of the building itself. And sunlight is an even bigger factor when considering a garden on a terrace or balcony. Many leafy green varieties can thrive with as little as four hours of direct sunlight, but if your unit’s terrace doesn’t get at least that much sunlight, it would be better to look around for a sunnier place to garden.

And for those rooftop gardens that actually do get a whole day of uninterrupted sun exposure, you may have to add shade to maintain a healthy growing environment. This can be accomplished by hanging shade cloth above the garden, by building a pergola with lath strips above the garden, etc.

Water Supply

Everyone knows that plants need water. Exposed rooftop gardens, with perpetually windy conditions, and many hours of direct sunlight, need a lot of water - usually every day.  Does your rooftop garden site have a convenient water supply? Is there an existing drip irrigation system that can be tapped into to water the new raised beds on a dedicated irrigation zone via the main control panel? If there’s not an existing irrigation system, can one be installed? If the answer is no, and there’s only a spigot and a garden hose up there, and the garden must be watered by hand, is there someone committed enough to carry out that often unglamorous and time-consuming task - every day during the hottest parts of the summer? Be realistic and carefully consider how the garden will be watered before investing the time and money in building your rooftop garden.

And if you’re considering an organic garden on your own unit’s terrace, watering logistics are especially important. Is there a spigot or would you have to water the garden with a watering can or bucket? Can a watering can be filled in the sink, or would you have to fill one in the bathtub? How many trips back and forth would it take to keep your balcony garden well watered?

Access

Your building or home’s rooftop might be the only place to put an organic garden. But do you have permission from the property management to even go up on the roof, let alone have a garden up there? And if you do have the property management’s full backing for your rooftop garden project, but how easy is it to get up there? Is there a  standard-sized door that leads out onto the roof? How would you bring you bring lumber for making raised beds, and other growing supplies from the ground floor up onto the roof? Large apartment and condo buildings have freight elevators that make this process possible, but sometimes tedious when multiple trips up and down are needed to compete the project. But a stairwell may be the only way up to the roof for smaller multifamily buildings and single family homes with a section of flat roof suitable for a rooftop garden. In cases like this, bringing building materials and soil up flights of stairs is unrealistic and a mechanical lift of some type is needed to bring these materials up to the rooftop.

Of course, once the garden is in place, it’ll need regular maintenance. Is there easy, open access to the rooftop for the ongoing maintenance that’s essential for a successful rooftop garden? Hopefully, the answer is yes, and you’re ready to begin designing your new rooftop garden.

Having an organic garden up on the roof is a lot of fun, and it’s easy to get caught up in the excitement and skip important steps in the process. Resist the temptation to “rush it”, and make costly mistakes that could have easily been avoided. Take your time, consider each one of these points thoughtfully, your rooftop garden is sure to be a success.