Rooftop Gardening

So want to have an organic garden, but, alas, your backyard is shadier than a politician. But you have noticed that the deck spends all day basking in glorious sunshine. Or perhaps you want a garden but have no backyard, placing responsibility for such an adventure squarely upon the roof. 

There’s a good chance that your deck is sturdy enough for the weight of garden beds, and it’s possible that even the roof is up to the task, but there are a series of essential factors to address that could very well mean the difference between pleasant mornings harvesting lettuce on the roof, and a fleet of emergency response vehicles, and perhaps a news reporter or two, racing to your home.

Simply put, you can’t just put what could add up to thousands of pounds of wood and soil above your living space, cross your fingers, and hope the roof doesn’t cave in.  Whether new construction, or reinforcing an existing building, a structure must be designed for the additional weight of a roof top garden, and any people using the garden. 

This equation involves not just the size and spacing of joist or rafters, but all the components of a building that are required to transfer that load down to the foundation, including: plywood roofing, rafter/joist sizes and span, girders, columns, walls and footings. 

For example, in order to evaluate a joist or girder, one needs to know the amount and type of load it supports, how far it spans and its size, type of material and its properties (allowable bending and shear stresses, etc).

It may not always be practical or cost effective to make changes to a structure in order to accommodate a roof top garden.  The main difficulty with a retrofit is the ability to determine if your home is actually strong enough to hold it up.  Most homeowners literally have to open up the walls and ceiling to find out.  If you live in a typical single family residence, chances are good your home’s roof  was not made to hold that much weight and you will need to add extra supports to accommodate a rooftop garden. Most decks, however, are capable of carrying the load of raised beds or other planters.

For a safe, rewarding rooftop gardening experience, follow these steps:

-determine that there is adequate sunlight and drainage on your roof or deck. Multifamily and commercial roofs and terraces will most likely have drains placed in low areas where water can be channeled into a gutter system
-determine how many people will be on the roof at one time, and how they will get up and down.
-address any HOA guidelines or concerns, and consider any other liabilities that could be an issue like your roof’s warranty, for example.
- Create a layout of the rooftop garden, including dimensions and volume of raised planters.
-Determine the weights and loads the rooftop garden design, as well as the estimated number of people would place on your building.
- Size or check the building members (beams, rafters, etc) from the roof down to the foundation

It’s always best to have a professional evaluate your structure to determine if it can safely bear the load of a garden, the basic science of it boils down to evaluating your roof or deck’s ability to support three weight categories :  dead, live or environmental loads.

Dead loads are weighted items that will be there all the time, including the materials in the actual roof or deck itself.  Typical rooftop garden  dead load types and their weights are:
        *Self weight of the structure - load varies
        *Conventional roof - 7 to 10 psf (pounds per square foot)
        *Small plants - 2 psf
        *Drainage layer - 2 to 3 psf
        *Soil density - 60 to 110 pcf (pounds per cubic foot) loads vary depending on soil mix

Live loads are dynamic or moveable loads associated with an areas usage -  In this case, mainly people.  Code dictates that different roof types be able to hold different amounts of weight.
        *In-accessible landscape roofs - 20 psf
        *Private or residential roofs - 40 psf
        *Multifamily or commercial roofs - 100 psf

Environmental loads such as snow, wind, seismic, rain, etc. Consider that your roof is most likely to fail during a rainfall when your garden’s grow medium is fully saturated.