Courtesy of James K., Virtually Green, San Francisco
There are two types of green roofs, identified by their soil depths:
– extensive (usually 2″ to 4″ in soil depth)
– intensive (“8 inches on up)
For SPIN farming purposes the intensive type is obviously the way to go, though it’s conceivable that in some limited circumstances one  might be able to do greens, such as lettuce, in shallow extensive  green roof soils.
There are also two different approaches to covering the rooftop with soil:
– membrane
– container
The membrane approach basically involves covering the roof with layers of waterproof membrane and drainage materials topped off by soil.
The container approach is just like it sounds. You garden in containers, though the containers on most green roofs are tray-shaped, often 2′ x 4′ width and length. 8″ or deeper depth trays are available.
In the membrane approach the green roof membrane is the roof surface. In the container approach the container/trays rest on top of the roof surface.
One way to do a tray-based green roof system is to buy commercial trays. Below are links to a couple green roof tray companies that have 2’x4’x8″ trays:
GreenGrid
https://www.greengridroofs.com/
Tournesol Siteworks
https://www.tournesolsiteworks.com/
Green roof containers/trays can be homemade. Commercial trays can also be modified to better suit the standard 2’x25′ SPIN bed.
At first glance the membrane approach might seem like the best way to go, and in a few situations it might be the best: for instance, you can create 2’x25′ continuous beds like you would on ground level. But, except for new construction projects willing to redesign the
planned roof, or for existing buildings willing to spend the money to essentially reroof their building with a soil membrane green roof, the container/tray approach enables a building builder/owner to test the green roof concept without needing to make any significant
changes to the designed or built roof surface.
Soil has weight, especially when wet. This usually doesn’t matter at ground level … unless you’re on a slippery slope or plagued by caverns.
Roofs have weight load limits. All legal roofs in USA and Canada have two load specifications:
– Dead Load
– Live Load
Dead load is the weight of the roof itself. Obviously a roof must be capable of handling its own weight. Live load is the ability of a roof to handle the weight of things that may come and go such as people or snow, or are there to stay such as PV panels or container plants.
In snow country it’s normal for roofs to have a minimum Live Load requirement of 30 lbs/psf (per square foot).
A 12″ depth of normal soil (not lightweight vermiculite etc) when saturated weighs about 100 lbs/psf: that exceeds a 30 lb/psf roof live load by 70 lbs/psf: definitely Oops!
And just imagine 12” of rooftop saturated soil in late Fall or Early Spring that unfortunately gets further loading from a heavy snowstorm. More Oops! By the way, for those of you who experience major winter snowfall, the trays can be removed from much or all of
the rooftop during the winter months. Â WORTH NOTING! This is a good moment to mention how nice it is to have freight elevator access directly to a green roof farm, such elevator access not unusual on large mid- and high-rise office buildings. Also, moving soil-filled trays around or off of a roof is easiest done using a standard pallet jack.
So one of the main things you must be aware of when considering a green roof is the Live Load of the roof and how best to deploy the green roof trays to accommodate the Live Load situation in your climate.
The container/tray approach enables the rooftop SPIN farmer to put  soil only where you want it. Path areas can be soil-free and therefore not contribute to live load. Trays can be spread out to lower the average roof Live Load to a safe average and point-load
level. The heaviest concentration of trays can be on top of the load-bearing walls and columns rather than over long unsupported roof spans.
As you’re no doubt realizing, rooftop SPIN farming is somewhat different from ground level SPIN. There are aspects of rooftop farming that a ground level farmer would never have to deal with and vice versa. Imagine farming that never ever has problems with
gophers, moles or deer, or via the elevator delivering the rooftop farm produce directly to your customers on the floors of a large high-rise building. Sweet!
