Rooftop Gardening

Rooftops are a city's greatest untapped resource. There are possibilities for all kinds of roofs. Whether yours is sloped or flat, large or small, industrial, commercial or residential, the possibilities are limitless.

How can rooftop gardens help our cities?
Urban greening, air cleaning, community building, and food production come to mind.

Rooftop Gardening Basics
You will find the links I have collected at the bottom of this article to provide ample information about rooftop gardening techniques, as well as inspiration to follow. To start, here are a few of the points that make rooftop gardening much different than gardening on the ground.

Ensuring a Safe Load
When gardening on a rooftop, load is a very great concern. Roofs that weren't constructed with gardening in mind must be checked for stability and leaks and then treated gently to avoid damaging the roof membrane.

A roof membrane consists of whatever it is that keeps the water out. My own roof is tar, with a slight slope. Others are shingled, tiled, or tar and gravel.

Once a roof is properly planted, the membrane can actually last longer than if it were left bare. The reason for this is that the membrane is insulated from damage. It is also insulated from heat loss and heat gain, making the building underneath easier to heat and cool. While a roof membrane in Vancouver is considered to last about 20 years in our climate, a planted one commonly lasts 50.

Regardless of how strong your roof is, it's a good idea to use materials that don't weigh very much. Soil can be weighed per cubic foot. Most garden soils weigh approximately 160 to 170 pounds per cubic foot. You can construct a soil that weighs less than that by using organic matter such compost, manure, or reconstituted food wastes and mixing it with pumice. If you mix 70% organic matter with 30% pumice, you can get the weight down to 60 pounds per cubic foot, and end up with a very light, well-drained mix.

The depth of the soil and the soil used should also be kept at a minimum while ensuring plant health. Not very many people know this, but the Vancouver Public Library has a rooftop garden consisting of 'Elijah Blue' Fescue (Festuca ovina var. glauca 'Elijah Blue'). It is not a public garden, and can only be viewed from the office and residential towers surrounding it. This roof-full of perennial grass grows in only 12 inches of growing medium, using the above recipe for soil. Most plants will need more than 12 inches, but careful attention to species and variety can mean less soil and water is needed.

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