The conventional approach to gardening has been to place a few “landscape” plants in the yard, then fill all the remaining area with lawn. The recent film Hometown Habitat advocates a different approach: deciding where people will need to walk or run or play, putting lawn there, and densely planting everywhere else with naturally-growing trees, shrubs, and flowers. Some people take this idea seriously by packing plants into every available space on their property, including on their roof. The result is called a green roof.
Aside from creating more room for plants to do all the wonderful things that they do, putting a garden on the roof creates some special benefits. First, it converts a typically impermeable area into a space capable of absorbing water. Thus, green roofs reduce runoff and flooding. Second, plants and soil are an excellent form of insulation, keeping the building below the roof both warmer in the winter and cooler in the summer. Third, far from damaging a conventional roof, a well-designed planting actually protects the roof underneath and extends its lifespan.
Of course, you can’t get these benefits just by climbing up on your shingles and scattering some seeds. A functioning green roof requires a little bit of planning and knowledge.
First, the aspiring roof gardener must check that their house, garage, apartment complex, office building, or other structure will be able to hold the weight of a planting, with all its supporting layers.
Second, the gardener must familiarize themselves with those layers: a roof garden consists of plants, of course, but those plants must be rooted in soil. A green roof, due to the challenges of weight constraints, doesn’t use soil dug up from the ground below, but rather incorporates a special lightweight planting medium. Below this is a root barrier, to prevent plants from growing too far downwards and working their way into places that are not improved by their presence. Next comes a drainage layer, to ensure that any water not absorbed by the plants will be channeled safely to the ground. Below that is an insulation layer, and finally a waterproof membrane, to make absolutely certain that no water will penetrate into the building underneath.
Finally, with all of that accounted for, the gardener must turn their attention to the most vibrant layer: the plants! A green roof can include more than just tough, low-growing groundcovers, but it won’t provide a suitable home for every kind of plant. The gardener must choose plants that can withstand the challenging conditions on a roof, such as harsh sun, strong winds, and only a few inches of soil.
Putting plants on a roof may seem like a crazy new idea, but in fact – as with many sustainable ways of living – it’s simply a return to the way things were traditionally done. In Iceland, houses were constructed with turf roofs for over 1,000 years, in part because of the superior insulation offered by sod against the frigid climate. This practice continued well into modern times; examples of turf houses can still be seen in Iceland today.