When installing a photovoltaic, solar heat, or wind generation system, one concept with which you’re sure to become familiar is that of structural load. The concept of a twenty foot wind tower on your roof spinning down free energy all year is nice, but in practice, you’d more likely rip a hole in your house without some careful consideration. Therefore, sustainable roof design has adapted to include a variety of green techniques, each requiring their own load profile. When used in combination, the elements can add a visual and technological depth to a space that is almost hard to describe.
Living roofs are required by law in some European cities, so it’s strange that so few people in the US have ever even heard of them. Basically, in a city, roofs cover between 30-40% of the available land acreage. Streets cover a good percentage more. By building a living roof, you offset the loss of porous surface area by simply elevating the layer above the structure. New sustainable design firms tend toward relatively autonomous plantings so that care needs are minimized. Varieties of drought resistant grasses or low-water plants like ice plants for a more spectacular display. Traditional examples of living roofs often display a more cultivated cover. Some are actually used as rooftop garden spaces, with fully functional plant beds in frames. They slow down water across their surface area and help promote local biodiversity.
The largest challenge in making a rooftop garden (besides keeping the frame watertight so it doesn’t leak onto your roof) is one of structural load. Obviously, cubic feet of dirt are heavy – just ask anyone who’s done construction or landscape work lately. On your roof, they bear down on the surface, creating stress on the seams between fastenings and structural supports. It is important to find ways to relieve this stress either in the building phase, or, as is more common, in the design phase of a remodel. Soil scientists have designed artificial soils that weigh less than traditional soils, and other growing mediums such as local crushed brick can be used. But usually this involves restructuring the load on beams so that the roof avoids carrying actual weight.
As mentioned earlier, a living roof may not be the only alternative energy installation vying for structural load bearing on your house. If you install solar panels or a solar heat collector, the same weight issues come into play, and careful siting along strong structural axes or retrofitting are necessary. With wind, add in the force of the tower’s rotation and the wind profile of the actual tower and it’s probably better not to site a tower on your house at all unless you like weird noises and warped beams. Save that for the back yard.
If you are considering installing one technology already which calls for boosting the load structure of your roof, why not design for the (future) implementation of another complementary technology now? As hurricanes so aptly illustrate, a little extra roof support ain’t gonna hurt you. With as much roof space as we have in this country, we could probably meet half our food needs if everyone started a garden today. Victory Gardens for a new millennium. Even just switching from a traditional tar shingle roof (made from petroleum) to a gravel-based cover slows water loss considerably across your whole property. Take a look at these examples of how nice living roofs can look, and consider integrating a little (more) green into your next roofing project.