The following is an excerpt from Serious Straw Bale: A Home Construction Guide for All Climates by Michel Bergeron and Paul Lacinski. It has been adapted for the Web.
As an alternative to conventional roofing, the idea of a green cover over a house can be very attractive. You might choose a living roof for aesthetic reasons, or to help buffer the building from the heat of summer sun, to grow flowers, herbs, or other edibles, or simply to blend the house with its environment. Covering a roof with soil may be the most natural way of not wasting the earth removed by the imprint of a new house. Take the turf or sod removed from the ground while excavating and put it back on your roof; instead of robbing the earth of its potential lifeproducing surface you will actually contribute to the planet’s life equilibrium, which is increasingly debilitated by the constant spread of asphalt roads, parking lots, and tar-covered roofs. Putting your local soil back on the roof is like a lung transplant for your environment.
Sod- or turf-covered living roofs will last almost indefinitely if laid over good-quality waterproofing membranes; in turn, they will prolong the life of the membrane by protecting it from sunlight and weather.
Sod or turf roofs and other living roofs don’t differ much in the way they are built up. They are quite simple to construct. Build a low-pitched roof frame, cover it with a solid deck, stick on a waterproofing membrane, and lay the organic material on top of the membrane. Little maintenance will be needed over the years to turn it either into a rooftop garden or a long-lasting shaggy blanket.
Archibio developed another living roof system. A basic substrate made of second-quality straw bales, laid side by side with the twines cut to loosen the straw, is placed on top of the water-proof membrane. Then a thin coat of manure, compost, leaves, or any other organic material is spread over the surface and left to grow on its own, or planted with edibles and flowers. The only maintenance required, besides the usual gardening work, is to add more straw periodically as the original layer decomposes and becomes thinner.
The temperature moderating effect caused by 5 to 6 inches of earth on the roof helps keep a house cooler in summer and warmer in winter, especially in extreme climates; 14 inches of decomposing straw will have the same effect while adding some insulation for a while. Such roofs are therefore a prime choice for cold-climate houses built with a high degree of insulation for maximum comfort.
The wind and noise protection qualities of a living roof are also worth considering in specific environments. A city house built with bales and covered with an organic roof will become a peaceful retreat at any time of day, even in areas with dense traffic. On particularly windy sites, such a roof anchors the house to the ground physically as well as visually.
Durable roofing membranes such as EPDM, Hypalon, neoprene, PVC, or modified bitumen are the best choices for low-pitched roofs. In conventional construction, these are left exposed to weather. Although they have some type of protective coating, they will nevertheless slowly but surely degrade under ultraviolet light exposure, and some of them will also erode over time from continuous heavy rains and ice buildups. In both cases the protective coat of organic matter will considerably increase their life expectancy.
Using a much lighter growing medium than soil, with enough volume for plant roots to stay healthy, was the origin of Archibio’s straw bale concept. Earth, the original base material for sod roofs, is ten times heavier per volume than baled straw. Moisture impregnated composted straw was tested to weigh approximately 30 pounds per cubic foot, while moist earth ranges in the vicinity of 100 pounds per cubic foot. With the addition of compost or manure on top, the straw roof will weigh a little less than the 5 to 6 inches of sod roof. It also seems to give more protection to roots in the winter because of its greater volume.
An interesting alternative to both of these heavy systems would be to lay down 4-inch-thick flakes of straw. That wouldn’t add more than 10 pounds per square foot to the whole structure, and it would still protect the membrane while offering the same natural look.
The main drawback of building a heavy roof is the additional 50 to 60 pounds per square foot that the structure will have to support. This might be too much on certain load-bearing walls and designs, so the bearing capacities will have to be thoroughly investigated. The need to shore up the structure would obviously increase the building costs, so that prospect should be crossevaluated with the positive aspects of the roof.
Although it may be reduced to a minimum, some seasonal maintenance will be required on a living roof. The type of maintenance will depend on the degree of refinement you want in the appearance of the roof. All the usual activities associated with gardening—weeding, watering, mulching—will be required if your roof is going to be some kind of garden. Otherwise you just need to keep a sufficient cover by periodically adding some material to the original layer. At worst, if no maintenance is performed, the membrane might eventually become exposed, as it would have been if not covered at all.
The last disadvantage derives not from the performance of your organic roof, but rather from your relationship with your neighbors. Social conformity may be something to consider in some areas. Taste and common sense may also be a factor in deciding whether such a roof will look completely out of place in a given site. If you go forward with a living roof, your design should emphasize integrating the building into its surrounding environment.