We are gradually in the process of greening up all of our roofs since we discovered to our delight that the indoor temperatures in structures we had built or retrofitted with green roofs were typically 15 degrees cooler than outside temperatures in the summer, as well as having enhanced insulation in winter. Only part of that summer effect is insulation. The other part is evaporation, or the transpiration of water from the roots to the leaves, dropping coolth into the building below. It’s the same way your fridge works.
We don’t have very big construction budgets, so most our materials are harvested or scavenged locally. The basic technique is to build a sturdy roof (one which can support the weight of wet topsoil and the maximum snowload), and then install a “carpet sandwich.”
The sandwich has a layer of carpet scraps (the dumpsters behind carpet warehouses are especially fertile sources), an impervious liner (old swimming pool liners and covers work well), and another layer of carpet. The carpet underlayer is to protect the liner from nails, screws, pebbles, or rough bits of roofing. The carpet overlayer is to protect the liner from ultraviolet light, limbs, hail, or anything falling from the sky, and also provides some structure for roots of plants to latch hold of so they don’t slide off the slope.
When we don’t have good scrap liner or want to go whole hog on a new building, we spring for new EPDM, which is the same material used for pond liner in your nearest garden store. If kept out of direct sunlight, it will last 1000 years, although we have to acknowledge it is a petroleum product (vulcanized rubber). Try to cut and splice it as little as possible, because leaks and penetrations are death to living roofs. You can add insulation to the area under the liner (flake straw, for instance) if you want to increase the R-value for heating and cooling.
On top of the top carpet we spread turf and mulch it really well so it can tolerate the low water regime.
A good place to get turf is by turning lawn into garden. Street/sidewalk medians are good for this.
Another possibility is sun-tolerant mosses. Here is a recipe for inoculating your top carpet with moss:
Serves 1 roof
1 clump of mixed mosses taken from a sunny area, dirt shaken outPlace ingredients into a blender and blend at low speed
½ pint buttermilk or thick soymilk
Insert mix into spray bottle and broadcast over substrate (carpet with light dusting of soil)
Moisten regularly until well established.
Today we took the roof on the greenhouse to the next stage by planting a summer garden there. The plants we chose had to be shallow-rooted, but there are no shortage of herbs and vegetables that can qualify. We made some bordered raised-beds and weeded out the random grasses that had volunteered since the roof was initially covered last year.
We now have experience with putting carpet sandwiches on all shapes and sizes of roofs — gable roofs, hip roofs, shed roofs, round-pole, even domed. In addition to the cooling effect in summer, they are beautiful, productive, and carbon-sequestering.
As the price and reliability of electricity becomes less predictable, and climate continues to warm, living roofs will undoubtedly become more popular for natural air conditioning.