Green roofs can be seen as flat or very low-slope roofs, partially or completely covered with vegetation; they are mostly of two great types 1) modular, that is, built with pre-vegetated plastic trays 2) or site-built with a layer of soil and vegetation.
What green roofs are not
Green roofs should not be seen as small gardens installed on the roof of our buildings, to attract birds or other small animals; or as a spaces for leisure or to provide beauty.
Why green roofs?
We can think of green roofs as an extension of an existing flat roof, based on a very good waterproofing membrane and a layer of plants over it.
Make sure whether a green roof is right for your climate and for you.
Consider potential problems involving their weight, leaking, maintenance, wind and long-term survival of the plants.
And do not forget the high costs of green roofs.
See: Green Roofs Pros and Cons
Green roof goals
Green roofs are mostly designed with environmental goals in mind, namely to reduce the very high summer urban temperatures (the urban heat island effect) and as part of a strategy to reduce heat gains through the roof.
In this last sense they can be seen as part of home low-energy strategies, intended to reduce air conditioning costs. But we should weigh their pros and cons, and consider their cost-effectiveness.
Building The most simple type of green roofs: modular green roofs
Modular green roofs are relatively simple to implement, compared to other types of green roofs. The plants come already grown in plastic trays, designed to allow direct attachment and easy drainage.
Obviously, the plants should be selected to withstand extreme weather conditions.
The advantages of modular green roofs
The advantages of modular systems over other more complex systems are obvious. They are simple to install, and much cheaper (expect prices of at least $8 per square foot).
With them it's easy to find leaks and fix problems with the roof. You don't have to remove the vegetation, soil and drainage system; just to remove some trays.
Be cautious, anyway. Consider carefully your climate, the conditions on the top of your roof, and the type of plants. Concerns about leaks and the weight of the system and its maintenance requirements are founded.
Modular green roofs are unsuitable for many cold and hot climates, or in windy areas. It's not easy to find plants able to survive in the extreme temperatures and weather conditions of rooftops, and it's very difficult or impossible to have healthy plants for long in plastic trays. And also do not expect a free-maintenance system.
A more complex and demanding type of green roofs: Site-Built multi-Layered green roofs
Site-built multi-layered green roofs are the "truest" type of green roofs. But even the simplest systems can be complex - and demanding and expensive.
Site-built green roofs have several layers of “materials”, as shown in the image below (from the GreenRoofs.org).
Site-built multi-layered green roofs are an expensive type of green roofs; expect prices at least three times higher than those of modular system, that is, prices above $25 per square feet.
Many systems have a wind erosion blanket, made of a thin fabric (burlap), to prevent the wind from blowing the soil away, and a root barrier layer, that is, some sort of foil or plastic material to protect the waterproofing membrane from the roots of the plants…
The filter membrane layer is a biodegradable fabric to filter the irrigation and the rainwater running down to the roof.
The waterproofing layer (or vapor control layer, in the image) is a typical roof waterproofing membrane. See: Roof Waterproofing Membranes
The drainage layer is a filter fabric or an egg-crate system, designed to prevent the water from sitting on the roof membrane; it directs the water to a drain.
The soil layer, and the other parts of the green roof, should provide high levels of insulation. But a specific insulation layer, typically made of foam rigid boards, combined with the soil, will provide extremely high levels of roof insulation, important for home energy efficiency and to reduce cooling bills.
Types of green roof plants
Pay special attention to the type of vegetation and the soil layerse.
You can't just select ordinary plants. You need very special types of plants, able to respond to extreme conditions.
The selected plants should be able to withstand extreme temperatures and winds, typical of rooftops in urban and suburban areas: for instance, mixtures of succulents, mosses and grasses found on shoreline coasts, or in semi-deserts and dry mountain landscapes.
The exact types of plants you can use on a green roof depend on your climate and factors like sun exposure and the type and depth of the soils.
Do not be let you be fooled by ideas of beauty, concerning the plants you would like to see on the roof...
You should think in other terms: what’s their mature weight, and how much soil do they require; or... what type of plants do well on extreme conditions (the conditions that are typical of a roof).
The type of green roof matters a lot when choosing the plants. In the case of modular green roofs, the sellers of the modules pre-vegetate themselves the plants; and they should have the experience and the know-how to choose the right plants, responding to your local climate and conditions.
Right plants for thin and deep growing medium (soil) for small green Roofs
For very thin layers of organic soil, consider hardy, shallow-rooting varieties, able to survive in poor and dry weather conditions.
These types of plants are low-maintenance plants and involve a small weight. Examples: stone-crop (Sedum) and ice-plant (Ddelosperma). They are succulents, which means that they will retain water during dry periods, surviving without irrigation for weeks. Sedums have the advantage of providing year-round leaves and flowers.
With deeper soil (say up to around 4 inches/10 cm), we can use a wider range of plants: several species of herbs, grasses, alliums, or wildflowers. For yet deeper growing medium, consider also grasses, but also black-eyed susans, columbines or asters.
As to the soil, it should contain proper mineral substrate, adequate depth and nutrients.
Obviously, young plants will require special care and maintenance during months (or years), contrary to what may happen with pre-vegetated modular systems.
See also: Green Roofs Pros and Cons.