Green roofs are a system in which buildings both large and small grow plants on their roofs. They can be edible plants, natural plants, even large bushes and trees to create a “floating park”. There are two main types of vegetative roofs; extensive green roofs which have shallow soil for smaller plants and intensive vegetative roofs which support larger plants. The plant choice depends on a few variables, environment, building structure and design, and what the inhabitants or builders want.
These roofs have seen increasing popularity but they aren’t a new idea. Green roofs have been around for thousands of years. The Hanging Gardens of Babylon are considered, by many, to be the first example of green roofs. More recently Scandinavian people used sod roofs on all of their buildings including personal residence and churches. Green roofs aren’t new to the United States either, during the westward expansion homesteaders used sod roofs on the prairie out of necessity, since there wasn’t and abundance of timber. The first examples of what is considered modern green roof technology emerged in the 1970’s from Germany in response to the energy shortage of that time. Germany has continued utilizing green roofs “By 2005, an estimated 13 million m2 roof area in Germany was covered with plants.”
The benefits of vegetative roofs are many. They improve thermal and noise insulation with the dense soil and vegetation, create an animal habitat, combat carbon pollution, increase longevity of the roof structure, and help storm water runoff management among other things. They also look outstanding when they’re done right. So are they a crucial part of combating climate change or does the cost outweigh the benefit?
Green roofs can be retrofitted onto pre-existing homes or buildings if they aren’t too heavy, but to have an intensive green roof system it has to be designed for the added weight. The simplified version of the process involves adding a waterproof layer to the surface of the roof, a drainage system, soil, and of course the vegetation. Both the retrofit, and the material intensive heavy load bearing roofs designed for a park like green roof cost anywhere from $10-$25 . The maintenance cost of a green roof is more or less $1 per square foot.
Other than the hard costs, it is hard to calculate the saving of a green roof in terms of storm water runoff savings and environmental impact saving, unless these entities are examined on a project by project basis. With that being said, Portland did a cost benefit analysis to discover whether it would make sense to start a green roof initiative in the city. The study investigated both the the costs and the payoffs on public and private buildings. The benefits that they used to calculate the dollar figures for the private side of the study include “onetime and ongoing reduction in storm water management fees, avoided storm water management facility costs, reduced cooling and heating costs, avoided roof replacement costs, and reduced HVAC equipment sizing costs. ” The figure they came up with over a 40 year period of having the “eco roof” was a staggering, $404,000.00 On the public side they determined that over 40 years the payback would be $191,421. With the cost benefit being close to $600,000 it is hard to imagine that all cities and private homebuilders wouldn’t support this advance in green technology. Unfortunately the upfront cost dissuades many private homebuilders, and cities alike, even with potential tax benefits and grants.