Roots in roofs


Extensive accessed 10/12/2013

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.


Intensive accessed 10/12/2013

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.”


Hanging Gardens of Babylon accessed 10/12/2013

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.


SImple construction example accessed 10/12/2013

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.

Aesthetically green roofs add value to a building. Landscaping, including green roofs and walls, add an estimated 15-20% to a buildings value. If a majority of the buildings both public and private built green roofs, it would be fair to argue that it would increase the property values in that area across the board. I would argue that these statistics are the best way to explain the true value of the green roof, it just looks better, and makes people feel better. With a decrease in pollution and cleaner air, green roofs could make a serious impact in the quality of life in a city.Green roofs are a booming business in the Netherlands, in fact the government has begun subsidizing builders with grants to create buildings with green roofs. The planning system in the Netherlands has been widely touted as progressive, successful and effective. If more countries supported a fairly simple innovation like green roofs, it would benefit the global climate and potentially the global economy. Even with the hard costs of vegetative roofing exceeding the costs of traditional roofing, the climatic, economic, and psychological payback are worth every penny.

5 thoughts on “Roots in roofs

  1. I have seen a marked increase in the number of green roofs in the U.S., in just the past ten years. Even the COBE Building has utilized green roof on a portion of its structure. It would be interesting to know what percentage of new building construction, especially commercial, is using green roof technology, and what incentives would make this more attractive to builders.

    • That is an interesting question, In terms of incentives I would imagine it comes down to reducing the initial cost, because from what I found out the long term payoff is there. I would like to see the payoff/cost along side that of solar panel roofs, i’d say that the green roofs are a more attractive option.

  2. There are several extensive green roof installations in the Boise area. One of the more popular uses drought-tolerant sedge planting material. These add only a minimal additional weight and (at least in commercial buildings with roof-top HVAC units) the supplemental irrigation needed for the plants during dry times (usually the summer) comes from the A/C-unit’s condensate. The Ada County Parks & Recreation Department’s administration building at Barber Park has just such a system.

    I designed a green roof installation for a friend’s residence (with such a sedge system in mind) and it only required moving the 11-7/8″ TJI’s (as I recall they were TJI 110’s) from 16″ on-center to just 12″ on-center. It was such a small additional cost, we went ahead and installed them for the system. We didn’t have the funds to buy the pre-trayed sedge plantings initially, so we just installed the required membrane roofing material and spread out some pea gravel for UV protection. If you use Google Earth and go to 1412 Promontory Road, Boise, ID (street view, west side of street) you can see the installation. This was a remodel of a mid-century modern ranch, that had been cut up pretty bad in the 90’s.

    • I think it would take an area specific cost benefit analysis to decide wether it would be worth it.

Comments are closed.