Like gardening but don't have the space for it?  Grow one on your roof!  While this is unfortunately not possible for everyone, green roofs are wildly popular in places like the UK and Germany and are picking up speed in America.  Green roofs are ideal for urban settings.  In dense cities like NYC, green roofs would help "mitigate urban heat island effect"¹.  One of the purposes of green roofs is to provide building insulation.  On those hot summer days, if someone lived in a building with a green roof, their homes would be much cooler without worrying about high energy bills.

What are green roofs?

To put it simply, green roofs are roofs that have plants growing on their surfaces.  Of course, it's so much more than that.  Green roofs come in three different types, intensive, semi-intensive, and extensive.  However, they are all designed to serve the same purpose.  Green roofs should, if installed correctly, help insulate a building, protect the roof and help with the environment among other things.

Types of green roofs

Intensive
Intensive green roofs are much like parks on top of buildings.  This type requires 8" or more of soil, and thus can support a variety of different plants, such as trees, shrubs, and lawns.  Intensive green roofs are much like parks or gardens in such that they may also include walkways, benches, or even playgrounds.  They are designed to support various different landscaping qualities. Also, because intensive green roofs can sustain a variety of plants, they can support a larger ecosystem for birds and insects.  However, all that additional landscaping means intensive green roofs can be extremely heavy, and will get even heavier when saturated.  In fact, the when saturated, the weight can increase anywhere between 60-200lbs/sf.² Intensive green roofs usually need to be built on concrete roofs in order to support all that excess weight. 

 
Because of all of these different qualities, green roofs require high maintenance and regular installation.  Intensive green roofs are also much more expensive due to the amount of maintenance required and the intensive installation process.  It is necessary to consult a professional before installing an intensive green roof.

Extensive

Extensive green roofs, though not as elaborate as intensive green roofs, require much less maintenance.  Since extensive green roofs only require 1-5" of soil they cannot support the same diversity of plants intensive green roofs can.  They are limited to alpine, dry land, or succulent plants such as moss, sedum, or herb plants.  However, extensive green roofs are much more accessible to the public than intensive green roofs.  Because they weigh much less than intensive green roofs, they do not need the same sort of support structure to hold their weight.  Even better, extensive green roofs only need a small amount of care and do not require irrigation systems.  After the plants have been installed on the roofs, it is not necessary to water or fertilize them.  In fact, the only maintenance required is a visit about two times a year for a check up and to weed out invasive species.

Semi-Intensive

Semi-intensive green roofs are somewhere between intensive and extensive green roofs.  They require less maintenance and are cheaper than intensive green roofs, but can support a more diverse amount of plants than an extensive green roof can.  Semi-intensive gardens require the same kind of work that intensive gardens do, just much less of it.  While one certainly wouldn't get the park-like quality of an intensive green roof, semi-intensive green roofs offer a wider selection of plants than an extensive green roof does.

Systems for green roofs

Aside from intensive, semi-intensive, and extensive green roofs, there is an option of built-in or modular green roofs.

Built-In-Place systems

Intensive green roofs are usually built-in-place. However, built-in-place green roofs are extremely complex, require several layers of materials (growing medium, filter membrane, drainage layer, and waterproof/root repellent layer³), and are expensive to maintain. Built-in-place green roofs are designed to be permanent, and not only entail a complex and lengthy installation process, but are also complicated to remove.  In addition, maintenance of the actual roof can be problematic. 

Modular systems

Since there are so many complications concerning built-in-place roofs, modular roofs are much more popular and manageable than built-in-place roofs.  Modular systems are especially used for extensive green roofs.  They are lighter than built-in-place systems and are quicker to install, and much easier to maintain.  Pre-planted modules are also commercially available.

Benefits of Green Roofs

Despite the effort and money needed to install and care for green roofs, they are beneficial in the long run.  In big cities, where there is not as much green space, green roofs are a great way to help with the environment.  With the amount of emissions pumped from cars in the city, we need as much green life as possible.

¹ Earth Pledge.  "Green Roofs."  Why Green Roofs for NYC. < http://www.earthpledge.org/WhyGR.html>

² Vasilnek, Nicole.  "What are Green Roofs?" <www.deq.state.mi.us/documents/deq-ess-p2-p2week-greenroofresources.doc>

³ Green Roofs for Healthy Cities.  "About Green Roofs." < http://www.greenroofs.org/index.php?option=com_content&task=view&id=26&Itemid=40>