What is a green roof?
While technically you could just hop on a roof with a big can of green paint, slap it on, and call it a green roof, that's sure not what we're talking about here!
The type of green roof we are concerning ourselves with is one that is covered by large amounts of foliage. Instead of a simple flat surface covered in gravel or tar or such, there is a covering of dirt or some other growth medium, and plenty of vegetation living in that medium. But understand, we're not talking just a few plants or a small greenhouse - it is an actual covering of vegetation, and for all purposes the plants are an extension of the roof. A proper green roof is completely designed around the foliage and for a number of things that happen because of their presence.
They are also sometimes called a living roof or a eco-roof.
Why bother with a green roof?
So, what is the value of placing a green roof on top of a building? Obviously there's extra cost associated with the additional materials - why not save money by just tossing on a thin layer of pebbles? Well, that's one of the cool parts. You can benefit the city in general, along with the environment, while at the same time reduce costs!
There are plenty of benefits to adding greenery on a rooftop. Remember, most likely the land the building is located on used to be vegetation. Turning it into concrete and steel is going to affect the environment. Vegetation lowers temperatures and absorbs water during rain, and by putting plants on top you can get some of the benefits back.
The 'urban heat island effect', where temperatures are hotter in the city than in surrounding areas, can be reduced by covering a building with plants. While each building may have a small effect, the more cover there is, the less additional heat in the area. Less heat, lower air conditioning costs and less energy usage. Oh, and less heat can also mean fewer problems with ozone.
A green roof also helps with temperature for the building more directly. The plants themselves take the direct sunlight, and instead of transferring it to the building like you'd get with sunlight right on the roof, the larger surface area helps radiate some of it away, and they also use some for evaporation. Larger plants such as bushes and even trees can help here, as shade is more effective at controlling temperature than internal insulation. But that's another great thing - the layers needed for the green roof also work as insulation, which you'll want on your building anyways.
Plants also do wonders for dealing with precipitation. Typically, you need a drainage system to channel all the rain on a building's roof, lest you get puddles and leakage. Even then, it still happens. A green roof can really help out here - a simple covering of grass can retain as much as 70-80% of the precipitation falling on it during non-winter months. You only have about a quarter of the amount to go through drainage - less wear on the system, and lower costs for dealing with the runoff. And because that water that does runoff has to go through the vegetation and such, it takes longer to drain, resulting in less runoff at peak times after a storm, meaning less stress on the sewer system, and the water that does runoff has already been at least partially filtered.
Vegetation provides benefits for air quality, and the more vegetation, the more benefits. First, there's the standard issue with plants using up carbon dioxide and producing oxygen, making sure the air is breathable, and helping to do a little to fight rising CO2 levels. Plants also act as a natural air filter, removing particulates which result in smog. The previously mentioned heat reductions can also decrease the amount of ozone held near the ground, helping fight pollution in another way.
Now, you get all these benefits even if you have a simple flat roof just covered in grass. But if you want to get a bit fancier, create your green roof to do just more than have plants. Perhaps you could make it safe for building inhabitants to spend time up there - if the building is large enough, add in walking paths, some benches or such, and make it like your own private little park. Make room for a garden - if a restaurant is in the building, the chef may love to grow their own herbs or vegetables - the Fairmount Waterfront Hotel in Vancouver saves $30,000 a year doing this. It's also great for an apartment building or condo, letting residents have their own growing spaces - without worries of vandalism that are concerns at ground level. Or if your building is large enough, perhaps a golf hole or two, for the executives to enjoy between meetings. (I'm not making this up!)
There's even better news, and another reason for adding a green roof. There is actually less wear and tear on a green roof than a regular roof. This results in a longer roof lifespan - you could find that you can go twice as long without requiring a re-roofing. Surely that speaks to your balance sheet when making a decision.
How do you create a green roof?
You cannot just throw a layer of dirt on top of a roof, unroll some sod or plant a bunch of flowers. You'll end up with structural damage and other unpleasantries.
For proper greening, you need to take a few details into account. First, can the roof support the weight of the plants plus all the materials they need for growth? Your typical roof isn't designed for that much weight. Second, there need to be drainage systems in place that can work with the new roof covering.
A proper green roof contains a number of layers. At the bottom is your building structure, the base layer. This is clearly the most important - if your base layer breaks, everything comes tumbling down and the building is open to the elements. Not good. So make sure it's strong. Above that is a waterproofing layer. While the foliage will decrease water getting through, it can never stop it. You want to keep the water from seeping into the structure and causing damage and leaks.
On top of the waterproofing is a drainage layer. If there's enough moisture to make it down to the waterproofing, then there's too much water to be absorbed - letting it sit can result in muck and dead plants, ruining the green roof. There are systems that can channel the water down to drainage pipes through combinations of membranes minor grades to prevent these problems. Above this is the growth medium, typically a soil. And then comes the vegetation.
You can't use just any plants. This is the key layer in a green roof, and proper plant selection is important, and depends on the purpose of the roof. If you're not creating a garden, just going for basic plant cover, choose plants that create strong root systems. They hold the soil in place better to prevent erosion, and have greater water retention capabilities. You also want plants that do well in the local climate year-round, and require minimum maintenance. Sod, for example, is a poor choice, as it requires frequent maintenance - think how often you need to water a lawn in the summer, for example. Mosses are also a poor choice - they may hold plenty of water, but they are also very flammable when dry, leading to a fire hazard, the last thing you want on your building's roof.
So what are the structural requirements? It depends on the type of green roof. A basic green roof, with just basic grasses or other plants fitting the area, which is known as an extensive green roof, has minimal requirements. There's only between 1-5 inches of growth medium, meaning an additional load of 15 lbs/f2 to 50 lbs/f2. They also require minimal maintenance - usually only once per year.
The other route, an intensive green roof, has much greater requirements. This type of roof includes shrubs, trees, and is more of a rooftop garden. This usually results in loads from 80 lbs/f2 to 150 lbs/f2. They offer more benefits, as they are usually designed for people to enjoy on a regular basis.
Where do I find green roofs?
Europe has long been the leader in green roofs, and Germany is in many ways the home. Countries such as France, Austria, and Switzerland also have significant numbers of green roofs, and they continue to pop up.
For the most part, the idea of a green roof is fairly new in the United States, though they can be found scattered around. Chicago's City Hall, for example, is greened. There are also quite a few in Portland, Oregon, and the city has incentives for using them in new construction - buildings are currently allowed a density bonus, meaning they can add 3 ft2 every ft2 of green roof. They're also implementing the Clean River Incentive and Discount Program, which will result in reduced stormwater utility fees for buildings with a green roof. Canada is also getting into green roofing, such as with the Toronto City Hall. Hopefully North America continues to add rooftop foliage, as it can only help out the cities.
Green Roofs for Healthy Cities, http://peck.ca/grhcc/
Environmental Design + Construction, Featured Item: Green Roofs, http://www.edcmag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,4120,18769,00.html