As "sustainable architecture" becomes a familiar term to many, we're starting to hear a lot about green roofing. It's a concept that seems to have an awful lot going for it, and -- added bonus -- it looks really cool, to boot (especially from a Google Earth POV). Via Ecogeek, here's a quick primer on just what they are and how they work.

First of all, there are two kinds of green roofs:
intensive, which are between two and four feet deep and support the growth of all kinds of plants and even some small trees, but require a great deal of structural support, because they're really heavy, and
extensive, the most common kind of green (or "vegetated") roof, which are about four inches deep at the most. Many kinds of grass and drought-resistant plants can thrive at that depth, which also naturally limits weed growth and tends to self-regulate. (Naturally, their weight is also a lot easier to support.)

The benefits
There are many, but a few of the most compelling include

Keepin' it cool:

Green roofs keep the roof cooler, which helps to reduce the heat-island effect, which contributes to cities being hotter than the surrounding countryside. This can be beneficial to the building in reducing its summertime cooling load.

Longer roof life:

Green roofs also protect the roof membrane from sunlight, which breaks down the roofing material. Having even a couple inches of soil helps to greatly extend the life of the roof, and a longer lifespan means less material ends up in landfills from re-roofing buildings after the membranes have failed.

Greatly reduced stormwater runoff:

In some cases, this can help reduce the size of stormwater pipes, and the amount of stormwater that needs to be treated by municipal water treatment. In a light rainfall, a building with a vegetated roof can have no stormwater runoff at all.

Everybody gets a yard:

In cities where concrete rules and green space is at a premium, how cool would it be for everyone to have a little park of their own on the roof? You can BBQ up there, stretch out for a nap in relative peace and isolation, and even, as some have done, graze farm animals!


The drawbacks
Right now, the main drawback is expense. It's definitely pricey to convert an existing "normal" roof to a vegetated one; the extra load necessitates extensive surveying and architectural rejiggering (and architects drive fancy cars for a reason). Cost is less of an issue with new construction.

Picture credits:
1) Green roof in Oswego, Illinois by Greg Robbins. Greg Robbins on Flickr
2. Goats grazing on a roof in Wisconsin, from Driftless Media on Flickr