4. Green / Living Roofs – Basics

I’m excited to see the rising popularity of green roofs, a great way to add planting and “green-ness” to a building in many ways.  The more accurate term is actually living roof, because we aren’t talking about green colored shingles or something, we’re talking about a planted roof top that is a living system of plants, soil, water, microorganisms, insects, birds – and maybe even a few humans.  Living roofs at their best are not only providing the many expected sustainability benefits, but also beautifying cities and providing additional outdoor space for people to enjoy.

Extensive (thin) versus Intensive (thick)
One key distinction to make in the definition of living roofs has to do with soil depth.  The technical terminology to distinguish these is Extensive and Intensive, which can be a little confusing.  The Extensive living roof, which is easier to remember as the “thin” version, is typically defined as a planted roof with a soil layer of 6”or less.  The Intensive living roof, or “thick” version, is anything with a soil layer over 6”.  This latter version has been around for a while, Architects and Landscape Architects have been putting pots and larger built-in planters on roofs, decks, and balconies for decades.  It is the Extensive or “thin” type that has recently had a rapid rise in popularity in the U.S., largely because of advances in the technologies that can allow plants to be happy in such a thin layer of soil in the harsh environment of a building rooftop. (I will get more into the technical aspects of living roofs in a later posting.)

Living Roof Benefits
Why have a living roof on your building?  Building roof tops are a very large untapped resource of space that receive ample sun and rain water.  Obviously, one way to take advantage of this resource is solar collectors, and ideally you have space for both, but in my opinion there is a pretty good argument that the sustainability benefits of a living roof are greater.  Here are a few of the main ones:

  1. Stormwater – One of the major issues in our continually expanding urban areas is the blocking of natural rain water cycles by paving and buildings.  A living roof is an elegant solution that can literally take the slice of land that the building has displaced, and replicate many of its functions up above.  The plantings and soil layer of a living roof can filter rainwater, reducing pollution of rivers and streams.  They also slow down the rainwater, allowing the plants to make use of some of it, and outputting the rest into local streams gradually, reducing flooding and erosion problems.
  2. Habitat – Carrying further that idea of the living roof as a mitigator of the land lost to the building footprint, there is great potential for it to provide natural habitat.  Of course the roof top climate is not exactly the same as it would be at ground level, and is usually vertically disconnected, so you’re probably not going to have a herd of elk ranging across your roof, but there are plenty of insects and birds that will use it.
  3. Insulation – traditional roofs are surfaces with dark colored, asphalt based materials which absorb a significant amount of solar heat.  This heat transfers to the building interior, adding load to the air conditioning system as well as radiating out into the local environment, contributing to the urban heat island effect.  A living roof has plant material to absorb this solar energy and convert it to plant growth, and transpire moisture into the air, cooling it.  The living roof’s soil layer is also an excellent insulator for the building interior.

Living Roofs Everywhere?
So, why aren’t all roofs already planted?  Certainly there are traditional building styles with vegetated roofs that date back centuries, but as modern construction techniques have evolved, roof design has been about maximizing the waterproofing and durability.  Nobody wants to see a leaky roof above their working or living spaces, which are full of things easily damaged by water.  Adding layers of wet soil, plants and even extra water piping for irrigation does increase the potential for damage should a leak develop.  It has taken some major developments in waterproofing technology – both products and techniques – to get to a point where design and building professionals (as well as insurance companies) can be confident that a living roof is a stable, leak proof system that will last the life of the building.

There can also be increased construction costs associated with living roofs.  At their best they don’t increase structural loads very much, but they do often lead to increased sizes for beams, decking, etc. plus the costs of the soil and plantings.  We won’t get into a cost versus benefit analysis here, but there are plenty of good arguments that living roof costs are quickly offset by energy savings and extended life of the waterproofing layers.

We may look back to this time in history as the turning of a corner -  have we finally reached a point in technological development and attitude where everyday buildings can be extrusions from the landscape that don’t wipe it out, only lift it up a bit?  A world where the natural landscape can flow right over our buildings would not only be more sustainable in a practical sense, but also re-connect us to nature in our daily lives.  Let’s do it!



I can’t put my finger on specific books that I referred to for this information; there are many good ones out there.  This writing is based mostly on my personal experiences designing and observing installation of green roofs, a good place to start is the organization Green Roofs For Healthy Cities:  http://www.greenroofs.org/
See also my Links page for other web resources.

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