14 September 2011

Low-Cost, High Performing Living Roofs


A living roof, or green roof, describes a system that allows plants to thrive on the surface of rooftop without access to groundwater.  The idea is to create a self-sufficient ecosystem that doesn't need you to water once the plants are fully established.  This type of roof does provide energy-efficiency benefits in the summer, because the plants provide a net cooling effect.  Even one inch of planted soil lowers overall average roof temperatures and reduces day-to-night temperature swings on roof surface.  A green roof does NOT provide insulation to speak of, so you still need to insulate your roof, just like you normally would.  Additionally, plants absorb and filter rainwater, reducing the negative impacts of excess storm runoff in watersheds, which is especially beneficial in areas with high percentage of impervious surfaces, such as cities and suburbs.  (Yes, I did write suburbs!  A mowed lawn only allows an average of 40% of the rain that falls on it to absorb into the groundwater table!)

The most important detail with a living roof is to select plants that will thrive in your climate and with the amount of sunlight striking your roof.  (See below for a living roof plant resource.)  First and foremost, your plants need to be able to survive without access to groundwater and rely just on your local rainfall.  If you get tons of annual rainfall, you will want to be sure you select plants that can handle "wet feet", and be sure your roof drains well.  If your roof is in full sun, you will need to select plants that will not wither under the heat of relentless summer sunshine.  Etc.

full shade allows greater variety of plants
Most planted roofs are installed on reasonably flat surfaces.  But this is by no means a requirements.  Steeper roofs (above about a 30-degree slope) do require additional erosion control, especially while the plants establish their root systems.  Most commonly, I use a wooden trellis type grid that rests directly on the drainage layer (not fastened to the roof).  Then plant in between the grid of the trellis.  As the plants establish their root system, the wood biodegrades, providing additional nutrients for the plants.  By the time the wood has composted, your plant roots become your erosion control.

plants thrive on shallow or steep slopes, even curves!

Here are the layers I have used with great success on numerous small scale roofs:
  1. sheathing (such as plywood) - typical roof sheathing is 1/2", but my engineer likes to bump it up to 5/8" to prevent any sagging from the weight of the soil.  The sheathing is part of your structure, so I make sure an engineer approves the roof framing as well as the sheathing.
  2. waterproofing membrane - the lowest cost option I've found for this that has high effectiveness is 60 mil EPDM (rubber pond liner).  I also recommend reading "Stoneview: How to Build an Eco-Friendly Little Guesthouse" (New Society Publishers) by Rob Roy for additional suggestions for waterproofing membranes.
  3. drainage/filter layer - I don't skimp on this because it keeps your soil medium up on your roof, even with heavy rainfall.  My favorite drainage layer with integral filter fabric is: Enkadrain 3615 (by www.colbond-usa.com) because it is easy to cut & easy to install, especially on curvaceous roofs.
  4. growing medium (soil) - I have used everything from compost to an engineer mix of expanded shale & organic soil; they all seem to work well.  The only advice here is to be sure if you are using compost, that it is sterile, ie, that there are no active seeds (or you will be up there weeding like crazy) and if you use a mix with high inorganic content (like expanded shale) be sure it's mixed with at least also 60% organic soil (like sterile compost)
  5. plants - I use ONLY sedums and other rock garden plants.  These are plants that don't rely on ground water, but instead have ways of storing rainwater (their leaves act like a cistern) or can pull humidity from the air for moisture.  Be sure to select plants that will thrive on your roof and in your climate, ie, whether your roof is in full sun or full shade and how much rainfall  you get in a year.  For recommendations on plants, see www.greenroofplants.com 
 
typical living roof detail
For additional information, including books and other resources, see my full article on Living Roofs at www.buildnaturally.com/EDucate/Articles/LivingRoof.htm

8 comments:

  1. Hi Sigi,

    What would be consider a too steep of slope for a green roof?

    thanks,

    Steve

    ReplyDelete
    Replies
    1. once you exceed about a 30-degree slope (around a 6-in-12 pitch), you need to install some kind of gridwork to prevent erosion. To me, I'd rather avoid that step, so I stick to the shallower slopes.

      Delete
  2. can a live roof be installed over a standard shingled roof, with no other underlayment? Blessings.M

    ReplyDelete
    Replies
    1. no, the water will run uphill under the shingles

      Delete
  3. i have a 12 degree slope and am doing a roof with GI sheet and pine rafters and planks below the insulation. can i do a living roof on top of the GI sheet, how do i prevent leakage?

    ReplyDelete
    Replies
    1. I am not familiar with what you mean by "GI sheet". But bottom line is that the waterproofing membrane below any of the living roof layers need to be your leak prevention layer. Everything else above is the living roof system and protects your waterproofing from UV degradation.

      Delete
  4. What size and spacing of rafters do you use for this roof?

    ReplyDelete
    Replies
    1. that depends on many structural factors. I would recommend to discuss your specific structure with an engineer.

      Delete