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|
|plants thrive on shallow or steep slopes, even curves!|
Here are the layers I have used with great success on numerous small scale roofs:
- 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.
- 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.
- 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.
- 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)
- 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