Low-Cost, High Performing Living Roofs
- Sigi Koko
- Jun 10
- 4 min read
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 on your rooftop that relies exclusively on rainwater once the plants are fully established. This type of roof provides 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.
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.
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). For oddly shaped roofs or larger roofs, I use a heat-seamed TPO membrane. 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 - The drainage layer lifts the soil up about 1/4" off the surface of your roof, allowing the water to drain off below the soil, instead of through the soil. 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. I have 2 points of advice here. First, if you use compost, be sure that it is sterile, ie, that there are no active seeds (or you will be up there weeding like crazy). Second, 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), otherwise your plants will take years to fully cover your roof area.
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
The best times of year to plant are during cool seasons with predictable rainfall. Where I live in Pennsylvania, this means March & April, or October & November. By planting during these cooler wetter seasons, you reduce the need to water your freshly planted roof.
I follow this watering schedule:
water the day you plant (always!!)
water again each day the first week your plants are on the roof
water twice a week for 2 weeks (if it's really hot and sunny, you might water 3x each week)
water once a week for 4 weeks (skip any week that it rains significantly)
Anytime it rains, you can skip the watering. Once plants are established (usually within 7 or 8 weeks), there is no need to water the plants.
Recommended Reads
Here are my favorite books about living roofs
(click the cover of the book to purchase)
Small Green Roofs: this book is digestible & easy to read, explaining basic green roof principles with case studies of actual installations. | Green Roof Plants: catalogs the type of plants that work on various green roofs, including what climate they thrive in. | Planting Green Roofs and Living Walls: This book has a bit of everything...design criteria, stormwater calculations, plant specifics, etc. | The Professional Guide to Green Roofs: Geared more toward designers, with design criteria, plus installation & maintenance considerations. | Green Roofs: This book has a bit more of a global tone, with examples from around the world |
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