When Sightline fan Matt the Engineer recently built a new garage at his Seattle home, he really wanted to put a deck on top of it. But city regulations prevented the fulfillment of his wish (there’s a ban on rooftop decks near alleys, the logic of which eludes me). Being a solutions-oriented fellow (he musta gotten that from us), Matt came up with an alternative. He built an ecoroof.
Now Matt’s garage top is a gorgeous extension of his yard, sprouting homegrown veggies that feed his family. And he’s helping the environment with planters that soak up rain and prevent stormwater runoff that harms salmon and causes erosion and flooding.
Ecoroofs (also called green or living roofs) are a proven strategy for cutting Northwest stormwater runoff by about 60 percent compared to a traditional roof, and local governments are trying to encourage residents and businesses to install them. But most folks aren’t yet ready to follow in Matt’s footsteps. For many building owners serious questions and concerns remain about cost of green roofs and whether the whole thing will spring a leak or collapse one day in a dirty mess.
Let’s see if the research that has been done on ecoroofs can help put some of those worries to rest.
First off, let’s tackle the easier structural questions.
Weight: A roof that supports an ecoroof needs to be able to handle an additional 10 to 30 pounds per square foot—which some roofs already do. Extra reinforcement is required in other cases.
Different kinds of ecoroofs will add varying amounts of weight. Some are “intensive” green roofs, like the one Matt built, in which the soil is deeper (about 11 inches). “Extensive” green roofs have a soil layer of about 4 to 6 inches.
Leaking: If the roof is well built, the main threat of leaking comes from the plant roots penetrating the roofing material. For sedums and grasses, that shouldn’t be a problem. Experts recommended making sure the roof is weeded to keep trees and other plants that could spring up on the roof as unwanted volunteers. If leaks happen, small, site-specific repairs should solve the problem.
Matt the Engineer, being a mechanical and not a civil engineer, was worried about the leak issue. His solution? He doubled up. On top of the drainage layer (also called geotextile material), he put down a second layer of waterproof membrane (5 mm polyethylene in his case) and then another drainage layer.
The material was inexpensive and light, so it seemed a good solution.
“Not being an expert in green roof design, I figured this was cheap insurance,” Matt said.
Lifespan: Green roofs are shielded from UV exposure, extreme temperature fluctuations, and damage—all factors that shorten the life of an asphalt, composite, or cedar roof. Conventional roofs have a 20 year lifespan, compared to 40 years or more for a green roof.
Cost: Here’s the biggie. Portland, the region’s leader in this field, did a detailed cost-benefit analysis for ecoroofs in 2008. The report concluded that a conventional roof for a large project costs about $10 per square foot, while a green roof on the same building would pencil in at about $15.75 per square foot (see Table 20). The range generally given for green roof costs is $10 to $25 per square foot.
The greatest costs come from the soil and plants needed for the roof. Here’s a breakdown:
| Component | Cost (per ft2) |
| Moisture mat |
$0.50 |
| Protection board |
$0.25 |
| 5-inch soil (with gravel drainage) |
$2.00 |
| Plantings (sedums and grasses) |
$2.00 |
| Irrigation system |
$0.50 |
| Plant establishment (labor cost) |
$0.50 |
| Total |
$5.75 |
While installation of the green roof costs more upfront, that skips all of the savings associated with the roof—primarily their extended lifespan. There are also heating and cooling costs savings that come from the ecoroof’s added insulation. And there are the less-economically tangible benefits (at least to a homeowner) of creating wildlife habitat, reducing “heat island” effects, and reducing air pollution.
Also, Portland and Seattle offer incentives for ecoroof installation. Folks in Portland can apply for a rebate of up to $5 per square foot for new green roofs (there’s a June 1 deadline, but the offer is repeated every six months until 2013; the next deadline is Dec. 1). And both cities offer a break on stormwater utility fees for larger projects, though it’s woefully difficult to figure out from the code what those savings amount to (Seattle here, Portland here).
When all of the costs and savings were tallied in the Portland study, a green roof cost $128,803 more at the 5 year point, but saved a building owner $403,632 at the 40 year mark (this calculation is for a 40,000 square foot roof, keeping in mind that the average residential roof is about 1,200 square feet, and does not include the potential rebate).
Matt didn’t calculate the cost of his ecoroof separate from the overall construction of the garage project. He’s thrilled to have created a whole new plot of growing space that gets loads of sun for producing a bounty of vegetables.
There are lots of resources available for folks ready to take the plunge. A great jumping off point is the city of Portland’s Ecoroof page and their fantastically easy-to-read ecoroof handbook. And Matt turned to a rooftop gardener at Portland’s Rocket restaurant for veggie growing tips.
More great resources include this Portland Tribune article on choosing a greener roof that goes beyond ecoroofs and also considers metal roofs and using recycled materials.
Also check out the city of Seattle’s green remodeling handbook on roofing, which includes a more detailed cost comparison of different types of roofing.
Ecoroof photos from Matt the Engineer. Ecoroof diagram is taken from the city of Portland’s “Ecoroof Handbook 2009.”
Rob Harrison AIA
(I’m going to try this again…my previous response seems to have vaporized—if it’s a double post please delete!) Good on ya Matt! The roof looks great! A couple clarifications and one comment. * Extensive vs Intensive These terms refer only to the depth of the growing medium, not the extent of coverage of the planting on the roof. * Weight Fully saturated, an extensive green roof with growing medium four inches deep will *ADD* 20 to 25 pounds per square foot to the roof’s structural requirements. Combined with the weight of the structure itself (ten pounds per square foot if it is light framing), the snow load (25 pounds per square foot in Seattle, significantly more in the foothills of the Cascades), that means a garage roof in Seattle with an extensive green roof will need to support more like 60 pounds per square foot, not ten to 30. A fully saturated intensive (thick) roof, with its 12 to 18 inches of growing medium, will be *much* heavier. One of the big advantages of an extensive (thin) roof is that an additional 20 to 25 pounds to the roof load is relatively easy to accommodate structurally. It is wise to consult a structural engineer before adding a green roof to an existing structure. * Leaking is no more (and no less) an issue on properly designed and detailed green roofs than on other low-slope roofs. The tricky part about green roofs is that they are covered with plants and soil, so finding a leak later is difficult, and disruptive to the plantings. The solution (as told to me by the head of the local roofers and waterproofers union) is to perform a “bathtub test” prior to placing the growing medium and plants. A temporary dam is built up at the low end of the slope, and the roof is filled with standing water for 24 hours. Any leaks will show up at that point, and can be easily fixed. After this is done, a green roof is less likely to have problems, since, as Lisa said, the membrane is protected from UV, the primary source of degradation of roofs. * Extensive (thin) roofs in particular are self-weeding. Any plant with a taproot won’t be able to establish itself. Our roof, for example, has no dandelions. Any tree seedlings die off over our hot dry summer. The root barrier, by the way, ought to go on top of the drainage layer, not below it as shown in the diagram. You want to keep that drainage layer clear, to allow water to flow to your drains or scuppers. * Rainwater Collection Exactly because green roofs are so good at absorbing rainwater, they don’t play well with rainwater collection systems—there just won’t be as much rainwater coming off of them! We recommend our clients collect rainwater off roofs first, to flush toilets and do laundry, and then do a green roof, preferably on a roof they will look out on on a daily basis. Seven years later, we are still completely enamored of the green roof on our garage: http://harrisonarchitects.com/projects/outbuildings/green_roof_garage
Eric Hess
Thanks Rob, sorry the original post disappeared. Sometimes we get funky comments that shut down the blog post. Everything looks good now.
Rob Harrison AIA
Thanks Eric, I think I recreated most of it. :)More about the root barrier: In an extensive (thin) green roof, the filter fabric above the drainage layer provides an adequate root barrier, so an extra root barrier is not necessary. In any case, the root barrier is meant to protect the membrane—to prevent roots from growing through the membrane—so should be above the membrane in the assembly, not beneath it. On our own garage, since it was our first one, we did put a bit of extra insurance in below the membrane, in the form of a layer of peel n’ stick membrane covering the entire roof. I’m not sure that’s necessary, but it doesn’t hurt.
Lisa Stiffler
Hi Rob –Thanks so much for your comments and feedback. I’ve checked out the ecoroofs on your Flickr page and they’re gorgeous. I updated this post to make clear that the weight requirements apply to additional weight, and corrected the intensive/extensive definitions. I’m changing the graphic to one that is more detailed and has the roofing layers in the order you describe. I’m surprised that the city of Portland got that reversed—one of their publications has it one way, the other flips it.
Matt the Engineer
Great comments [Rob]. The process would have been a whole lot easier if I’d used an architect with green roof experience (ok, using an architect at all would have been a whole lot easier).
Rob Harrison AIA
Lisa, I decided to dig a bit deeper on the root barrier question. I started to think, hmmmm, Tom Lipton in Portland is a real authority on greenroofs and probably knows more than I do…his group wouldn’t publish an incorrect diagram…. As it turns out there are two ways to do it, depending on the materials you use. In the first (what we’ve done) a synthetic filter fabric that acts as a root barrier is bonded to the top side of the geotextile drainage layer, and the waterproof membrane is inorganic TPO—also root-resistant. The second approach, used when the waterproofing membrane is organic (asphaltic or bitumen), is to have a secondary root barrier (like polyethylene) as the current diagram above shows.Sorry for not researching this more before responding!
Michael
Any design for a green roof with the insulation board installed below the waterproofing membrane is a recipe for disaster. We have experienced this problem on some projects not to mention the fact that the insulation board is an additional protection if anyone is probing into the soil.