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.

Ecoroof_Matt the EngineerNow 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.

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  • 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.

    Ecoroof diagram_city of Portland

    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


    Protection board


    5-inch soil (with gravel drainage)


    Plantings (sedums and grasses)


    Irrigation system


    Plant establishment (labor cost)




    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.

    Garden Matt the EngineerAlso, 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.”