WHAT IS A GREEN ROOF?

Shh Ish’t Kin — The term was used by Native American and Canadian First Nation tribes living within the Salish Sea region to denote the fecund layers of growing ferns, mosses, and opportunistic saplings that colonized their roofing materials. An esteemed longhouse might advertise its communal staying power and harmony with natural spirits by having an impressive layer of forest duff and riotous greenery,  the ‘Shh Ish’t Kin’, living upon its rooftop.

Well … it seems somewhat probable.  ¤

Some version of green roofs have been around, in multitudinous forms, for as long as homo Sapiens have dunced their way into seeking shelter. Why not here?

Green Roof Systems

A modern green roof is composed of layers. The green part is the vegetation layer, or plants, and it obviously needs to be on top. Depending on your soil depth, the chosen plants can range from ground-hugging sedums to outright trees. For engineering and expense reasons the thinner soil, and thus the smaller plants, are the norm. Rooftops are harsh, exposed, environments and generally not conducive to heavy maintenance.  The plants, therefore, need to be hardy little beasts that like abuse and can handle long spells on their own.

An example of a basic green roof system for flat roofs

Beneath the plants is a layer of growing medium — a.k.a., soil. This material needs to be fairly specific and is engineered to do many critical things on your roof. Well designed soil mixes will feed your plants nutrients, allow water to rapidly drain off your roof (but not too rapidly!) and be composed of lightweight materials so as to avoid structural stresses on the underlying house or building. No, you cannot just yank hunks of sod from your lot and toss them on your roof. No, fertilizers and chemicals should not be used except, perhaps, during the plant’s initial establishment phase.  Green roof soil, in many cases, is largely light pumice or perlite. Don’t call it dirt.

A root barrier — typically a dense cloth screen — is the next layer beneath the soil. This layer is important for keeping the roots out of the drainage layer, protecting it from clogs,  and thereby allowing the water to flow off the roof.

The drainage layer is next and this element can get a bit exciting. Picture a thin plastic sheet with a million dimple-sized cups molded into it. Originally designed for vertical applications to release hydrostatic pressure along retaining walls, the producers of these drain curtains  now cater to the green roof market by selling their sheet product with a root barrier attached.  Where a vertical application was designed to quickly move water away from a building or wall foundation, the horizontal application allows the water to loiter a bit by flipping the cups and letting them retain the water for the benefit of the plants. Being plastic, this layer in and of itself provides significant waterproofing.

An example of the drainage mat, also called "dimple board."

The membrane, or waterproof layer, keeps water out of your house. Typically the plants — which are currently not widely cultivated — are the most expensive layer of a custom-designed green roof. Since fixing leaks would be hugely problematic with these sorts of systems, the membrane is usually the second-most expensive element. These polymer or rubber layers are bomb-proof, however, and should last forever as they are safely entombed beneath all those protective layers; they are never exposed to pounding sun or rain and the temperature extremes they endure will be minimal compared to a typical shingle or laminate roof. Unlike the plastic (technically it is EPDM, but more about that elsewhere) drainage layer, the membrane is flexible and can be heat welded or glued at its seams if it is not delivered outright in one gigantic sheet.

Green roof plants, if selected well, should be stout enough to endure our region’s occasional cold snaps. Still, many commercial roof designs seem to like putting a layer of rigid foam directly under the waterproof membrane to help protect it from nails or splinters that might sneak up from the plywood roof sheathing. This, incidentally, is also a cheaper and easier way for contractors to install insulation — toss it on the roof top, let the plants hold it in place. If your structure already has insulation snuggled into the rafters, then a simple layer of felt can offer the same puncture protection for membranes.

(This example of a green roof system was installed in the Miller Hall renovation — image courtesy of Sika Sarnafil)

When one also considers that a certain amount of structural engineering may need to be done before the added weight of all these plants and soil can safely go above your head, then it becomes clear that green roofs can cost a lot of money, literally, from top to bottom.

It can be done, however.  In Germany, where green roofs have been available for the past 30-40 years, the “fad” has quickly become just another mainstream option. (One popular citation claims that green roofs in Germany cover %10 of the available flat commercial rooftops. In some cities, such as Stuttgart, that number is considerably higher.) The technology, especially in Europe, has followed pace and there are now pre-package “systems” that can be installed with ever-increasing ease and ever-decreasing expense. A few of these system manufacturers have offered formulas for projects to estimate off of  — cost: $18-$22 per square foot, and weight: 6 lbs. per inch/square foot. Every roof will be different, and sloped roofs can add complexity, so don’t presume those loose numbers to be gospel.

The expense of green roofs in America, due to competitive zeal and civic enthusiasm, seems to be rapidly coming down. At the 2010 GRHC Green Roof and Wall Conference, Tom Liptan, who works for the stormwater division of the City of Portland, received gasps (and some guffaws) from the audience when he announced that he knew of installers offering bids at $5.00 per square foot. The Portland case study roster currently has examples as low as $6.81/s/f which includes everything above the waterproof membrane: products, delivery, installation. Many of the Portland examples are cost-competitive retrofits, with the City’s incentive program, which convert existing commercial roofs into green roofs.

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Truth in advertising.

“Green” is a very over-marketed word nowadays. Sometimes the label is meant to stimulate innocent shoppers who need the comforting nudge of “eco-friendly” or “environmentally manufactured” to tickle them toward the check-out lanes. An ideology that fervently believes in global warming or greenhouse gases hardly matters since, realistically, the daffy labels are here either way. It really could be the dinosaur’s fault. Go green!

The way this new reality applies to your future roofing choices should be nothing new to the careful and conscientious consumer. They’ll do their research first, dodge a few magical “eco” bullets, then they weigh their best options. Every green roof is going to be site specific and will depend heavily on the structural design of the underlying framing.

Today you can purchase hundreds of different roofing products — metal, asphalt, cedar, clay, plastic, rubber, and various composites too numerous to mention. As is true with almost any common construction materials, some of these roofing options can argue their way into a “green” category. The LEED (Leadership in Energy and Environmental Design) rating system for sustainable building practices acknowledges some of these roofing products and gives credit for their use. Recycled tires, post-consumer metal, and globs of reflective material crammed into asphalt — apparently to reflect heat away from the roof — can all merit some consideration in a quest to use less of the planet’s resources and use them in better ways.

For the most part, however, I don’t care about any of these products.

I care about green roofs.

They have plants on them — living, green, chlorophyll-producing, plants. They are an oddity, to be sure, and likely the sort of windmill that only eccentrics and fanatics will lean toward. They are a fantastic pain in the arse to build, require engineering on many levels, and seem not to be even vaguely economical in the grand pantheon of roofing options for normal folk.

Still, when a buddy of mine recently poo-poohed the mere notion of these roofs, I got my dander up. (He’s a Buddhist — he should know better than to make waves of this sort!)

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Why?

To build a green roof in America today will require: A) that a list of somewhat exotic materials be shipped to your project site, B) increased consumption of natural resources for building materials for the structural weight-loads and, perhaps, just for the added layers and density of the roof itself, C) a lot of human energy that might be more productively expended elsewhere, and, D) a chunk of cash that could be invested in other, less convoluted, planet-saving measures.

The direct tangible benefits that green roof cheerleaders chirrup about are that green roofs are great insulators for a building and thereby lower energy bills. They also filter and slow water run-off that otherwise might just be grinding your asphalt shakes into atoms and lobbing them into the nearest water supply. They could help reduce the urban “heat island” effect — the plants absorb and use the sun’s radiation rather than retaining it and slowly baking city-dwellers into submission. A green roof, being composed of plants and organic matter, also can provide some habitat for bird and insect species as compared to the utterly hostile environment of a typical roof. Because they are alive, green roofs automatically network with the natural cycles of carbon capture and storage, oxygen creation, the nitrogen cycle, and other “web-of-life” elements that dour environmental scientists claim are important. Green roofs look pretty: they are a pleasant break from the horrid monotony of thousands of square feet of shingles or rippled, painted, steel. Aside from occasional weeding and soil tweaking,  a well-made green roof should last for 50 years or longer and would require none of the pressure washes or chemical bombardments of the other options’ maintenance regimes.

All of that is nice. My inner hippie can easily fathom the direct benefits of a green roof without too much dabbling into the environmental science libraries.

But the one selling point for green roofs that I find overpowering, and the bastion I defended against the attacks from my Buddhist buddy, is that nearly every single action we take as modern humans is an act of natural resource consumption and natural resource pollution. Our cars and houses bellow this point loudly, but I could go further (too far?) and consider that — beyond a few pounds of flaking skin each year — we rarely allow anything organic to go back to this planet before molesting it with a slurry of chemicals and sterilizing agents.  We are not generous contributors to the planet’s biologic community. From cradle to grave,  from our first purchase to our last, we humans offer very little in the way of amends for the insults and injuries we dish out in every direction.

Shuffling carbon credits in existential circles, or buying solar panels and hybrid cars, is a nice start for our evolution. But I would argue that planting a garden in your yard instead of mowing it every week is a more direct fix, a more fundamental interaction with “progress,” than our techy widgets and gee-gaws.  Reclaiming your roof, therefore, is even better still.

My point is simply that the shelters we live and work in are wretched moonscapes for a vast majority of the critters and natural systems on this planet. They take away, rather than give, for every second of their existence and, for the most part, long beyond the time-frame of our individual selves.  Shelter, however, can also be a verb. Those same seconds or decades can also be used to give something back in return  — even if it is a mere nod of recognition — to this jolly blue and green orb that sustains us. And the psychological rewards of getting your hands dirty, of actually caring for and investing in your roof, will likely be more priceless than simply dusting off your credit card to buy 2,500 square feet of used car tires.

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¤(This term, “Shh Ish’t Kin,” comes from Coast Mountain Salish First Nation member Jerry Old-Man. Old-Man announced this before 700 people, and the mayor of Vancouver BC, so I will presume that he knew what he was talking about.)

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