Grow a Green Roof

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Living roofs blend beautifully into the landscape, and they can be cheap or even free if you’re resourceful.
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A well-built roof will keep you warm in winter and cool in summer.
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You can focus on beauty as well as brawn by choosing plants with alluring blooms.
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Insulation under your living roof is essential, this is an example of a standard insulation setup.
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Consult with an experienced designer or engineer on bigger projects.

I like buildings to “fit in” like they’ve always been a natural part of the landscape. I build walls out of natural materials, such as mud and straw, and use posts and ceilings made from local wood, with the bark left on when possible. My favorite foundations are stone with the mortar you can’t see. It bothers me when the biggest, most visible part of a house — its roof — sticks out of the landscape glaringly, as modern materials tend to do. Maybe there’s something to my grandmother’s claim of being half-hobbit — and not just because I’m short and plump and like to go barefoot. In my opinion, there’s no more pleasing way to cover a shelter than with a green roof of living flowers, vines, and grasses.

A lot of roofs seem to be built primarily with economy in mind. Not the kind of economy that looks for overall cost-effectiveness through a long life free from trouble — just economy in the sense of affordable installation. Let someone else worry about whether it needs to be replaced in 15 years instead of 25 or 50, or where to dispose of the tar-impregnated remains. Of course, I’m speaking of asphalt roofing. You see it everywhere because it’s cheap and so ugly that everyone has tacitly agreed to ignore it. People inclined to put lipstick on pigs can order “architectural-grade” asphalt shingles. They don’t last much longer, but they come in different shapes and colors, and they cost more.

Metal roofs are a little better environmentally. They tend to last, are recyclable, and allow you to catch and use the rainwater that would otherwise go down the gutter. Wood shingles are natural and renewable if made from sustainably grown wood. Slate is also natural and very long-lasting. Reed thatch is long-lasting with a very low environmental impact. All of these options are fairly expensive, especially if you want your roof to be any shape other than a rectangle.

My friend Ianto Evans likens the roof of a building to a good hat. It has to keep out the rain, wind, and sun. It must keep you warm in winter and cool in summer. In climates like mine, your roof may also have to hold up several thousand pounds of wet snow in winter. Of course, it also ought to look good because the first thing people see is your hat. A roof isn’t as easy to change as a hat, so it needs to be a hat for all seasons.

If you wander through the cottages and courtyards of Ianto’s Cob Cottage Company campus — affectionately known as “Cobville” — you’ll see that the hats on his buildings consist of nothing more than a waterproof membrane topped with a growing medium and plants. Most builders add a layer of padding to protect the membrane, as well as a drainage layer — usually small rounded pea gravel — to get rid of excess water. Pools of water invite leaks and mosquitoes, and water and wet soil can get dangerously heavy, so drainage is important.

Fancy architectural magazines advertise complicated living roof systems, and heavy roofs with large spans do need special engineering, but a simple living roof on a small building needn’t be heavy if the plants and their growing medium are chosen carefully. Such a setup can be less than the weight of three layers of shingles, which is the normal load design of a conventional roof. Special potting soils made of lightweight perlite aggregate are available, but 2 to 3 inches of compost or aged goat or horse manure mixed with straw will also do the trick. Regular soil is generally too heavy because its aggregate (sand, silt, and clay) adds weight without adding the nutrient- or water-holding capacity the plants need most. Anything more than 2 to 3 inches of a very light medium would require a stronger roof structure than normal.

A living roof doesn’t need to be expensive — it can be nearly free if you’re resourceful. I use sticky-back plastic roofing membrane scraps because they seal to each other and work with complicated roof shapes. Plus, I can dumpster-dive for them. Other folks I know have used multiple layers of recycled billboard tarps padded with used carpet or even cardboard.

Tony Wrench, author of A Simple Roundhouse Manual, used a piece of EPDM pond liner on the roof of his cottage. It wasn’t cheap, so he was quite frustrated when it started leaking after eight years instead of its expected life span of 50-plus years. Apparently, ants were biting holes in it. He’s had no problems since relining his roof with a UV-protected silage tarp. Whatever membrane you use, you must guard against puncture from above or below. After you get a good jungle growing up there, you may not be able to tell where a leak originated. Tony’s roof is a jungle indeed. In addition to native sod, he has arctic strawberries, sedum, and grapevines that provide a lot of shade and about 35 pounds of grapes each year.

Sedums are a favorite plant for living roofs because they require very little soil and develop a dense root structure that helps them stay happy in extreme hot, cold, wet, or dry conditions that would kill most other plants. They also grow beautiful little flowers. If you intermix several varieties, you can get a constantly changing display of colors that lasts most of the year. Or, if you plant only one variety, like a friend of mine did, you’ll get a drab brown roof for 50 weeks of the year and an amazing purple carpet for the remaining two.

Deanne Bednar of Oxford, Michigan, likes to use local wild plants for her living roofs. “[Native plants] are generally very well-acclimatized to that specific spot. They’re going to do very well. Plus, they’re usually free for the gathering,” she says. The wild geranium she found in her woods has completely colonized the compost layer covering her woodshed, along with fiddlehead ferns and local sedum. Like Tony, Deanne mixes pieces of rotted wood into her straw-manure mix to help hold it in place while the plant roots are establishing themselves. The rotting wood also acts like a sponge to hold additional moisture and nutrients where the plants can access them. You can read more about Deanne’s techniques by visiting her blog, Strawbale Studio.

Architect Sigi Koko takes a more studied approach to her living roofs. Her buildings are beautiful, but the benefits she sees in living roofs go far beyond looks. She explains how a living roof catches and slows down the runoff from a rainstorm that would normally rush directly off, causing flooding and erosion downstream. The plants filter the air around the house and help keep the temperature more comfortable year-round, particularly in summer, when they block much of the sun from even reaching the roof. And when they “sweat,” they radiate even more heat back into the sky, like a big evaporative cooler.

An example of this is a real-life hobbit house tucked under a canopy of oaks in a suburban backyard in Texas. The neighborhood children love how the 2-foot-thick earthen walls and a living roof keep the playhouse comfortable through hot summers and cool winters. Builder Tracy McCloud of Belton, Texas, loves the way living roofs allow her tiny cottages to blend into the landscape, where before there was only concrete and steel. Local birds love the way they can nest on her living roof safe from predators, and seeds from their droppings have helped cover the roof with a variety of drought-tolerant native grasses. “It’s its own little bioregion up there,” Tracy says. “A lot like the prairie that used to cover this part of Texas.”

Keep Your Cottage Cozy with Insulation

Like any roof over a living space, a living roof needs insulation under it. On the cheap end, I’ve insulated with used carpet and cardboard. Builder Tony Wrench keeps his cottage cozy with entire straw bales tucked under the membrane. It works, but when he had to go in for repairs 20 years on, he noted that condensation under the membrane had caused some decay in the outer 2 inches of the bales. Some types of insulation, such as straw bale, fiberglass, or cellulose, need to “breathe,” so the inside of the roof has to be vented to allow condensation to escape. Foam insulation doesn’t need to breathe and is thus far more effective per inch, but it’s also expensive and has a high environmental cost.

Build Your Own Green Roof

Architect Sigi Koko recommends consulting with an experienced designer or engineer, especially on bigger projects. A living roof can add 10 to 50 pounds per square foot to your roof load, so you must find out whether your roof can handle the additional weight. Existing cottages or sheds with small roofs and small spans may take a living roof with no additional structure. If you’re planning such a building, boosting your roof sheathing from 1⁄2-inch to 5⁄8-inch and your rafters and beams from 2-by-6 to 2-by-8 may be enough.

Chris McClellan (aka “Uncle Mud”) raises free-range, organic children in the wilds of suburban Ohio. Building with mud and junk is his soapbox for preaching self-empowerment. He writes, builds, and teaches workshops.

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