Grow a Green Roof

Turn your roof into a canopy of lovely plants that will keep your house comfortable year-round.

| February/March 2018

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

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.






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