RETHINKING GREEN ROOFS: 9 points to consider before greening your roof

In this age of energy-efficiency another method of building the top of a building is taking root: green roofs. On such “upland” terrain one can grow foods from strawberries to cornstalks, enjoy the view, even have fountains and topiary—whatever one’s fancy and budget will allow. LEED awards points for this construction on the grounds that it “can reduce roof temperatures from summertime highs of 150 degrees to less than 80 degrees, can reduce energy demand by more than 50 per cent annually, can minimize impact on microclimate and natural surroundings and can increase oxygen through photosynthesis.”

“Can”? That little three-letter word as recruited above could also mean “might not,” probably won’t,” “wouldn’t in a million years,” and a host of other adverse implications. In fact, each of the four conditions listed above “can” under a minority of circumstances prevail. However, while LEED awards points for green roofs, it does not make the following points about certain majority aspects of this construction:

1. Green roofs cost a lot. Typically 15 to 25 dollars per square foot on top of one’s initial cost. This could tilt the scales of this shelter’s potential cost-effectiveness from positive to negative.
2. These roofs weigh a lot. Usually 40 to 80 pounds per square foot, some as much as 200 psf; and all supporting structure below down to the ground must be made larger. To indicate how deceptive some promotions of green roofs have been regarding this, consider the recently built green roof on the Chicago City Hall, an impressive project that received rave reviews in numerous architectural magazines. However, only one of these periodicals mentioned (in a short sentence sequestered deep in its text) that what made all this heavy construction possible was that the building was originally designed to support the construction of an added floor; and the weight of the green roof replaced what would have otherwise been the weight of the future floor without requiring any revisions in the structure below. Every article about this roof should have made this vital point clear. Or else—again—when someone who constructs this idea learns that it or some other ecologically promoted concept isn’t what they were led to believe, the tide of one’s opinion will likely turn from delight to dissent, and in the company of one’s colleagues they will colloquially cast a vote against rather than for the reality—often with a fervour that far outweighs any feeling they might have otherwise expressed; for in such scenarios, censure generally outpaces praise.
3. Topsoil can erode and subsoil can slip during heavy rains. Though LEED’s guidebook says, “All garden roofs decrease stormwater runoff volumes substantially,” this is false. Garden roofs may decrease stormwater runoff during short light rains; but during severe storms when the roof’s soil has become saturated, stormwater runoff will not be less than on standard roofs and this is when topsoils and subsoils will likely erode and slip.
4. After construction most green roofs need almost daily horticultural attention: mowing, weeding, erosion control, and precise watering (enough to encourage growing but not enough to initiate erosion and slippage).
5. Green roofs attract bugs, rodents, and other members of a thriving ecosystem, even on tall buildings.
6. Green roofs require access—not by a ladder over a gutter but by a railed central staircase. A good central entry is a stairwell that rises into a small greenhouse with a door from which you step onto the roof. Also, areas that can be occupied should be enclosed by chest-high parapets; and any walkways or terraces should be masonry and not wood, which requires waterproofing above, airspaces below, and toxic rot-proofing all around.
7. These roofs often require electric outlets, landscape lights, hose outlets, and related mechanical equipment.

8. The roof substrate can leak, and when it does it is difficult and expensive to repair.

9. A green roof can be a fire hazard. If the roof’s vegetation turns brown due to lack of water, a brush fire could start there and spread indoors or to other buildings.

LEED’s guidebook also says:

a) “Some green roofs have grasses and plants that require no watering,” (False: all plants, even cactus, require some watering, which means monitoring, which means access).

b) “All types of green roofs have longer lifetimes than conventional roofs,” (False: if they do last longer it is because they must be constantly maintained in ways that are not required with standard roofs).

c) “Green roofs provide lower maintenance than standard roofs.” Hah! See #4 above.

If you want a green roof to increase a building’s insulating ability and decrease its water runoff, there are better and cheaper ways to do these things. Esteemed building scientist, Joseph Lstiburek, Ph.D. says:

Vegetative roofs? Grass and dirt are not energy efficient. Work with me here. Which saves more energy: two inches of dirt or two inches of insulation? Which saves more energy: grass or a white-coloured membrane? Which is more expensive and does not save energy: grass and dirt, or insulation and a white-coloured membrane? Which needs to be watered to keep the grass from dying and blowing away?

On the other hand, if you love gardens and want to transform a barren tract wracked with violent temperature extremes into a meadowy landscape that blooms with wildflowers, bustles with butterflies and birds, is threaded with pleasant walks and patched with relaxing terraces, and offers fine views above the bugs and closer to the stars, then you deserve all the joys that gardens can give you, all the praise that periodicals may rain on you, and all the points that LEED may award you.

One bona fide technique that will improve the thermal performance of many a roof is to do what Joseph Lstiburek mentioned above: paint it white. According to an article in The New York Times, such roofs not only are “An energy saver but also a way to help cool the planet… These materials reflect as much as 90 per cent of the sun’s heat energy… Studies show that white roofs reduce air-conditioning costs by 20 per cent or more.” Even Frank Lloyd Wright got into this act—years before anyone else did—by saying: “A white-topped roof is economical partly because white, of course, reflects heat rather than absorbs it.” As for the possibility that white roofs could lead to higher heating bills in winter, Home Power magazine says: “Summertime air-conditioning savings from choosing a light-coloured roof will most likely outweigh the heat gained in winter by using a dark-coloured roof [because] the winter sun is available for a shorter part of the day, is lower in the sky, and its light passes through more atmosphere than that of the summer sun.”

Three months before the New York Times article appeared, I painted 2,400 square feet of my house’s roof with SolarFlex 287 SF, a thick white elastomeric roof coating made by the Henry Company of El Segundo, California. During the past two summers the rooms below have been notably cooler than in previous years, and they weren’t any cooler during winter.

After graduating from the Cornell School of Architecture in 1964, Robert Brown Butler has worked as a carpenter, contractor, and registered architect. Through the years Mr. Butler has received a variety of honors for his creative work. He is the author of The Ecological House, which introduced many ideas that are further advanced in this volume, among other books. This excerpt reprinted with permission from Architecture Laid Bare!: In Shades of Green, by Robert Brown Butler. © 2012, Robert Brown Butler.

photo courtesy 416style