Vegetated Roofs

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Vegetated Roofs

As California homeowners are urged to replace the grass in their yards with decorative gravel, why would anyone try to grow grass on their roof?

Posted on Apr 22 2016 by Martin Holladay

Vegetated roofs are low-slope roofs the have enough soil (or soil-like growth medium) on top of the roofing to support the growth of grass, wildflowers, or shrubs. Although some people call this type of roof a “green roof,” the term “vegetated roof” is more accurate and less confusing.

Vegetated roofs can be humble or sophisticated. In Scandinavia and the Great Plains of North America, these roofs used to be called “sod roofs.” Most sod roofs were installed by poor people in rural regions, and most sod roofs leaked. Later versions of the sod roof included a waterproof membrane under the sod, improving performance.

In the early 1970s, Malcolm Wells, a widely respected green architect, began preaching the gospel of underground architecture. Wells took his vegetated roofs seriously; he famously noted that his definition of an underground house was a house with enough dirt on top to grow mature trees. Interviewed at his underground house in 1999, he said, “I have three feet [of dirt] here, but if I had to do it again, I’d put about 10 feet of earth up there, so it could be a truly natural habitat for all plants.” He bragged that his roof had “a couple of pine trees, now about four feet tall.”

These days, most vegetated roofs have better specifications than the old sod roofs, but less impressive specifications that Malcolm Wells’ 10-foot-deep ideal. Most vegetated roofs have between 3 inches and 6 inches of soil. That’s not enough to create a natural habitat; it’s just enough to create a kind of ersatz pasture — one that looks green if you remember to water it.

Why install a vegetated roof?

Vegetated roofs are quite expensive. While some architects estimate that these roofs cost only $15 per square foot, others estimate that a vegetated roof, including roof framing, roofing, root barrier, and growing medium, costs about $110 per square foot (10 to 20 times the cost of a built-up roof).

Because the cost of these roofs is so high, few people will install a vegetated roof unless they are strongly motivated to do so. There are two main motivations:

  • Some people think these roofs are good for the environment.
  • Some people think that vegetated roofs are beautiful.

Of course, if you want to enjoy the beauty of your vegetated roof, you either need to make sure that the vegetated roof is dominated by a second or third story, so that building occupants can look down on the roof from above; or you need to design your building so that you are able to walk on your roof. Most low-slope roofs, including vegetated roofs, are rarely seen.

How did vegetated roofs become hip?

Vegetated roofs are rarely installed, so how did they gain such an outsized reputation in the green building community?

The main reason appears to be because that the LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. rating system awards points for vegetated roofs. That motivation is all it took for point-checking architects to specify a vegetated roof on their next LEED project.

Do vegetated roofs provide good insulation?

No. A typical R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. for soil is R-1.4 per foot (or R-0.12 per inch). As building scientist Joseph Lstiburek puts it, “If dirt were energy-efficient, we would call it insulation and put it in walls. It is just dirt. Insulation is better insulation than dirt.”

By the time you beef up the roof framing to handle the additional load created by 6 inches of soil, and haul the soil to the roof and spread it, you’ve added a lot of expense to your roof assembly without improving the R-value of the roof by much. Cellulose is cheaper and easier to install.

Do vegetated roofs help address global warming and the urban heat island effect?

Yes, somewhat. Most researchers have concluded that, compared to conventional black roofs, vegetated roofs are a net plus when it comes to global warming and urban heat island effects — but that white membrane roofs (which are much less expensive than vegetated roofs) are even better. For more information, see Do Green Roofs Temper Urban Heat?

Here are more links and quotes for readers interested in pursuing this topic:

  • “Unlike white roofs, green roofs do not offset climate change. White roofs are more reflective than green roofs, reflecting roughly three times more sunlight back into the atmosphere and therefore absorbing less sunlight at earth’s surface. By absorbing less sunlight than either green or black roofs, white roofs offset a portion of the warming effect from greenhouse gas emissions. Both white and green roofs do a good job at cooling the building and cooling the air in the city, but white roofs are three times more effective at countering climate change than green roofs.”
  • “Rooftop gardeners don’t win when it comes to global warming. ... White roofs reflect more sunlight back into the atmosphere, so they absorb less sunlight at the earth’s surface, offsetting a portion of the warming effect from greenhouse gas emissions. ... Green roofs with gardens or plantings of any type do have benefits, even though they don’t win in the cost-savings category.”
  • “We find that relative to black roofs, white roofs provide a 50-year net savings (NS) of $25/m2 ($2.40/ft2) and green roofs have a negative net savings of $71/m2 ($6.60/ft2). Despite lasting at least twice as long as white or black roofs, green roofs cannot compensate for their installation cost premium.”
  • According to Art Rosenfeld, when it comes to “‘green’ (vegetated) roofs, their albedo is only ~20%, so they absorb sunshine, get warm, and then cool by evapo-transpiration. The absorbed heat is trapped by the greenhouse effect and the cooling by evaporation is cancelled within hours or days by the condensation of the water vapor into rain. For global cooling they are only one-third as effective as white roofs.”

Do vegetated roofs help store rainwater and reduce runoff?

Yes. In most cases, a vegetated roof reduces stormwater runoff compared to a conventional roof, and that’s beneficial in many communities.

Not all studies agree, however. One study cited by an article in Environmental Building News came to a different conclusion: “Extensive vegetated roofs provided net benefit for most impact categories, though surprisingly, the green roofs’ influence on stormwater and air quality were ‘essentially negligible,’ according to the study. Green roofs reduce eco-toxicity by avoiding pollutants like metals and polycyclic aromatic hydrocarbons, which can seep from shingles and tar on a gray roof. However, nitrogen and phosphorus can leach from the growing medium, especially in the first year that it is installed.”

In any case, it’s not clear that this method of storing rainwater makes sense. Joseph Lstiburek, the master of the memorable quote, doesn’t hide his feelings when he discusses this issue. “Folks also like green roofs because you can store water in them; you can control the rain runoff from your building.” Lstiburek writes. “You’re kidding, right? Storing rainwater on top of your building? Are you on crack? The whole point is to get the rain off of your roof, which is why we slope them.”

In other words, there is no reason to confuse a roof with a cistern. If you want a include a cistern in your project — and in many areas of the world, it’s a good idea to do so — then by all means include a cistern. But you probably want to put the cistern in your basement, not on top of your roof.

Do you have to water the plants on your vegetated roof?

If you are lucky enough to live in a region with ample rainfall, you probably won’t have to water your vegetated roof.

But if you live in a dry area, or if your region is suffering a drought, you’re going to have to water your roof.

A roof is not a pasture

Leaving aside environmental considerations, the two main reasons to consider a vegetated roof are: (a) A vegetated roof is a nice surface to walk on and a nice place to visit, and (b) A vegetated roof is beautiful.

Vegetated roofs are, indeed, nice surfaces to walk on and visit. But so are wood decks, and it’s certainly possible to install a wood deck over a low-slope roof for a lot less money than a vegetated roof. That said, if you want to walk on grass installed on top of your roofing, then a vegetated roof is the way to go.

Is it the most beautiful type of roof to look at? Opinions vary. Personally, I like to look at red tile roofs and slate roofs. But everyone has a different idea of the most attractive type of roofing to look at.

Installing a lawn on top of a low-slope roof is an architectural tour de force, similar in some respects to installing a swimming pool or a trout pond on top of a roof. Of course, many Americans would love to look out of their third-story window and see a trout pond on top of their garage roof — and, with a large enough budget, architects and builders could come up with a way to make it happen. It’s a little like teaching a dog to ride a bicycle. Sure, it’s possible — but why would you want to do that?

Pastures are very nice. If you’re lucky, there’s a pasture or a lawn near your home, close enough to visit. But just because pastures are nice, doesn’t mean that you need to try to recreate one above the top story of your house. Even if you almost succeed, it won’t be a real pasture, and you’ll need to water it.

[Photo credit: CNN]

Last summer in California, as the state’s drought intensified, several municipalities created financial incentives to encourage homeowners to tear up their lawns and replace them with gravel-covered landscape fabric (often with a few drought-tolerant plants poking through the gravel). To my eye, many of these yards ended up looking like an old-fashioned tar-and-gravel roof.

Meanwhile, elsewhere in California, other building owners were trying to figure out how to grow grass on a new low-slope roof. Logical? You decide.

Martin Holladay’s previous blog: “Is Your Ventilation System Working?”

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  1. Image #1: Martin Holladay

Apr 23, 2016 3:04 PM ET

Edited Apr 23, 2016 3:21 PM ET.

Roof top gardens
by Malcolm Taylor

I'm not sure you can even consider them a sub-set of "green" roofs, but where roofs are already designed as patios or terraces, roof top gardens in planters have traditionally been used to both improve the ambiance and produce flowers and vegetables. These seem like the only variant that has a long track record of making sense.
I'd wager the group least likely to champion vegetative roofs are builders like me, who have already spent enough sleepless nights worrying about potential leaks in their conventional roofs, without adding more complexity to their projects.

Apr 23, 2016 3:46 PM ET

Edited Apr 23, 2016 3:48 PM ET.

Response to Malcolm Taylor
by Martin Holladay

You're right that there is a spectrum of green roofs, and that one end of the spectrum includes a wood deck or stone-flagged rooftop patio with some planters.

In Portland, Maine, I saw a roof that is hard to categorize, but it's somewhere along the vegetated roof spectrum. It's the roof over the architectural office of Rick Renner. It's a black membrane roof -- not necessarily the type of roof where most people would want to hang out, but at least there are stairs leading up to the rooftop. Renner has installed shallow trays filled with soil; he describes the plants in these trays as "low-maintenance perennials."


Renner building rooftop.jpg

Apr 23, 2016 7:38 PM ET

Edited Apr 23, 2016 7:46 PM ET.

by Malcolm Taylor

The criteria I would suggest for including vegetative roofs is probably as far from a logical building science perspective as is possible. It would be that it was a necessary component to the building design. That it "just had to be". That means they would be reserved for those very few projects that cried out for them, not adopted as a matter of necessity or principle. I think this cabin by Snohetta embodies this idea.

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