Op Ed: The “Greenest” Roof is White and Metallic

by John Hammerstrom. Updated with Q&A from reader comments.

Among the goals of what are called “Green” roofs, vegetated roofs or roof-top gardens are reduced building energy costs and reduced urban-heat islanding. For both goals, white metals roofs are superior, particularly in the “sun belt” where cooling is a year-round task. If needed, the insulating properties of a vegetated roof can generally be provided underneath the roofing layer with radiant barriers or other insulation. But it can also be argued, particularly in the “sun belt,” that excess insulation blocks the advantageous nighttime building-cooling benefit of a metal roof. Insulation aside, this writer believes that the most important disadvantage of a vegetated roof is that it is designed to absorb water that should be collected to meet the needs of the buildings inhabitants.

Roof-top gardens and rainwater harvesting both contribute favorably to storm water mitigation, though roof-top capacity is limited compared to the “downstairs” cistern-storage capacity options.

This writer believes rainwater should be dedicated to the highest human uses first. Since the worldwide demand for fresh water exceeds the supply, it is counterproductive to purposely decrease the harvestable fresh water that would close that gap. On-site harvesting and use of rainwater displaces both the water and the energy required to deliver that water. Therefore, in most non-urban settings, the expense of a building’s vegetated roof systems should instead be applied toward systems to deliver and treat rainwater for potable and non-potable uses.

The quantity and quality of water that can be collected after being exposed to the unavoidable contaminants of a garden both significantly diminished compared to the rainwater that can be harvested using a well-designed rainwater harvesting system. If intended for potable uses, such a system would collect, convey, store and distribute rainwater using surfaces and materials designated by the National Sanitation Foundation to meet the EPA drinking-water standards. When rainwater is properly harvested and treated, the result is the best water you’ve ever tasted

As a raw material, rainwater is far superior to nearly all ground- or surface-water sources. There are fewer airborne potential airborne contaminants in rain compared to water that has come in contact with the plethora of serious contaminants in surface and groundwater. Furthermore, the bulk of any airborne contaminants in rainwater can be eliminated before storage with a well-designed and maintained first-flush system that eliminates the initial rainfall. On the other hand, rainwater collected after contact with the soil, materials and surfaces of a roof-top garden is no better than common surface water.

Admittedly, the definition of “highest human use” is subjective, and in urban settings the highest human use may in fact be the creation of an oasis of vegetation. However, this observer believes that aside from urban oasis creation, wherever feasible, rainwater should be harvested for potable and non-potable purposes in the greatest quantities possible. In most circumstances, vegetative uses for rainwater should be subordinated to human uses.

Besides, unlike plants, we don’t actually consume much water, but we do waste a bunch in the form of gray water and plants thrive on gray water. Furthermore, plants are much more capable of using our gray water then vice versa, so the order of use is important in order to derive the greatest utility for both plant and human.

Compared to a vegetated roof, a white metal roof is more energy efficient because it has greater solar reflectance and is highly emissive, is less expensive, is easier to maintain, can deliver more high-quality water for human purposes and would not diminish significantly the water subsequently available for plants. The “Greenest” roof is white and metallic.

Opinions expressed above are those of the author. Send comments to John who can be contacted at johnhammer at bellsouth.net

Reader Reply to Article:

While I have no financial interest in either of the roofing options discussed and have never been directly involved with a green roof project, I am a bit of a plant nerd (technically a landscape architect), so I suppose therein lies any bias.

Some of your statements, including your first, I agree with quite strongly. Others, I doubt - including your assertion regarding rainwater's superior cleanliness to other water sources, again because it's too broad of a generalization, and even as a generalization, may be incorrect. While it's possible that in rural or remote areas, exceptional air quality allows for superior rainwater quality, unfortunately this does not appear to be the norm. The excerpt below is from this article published by West Virginia Univ. (p.20): http://www.nesc.wvu.edu/pdf/dw/publications/ontap/magazine/OT_FA08.pdf
"Rainwater provides an excellent primary, supplementary, or alternative source of water. In terms of quality, it generally falls between groundwater and surface water."

As a landscape architect, my water related goals on a site typically relate to mitigating stormwater runoff, preventing erosion, using rainwater for plants on the site (instead of potable water) and helping to recharge the water aquifer. The biggest affront to my sensibility with the current state of water use is that we are feeding our landscapes water that has been treated to the standards necessary for human consumption, while the traditional paradigm for stormwater management has been to eliminate all runoff from the site to the storm sewer as quickly as possible. Therein lies a huge amount of wastefulness and a missed opportunity.

Regarding the matter of utilizing vegetated green roofs for rainwater filtration, there is some evidence that it is superior to other roofing materials in providing water that leaves the roof comparatively clean (see article below). However, a coated metal surface isn't included in the following study (only bare galvanized metal), but I understand that coated metal appears to be the superior choice for hard surfaces in potable water scenarios. So a key consideration is whether harvested rainfall is to be used for direct human consumption, or for other purposes. The full article of the excerpt immediately below is located here www.harvesth2o.com/RWH_good_bad_ugly.shtml

Potential of Extensive Green Roofs for Rainwater Harvesting

Extensive green roofing, where the media depth is shallow and the plants are typically drought-and heat-tolerant alpine species, has been examined as a means of reducing the volume of and improving the quality of stormwater runoff. The research at Penn State Harrisburg by Long et al. (2007, 2008) has confirmed the ability of the green roof, both through potential plant uptake and growth/drainage media filtration of the rainfall, to neutralize acid rain and to remove certain pollutants from the rainfall. Extensive green roofs, for example, in central Pennsylvania, are documented to return approximately one-half of the annual rainfall to the atmosphere through evaporation and plant transpiration. This is a substantial reduction in runoff volume when compared to a traditional roof. However, this still leaves 50% of the rainfall to become runoff. This runoff, if it is of appropriate quality, may be available for capture and reuse with minimal treatment after passage through a growth and drainage media that also filters out pollutants

eFor green roofs, the early stage encompasses plant establishment, washout of fine particles from the media, ... (what we are trying to say is that it takes a few storms for the finer grains of media to wash out. It's like planting a container plant - the first few waterings see some of the soil washing out). The late life definition is still being studied/defined, but it may be when either the plants do not have sufficient nutrients left in the media to be healthy or when the removal capacity of the media for rainfall pollutants is exhausted. Penn State Harrisburg has several pilot-scale green roof tests ongoing to evaluate both runoff generation (compared to rainfall quantities) and pollutant removal of green roofs (Figure 5).
From this study the main bi-product of vegetated green roof runoff appears to be small quantities of phosphorus (approx. .5 mg/liter). According to the EPA, "The age-dependent recommended dietary intake of phosphorus ranges from 100 mg/day for infants under 1 year old to 1,250 mg/day for children 9-18 years old and pregnant/lactating women younger than 18. The recommendation for adults is 700 mg/day." According to these numbers, an adult could theoretically consume an estimated 1400 liters of water per day, produced from the average vegetated green roof per day and still only be getting the RDA of phosphorus. www.epa.gov/dclead/phosphoric_acid_health_effects_sheet_FINAL.pdf

gI don't claim to be an expert on any of these subjects, and only put this information in front of you for your own further study and inquiry. I appreciate your reply, and I feel that the statements that you stand by, particularly the ones backed by substantive evidence would make for a more compelling case in your articles.

Good luck to you in your work.
Sincerely,
Jason

John's Reply:

lI live in the Florida Keys where we receive approximately 35 inches of rain a year. Irrigation is still used here, although it can be minimized by use of native plants, as you know better than I. We have a relatively dry season approximately six months out of the year and harvesting rainwater can help dampen (so to speak) the impact of that dry season on utility water usage at the same time it dampens stormwater impacts. The pH of my water is nearly neutral for two likely reasons - I live near the salt water, and I'm told by better-informed authorities than I that the salt environment mitigates acidity. On top of that [or rather underneath that ;-) ], the concrete tank buffers any residual acidity.

I test the water annually (see test results below) now that I'm comfortable with the stability of my system and the periodic maintenance of it. My rainwater is "treated" many ways from the sky to the glass, by imparting the fewest impurities during contact with surfaces and by diverting unavoidable impurities whenever possible in the collection process -- through the use of coated galvalum roofing, well-designed and maintained copper gutters, stainless-steel leaf screens, SafeRain first-divert devices, potable-quality lined concrete tank (check with the National Sanitation Foundation for current authorized materials), foot valve placed approximately 6 inches from the bottom (another method would be a floating pickup in conjunction with a calmed inlet) and then a Pura BigBoy triple-filtration bank, equipped with a 1-micron sediment filter, carbon block and ultraviolet light.

Fifteen inches of rain is plenty to harvest substantial quantities. My roof is approximately 1,700 square feet. That means that I'd be able to harvest 15,000 gallons of water a year if it were in Colorado, which for two people, assuming conservative indoor use (using the low-end per person use of 75 gallons per day) means 100 days of water.

I am aware that Colorado has rainwater-water-rights issues, but I think you'll find that they've recently been relaxed somewhat. Here's an article on the subject. http://earth911.com/news/2009/07/03/colorado-bill-legalizes-rainwater-harvesting/

Thanks again for the informed and civil exchange.

John

Related Links

Green Roofs article
The Good, the Bad, and the Ugly side of Roof Runoff
Coupling Rainwater Collection with Living Roofs: A Water Quality Review

Water Test Results

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