December 3, 2008

When designing green roofs, think ‘stormwater retention’

Construction Corner

Korky Koroluk

We’ve all heard by now of the benefits of green roofs — how they can save some of the energy that would otherwise go to heating and cooling, how they can be used to create park-like spaces for building staff, and how they can contribute greatly to stormwater retention and management.

All true, of course, but there can be a lot of variability in that last category — stormwater retention.

A recent study done at the University of Texas at Austin, shows that while green roofs are capable of significant thermal moderation, not all are equally capable of retaining stormwater.

Researcher Mark Simmons and his colleagues compared two conventional roofs — one white and one black — and six green roofs extensively planted with perennial plants native to the area and mounted on test platforms in Austin’s humid, subtropical climate.

Their measurements included temperatures at several points in and under each structure and water runoff during and after storms.

They found that the thermal performance of all the green roofs was pretty good. They also found that the white roof was cooler than the black roof because of the differences in reflectivity, but not as much cooler as they had expected.

What really surprised Simmons and his team, though, were the differences they found in the stormwater retention capacity among the various the green roofs.

Going into the project, they had examined the six different green roof designs and expected that all would be effective in retaining stormwater.

But during three heavy rains, they found that the most effective roof captured roughly four times as much rainwater as the least effective—which turned out to be little better than conventional roofs.

All the roofs were planted at the same time and the study was done during rains about eight months later.

So what is the lesson here?

Well, just as all climate regions differ, so, too, do many green roof designs.

Substrates equipped with large cups or depressions for retaining the planting medium, and a relatively smaller area lost to drainage holes in the drainage layer, held water better.

Another variable is in the planting medium itself, with mixes containing a high proportion of perlite holding moisture better than those with less perlite.

Horticultural perlite is an expanded form of siliceous rock that is frequently added to growing mixes to help hold water and avoid soil compaction.

Although the tests were done in a subtropical climate, the researchers said they believe a green roof that performed well in Austin would also perform well in more temperate zones.

In Canada, heavy rainfall is common on both coasts, so stormwater management is an important factor in green roof design.

But, as the study suggests, no one should think that a design that performs well in one place will do equally well in another. Green roof professionals probably know that from their own experience, but it’s always nice to have experimental proof.

Still, although they are moving into the mainstream at a satisfying rate, green roofs remain a mystery to many building owners and managers, and many contractors.

The two best websites I’ve found are operated by Green Roofs for Healthy Cities, in Toronto, and Greenroofs.com, LLC, in Alpharetta, Ga.

Both serve as industry portals, which means they have solid libraries of non-commercial information, as well as heavy involvement by the research communities, various levels of government and participation from suppliers and contractors. They can be found at www.greenroofs.com and www.greenroofs.org.

Korky Koroluk is a regular freelance contributor to the Journal of Commerce. Send comments to editor@journalofcommerce.com

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