published Tuesday, August 16th, 2011

Time for green roofs

  • photo
    Staff Photo by Robin Rudd Hamilton County Health Department.

The County Commission will be asked Wednesday to approve the county Health Department's plan to install a green roof to replace a section of worn-out roofing at its Third Street center. Approval should be an easy call. For a number of cost-efficient environmental reasons, roofs covered by vegetation are bound to become a national norm in the future, especially in cities. Hamilton County government should already be headed in that direction.

The lifetime value of green roofs reflects a range of benefits that easily offsets the marginal difference in up-front costs. Among the chief factors are longer life than traditionally finished flat roofs, energy efficiency, reductions in municipal costs for managing urban stormwater runoff and reductions in the "heat island" problem in cities.

The latter two are especially significant. And their advantages accrue to local governments and taxpayers in reductions of their tax costs and power bills. In fact, the potential savings are significant enough that progressive cities in North America and governments in other advanced countries have wisely begun requiring green roofs as a matter of public policy.

Properly constructed, green roofs last significantly longer than regularly constructed flat roofs because the plants used to vegetate the roofs protect the roofing material from the baking effects of the sun, and moderate the effects of winter cold. Roof vegetation also helps absorb and divert rain from storm-water runoff systems. It also reduces a building's contribution to the "heat island" effect in cities. Those are hugely significant benefits. (For more savings, building owners often store rain-water in cisterns to serve roof-irrigation systems, gardens and gray-water use.)

Chattanooga, for example, has had to raise its stormwater fees and on-site stormwater retention requirements substantially because of the costs associated with treatment and overflow of the city's combined stormwater and sewer system.

That problem is not going away. If anything, standards will continue to be tightened both to prevent sewage overflows, and to remedy the significant runoff of toxic chemicals contained in urban stormwater runoff into rivers and streams and other public-water resources.

Similarly, consumption of electricity for air conditioning in urban areas can be significantly reduced by mitigating the heat-island effect of cities, where so many paved streets and layered flat roofs capture and retain solar heat. Even cities in colder climes -- Toronto and Philadelphia, for example -- have begun mandating the installation of green roofs to mitigate both storm-water runoff and the heat-island effect that drives up air-conditioning costs and forces electric utilities to add more costly capacity.

In the South, electrical demand for air conditioning is especially high. It has caused many utilities, including TVA, to become summer-peaking systems, in which consumer demand for electricity is higher in the summer than in the winter. TVA, in fact, often has to purchase additional power from other utilities in the summer to meet peak air conditioning demands.

Such peak-hour purchases cost significantly more than off-peak generation. Reducing municipal heat-island effects by several degrees through wide use of green roofs would help limit customers' peak-power demands and utility bills.

County government's approval of the health department's plan for a green roof, to be sure, will not by itself make much of a dent in local power use and stormwater reductions. Its benefit is mainly educational and symbolic. It would also support a leading-edge trend, one that promises great value as building owners and cities learn to appreciate and replicate the use of green roofs.

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