This green roof in Toronto covers an area of almost 10,000 square feet. Half of the roof surface is covered in shrubs, perennial grasses, fl owers, herbs and groundcovers. (Photo courtesy of Susanne Jespersen, Mountain Equipment Co-op)
: How They Top Out these two environmental options
to oxygen, provide insulation, create a natural wildlife habitat, process airborne toxins, offer aesthetic qualities and like cool roofs, mitigate the heat island effect. A vegetative covering protects the waterproof
membrane from daily temperature fl uctuations and the ultraviolet radiation of the sun that breaks down conventional roofi ng systems. Similar to a green roof, a vegetative roof’s lower surface tem- perature reduces the heat island effect in urban areas. Their increased insulation values stabilize indoor air temperatures and humidity, and reduce the heating and cooling costs for a building. There are two types of green roofs: inten-
sive roofs, which are thicker and can support a wider variety of plants but are heavier and require more maintenance, and extensive roofs, which are covered in a light layer of vegetation and are lighter than an intensive green roof. Green roofs are most effective on fl at-roof sur- faces (maximum slope of 1:12) using the largest continuous roof area. Slightly higher slopes up to 2:12 are feasible but may require additional engineering controls.
and must be considered in initial planning. It must be chosen with regard to the amount of care it needs, its mature size and roof wind con- ditions. Vegetation that is oversized may suffer in shallow green roof soils and become uprooted in high winds if there is no shelter. Plants must be able to withstand the harsh climates and weather extremes found on roofs.
Maintenance Cool metal roofi ng is virtually maintenance free and can last 30 to 40 years. “Coatings are considered ‘self-cleaning’ in that they resist dirt pickup,” says Lanza. “Some paint manufacturers recommend an annual sweet (tap) water rinse in highly corrosive environments, but unless your building is in a heavy industrial or seacoast envi- ronment, no maintenance is required.” Thimons agrees, saying cool metal roofs require
minimal maintenance and cleaning. “Research at the Oak Ridge National Laboratory demonstrates that metal roofi ng retains its solar refl ectance over time better than other roofi ng products because it
resists the growth of organic matter and sheds dirt more readily than other materials. It maintains its surface properties and resists soiling.” On the contrary, a green roof is a highly engi-
neered living system needing a routine and con- sistently executed maintenance protocol including watering, weeding, fertilizing, replanting, and replac- ing dead or dying vegetation. An ineffective mainte- nance program will result in a wild and overgrown look that lessens the green roof’s success. Weeds, discolored or scorched plants, small plants and plant loss are signs that a green roof is not being correctly maintained. Fungal diseases and insect problems must be checked on a regular basis.
Initial expense According to Thimons, there is typically no ad- ditional expense for installing cool metal roofi ng when compared with non-cool metal roofi ng. And, “it pays for itself over time with its energy- effi ciency and durability benefi ts,” he says. However, the initial expense of green roofs
is higher than cool roofs. This is because they require more extensive design, careful struc-