As politicians debate back and forth on the subject of whether or not carbon dioxide is causing the earth’s glaciers to vanish at record paces; let’s visit the option of installing “Green Roofs” and their benefits to scrubbing the atmosphere of this pesky greenhouse gas. In 3rd grade science class we all learned that plants take in CO2 and pump out fresh new O2 so that we can exist. After all, politicians are not scientists, albeit they do expel tons of their own gasses on this “heated” subject.
Green Roofs gradually appearing in the Southern California area are going to affect the environment in many positive ways. But first, let’s get one thing straight about the economy of being environmentally conscious: yes, it’s going to cost green to make a roof green. But the energy savings far outweigh the cash needed to retrofit the top of a structure. For example, costs generally vary from $15 to $25 per square foot for replacing a conventional roof with a green roof, and from $10 to $15 per square foot to install a green roof as specified with new construction.
Dirty Waters
Storm runoff in Los Angeles is also another environmental concern. Ninety one billion gallons of rainwater cascade off of our roofs during an average stormy January day. All of this runoff picks up solvents, petroleum products and debris that are then directed out to pollute LA’s world famous beaches. Vegetated rooftops mitigate this runoff because the water is absorbed quickly and releases it slowly.
However, the roof design dictates how much water is absorbed before the roof becomes saturated. This is where the implementation of a cistern comes into play. This system can be designed to capture excess rainfall and store for later irrigation of the roof during the rest of the not-so rainy seasons of Southern California.
Green Roofs not only assist in air conditioning the confines of the building that it tops, but also make the outside cooler. “Heat islands” are common in large cities and Los Angeles boast the highest heat island affect in the nation. Green tops reduce this affect by absorbing the sun’s rays and not reflecting it back around the neighborhood like concrete, tar and steel. Recent figures promise that urban temperatures can be reduced as much as 7 degrees as more and more plants appear in place of flat heat reflecting roofs.
Attics full of Roots?
The structure of a green roof departs from that of a conventional roof beginning with the roof membrane. Bituminous membranes (organic content ) are vulnerable to root penetration and/or micro-organic degradation. Most green roof installers will only guarantee their work if a new membrane is installed along with the green roof. As a result, the most advantageous time to install a retrofit green roof is when the existing membrane is scheduled for replacement. Although a green roof serves the function of protecting the membrane from exposure to the elements, it also exposes the membrane to potential penetration by roots.
Other membranes, such as those composed of synthetic rubber or reinforced PVC, generally do not need a root barrier.
An experienced green roof contractor or green roof materials provider will be able to determine if a root barrier is required for a given roof membrane. American Hydrotech Inc. offers a self-sealing monolithic membrane in their “Garden Roof®”, system that now supports over 2.5 million square feet of Chicago’s roof greenery.
The Green in Green Roofs
The climatic conditions in Los Angeles limit the selection of plant materials for your green roof. It is recommended that vegetation be drought/wind tolerant, perennial or “self-sowing” low-maintenance stock with a shallow root structure.
Temperature extremes must also be taken into consideration since the Los Angeles climate tends to spike into the 100’s with minimal precipitation.
One group of plants which commonly have the above characteristics is referred to as succulents. Sedums and sempervivums are commonly used on green roofs. Of course, the list o
f plants that will survive on a green roof grows longer as the budget for caring for these plants progresses.Two California projects in particular offer examples of appropriate plant materials. One is the green roof at the Premier Automotive Group headquarters in Irvine where the roof is planted with Sedum, Echeveria, Lampranthus, Delosperma, Agave, and Aloe.
The other is the GAP Headquarters in San Bruno, which is planted with native grasses and wildflowers.
Salad Days
Green roofs fall into two categories: extensive and intensive.
Extensive green roofs are lower in weight, cost, and maintenance than intensive systems. Plants for extensive green roofs tend to require only a few inches of soil and little additional irrigation or care. Typically, extensive roofs are often unable to accommodate regular human traffic.Intensive green roofs are heavier, more costly, and require additional maintenance than their extensive cousins. However, they can accommodate vegetables, shrubs, and trees. Because these are deep-rooted plants, at least 12 inches of soil may be required. This presents a weight issue that must be considered in the building/ planning stages, so as not to put stresses on the home or building underneath.
Switzerland has passed green roof legislation that requires any new construction that invades upon vegetation to replace it 100% through the use of roof top landscaping.
Green roofs not only provide a sensible, beautiful, architecturally savvy process to address some of the most urgent environmental issues facing urban life today, each molecule of CO2 converted into breathable, global cooling oxygen may slow the advance of Greenland’s ice sheets into the North Atlantic.
