The green roof soil or “media” is one of the most important elements of a healthy green roof. Typically, most extensive green roof soil has 10-15 percent organic material and the rest is some sort of light-weight aggregate.This allows the roof to drain and not become water logged. The best mineral content is pumice. Pumice is the only truly natural light-weight aggregate. Other light-weight aggregates are man-made and have a large carbon footprint due to the use of fossil fuels in the manufacturing process. Research has documented that the carbon footprint of pumice is approximately 1,300 percent smaller than man-made aggregates like expanded shale or calcined clay. Since the single largest ingredient on a green roof is soil, it’s important to use a natural product like pumice. Otherwise, the positive benefits of a green roof are overshadowed by the carbon dioxide released into the atmosphere during the man-made soil manufacturing process.
Pumice is found all over the world and is a natural by-product of volcanic activity. No manufacturing is required. The pumice is simply loaded, graded for size, and delivered to the green roof site. Thus, the carbon footprint is small.
It is nature’s perfect soil conditioner. It is very light-weight, but it is heavy enough not to blow away with strong winds. It holds water – up to 35 percent by volume – and the microscopic pores store oxygen and nutrients. As plants require water, air, or nutrients, the pumice releases these vital components. Pumice is also very hard (6.1 on the Mohs hardness scale – diamonds are a 9 on the Mohs scale).
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There are two main types of green roofs. The first is called an extensive green roof. This type of roof has only 3-6 inches of soil. The other is called "intensive". Intensive green roofs have more soil and can sustain a wider range of plants, shrubs, and trees.
For both types of roofs a high quality water proofing system is essential. The water proofing system is like a rain coat that keeps us dry. The better the waterproofing system, the longer the roof will last before it needs to be replaced. The most durable waterproofing system is called a built-up modified system. This type of system creates a very thick continuous waterproofing barrier across the entire roof. Other popular systems include PVC, TPO and EPDM systems. These sheet membranes are less expensive, but still have excellent longevity. Again, the better the waterproofing system the longer the green roof will last.
The most common mistake on any green roof is not having proper drainage. Below the soil, some form of drainage board or mat must be installed. Without a good drainage system water sits on the roof and the plants die. Usually, a root barrier material is also installed with the drainage material. The root barrier is an added protection for the waterproofing system – guaranteeing that the plant roots don’t damage the waterproofing.
In summary, green roofs are highly technical roofing systems that can provide huge practical benefits to the building owner and important benefits to the surrounding eco-system. Green roofs are also beautiful and research has proven that the natural beauty of green roofs can be a powerful and sustaining healing force in our lives.
Green roofs are new to our city, but they have existed for thousands of years. A good example is the Hanging Gardens of Babylon – one of the Seven Wonders of the Ancient World. These gardens created a “green mountain” up to 320 feet tall with an elaborate irrigation system to keep the massive trees and exotic plants healthy and green.
One hundred or more years ago, green roofs were a common sight in Northern European countries like Norway, Iceland, and Switzerland. Most people have seen pictures of “sod” roofs that helped to keep the simple homes warm, dry, and cool in the summer.
Today, green roofs are everywhere and they are highly technical. In cold climates, desert climates and even tropical climates like Guam, people are building green roofs. They are turning to green roofs to help manage storm water, reduce air conditioning costs and extend the life of the waterproofing system on the roof for up to 40 years. In some countries, like Germany, 14% of all roofs are green roofs.
Green roofs are expensive, but the long-term economic benefits outweigh the additional costs. One of the most important benefits of a green roof is the waterproofing system can last 2-3 times longer because the roof is protected from harsh weather and ultraviolet rays (UV). That means rent for apartments and businesses can stay lower since the maintenance costs are lower. Another key benefit is that green roofs insulate the building from big temperature swings. According to one study, even a six-inch deep green roof can reduce summer energy costs by 75 percent. Again, this helps keep maintenance/operating costs lower which helps families and businesses.
The environmental benefits are sometimes harder to quantify financially, but they are just as important. The pumice soil and plants act like a sponge to absorb and filter the rain water that would normally plunge down gutters and into streets and sewers, which often causes flooding when storms stall. The plants also filter the air and produce oxygen and evapotranspiration occurs when water evaporates from plant leaves and cools the air. The air is cleaner to breathe and cooler - helping to reduce the uncomfortable effects of Urban Heat Island Effect.
Some green roofs capture the water that lands on the roof in holding tanks called cisterns. Sometimes these cisterns are located in the ground or basement of the building. The water that is captured can be filtered and pumped back up to the roof or the ground level landscaping for irrigation purposes. This way, essentially all the water that falls on the green roof is used and kept out of the storm sewer system. In many parts of the world, water is scarce. Using green roofs to capture and reuse rain water is a simple way to conserve water and increase healthy vegetation.
Most green roofs are designed as extensive green roofs. Shallow soil depth requires plants that can tolerate dry periods and lots of sun. The sedum plant family is widely used throughout the world because of its ability to withstand heat, wind, and drought. On top of that, the many sedum varieties are colorful with outstanding blooms.
When the soil depth increases, a wider range of plants and trees can be used. Roof top vegetable gardens are even being planted. The famous San Marzano tomato prefers volcanic soil and is a favorite for green roof farmers. Like any gardening, there is some trial and error with green roof plants. Conditions vary from roof to roof and plants that do well on one roof may do poorly on a similar roof just a few miles away.
The saying, “it ain’t green if it ain’t green” is important when considering the types of plants on a green roof and the maintenance regiment developed for each roof. Irrigation and fertilization are as important to green roofs as they are for backyard gardens and commercial farms.