Water Resources Program  |  Green Roof -- System Types
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GREEN ROOF SYSTEM TYPES
The roof of the County Office Building was replaced as part a scheduled roof replacement. The County's green roof is an extensive application of green roof technology. That is, the roof is not designed for active recreation or use. Extensive green roofs supply the technology and the benefits of green roofs in a cost-effective manner. The other roof type, intensive green roofs, are designed to provide outdoor places for recreation or contemplation and are generally more expensive due to engineering requirements, a deeper soil medium, and the need for fixtures such as walkways, patios, benches or open shelters.

The County's green roof project, which is a demonstration project to illustrate and evaluate the value of green roofs, utilizes two extensive system types. The first is a system of 18"X24" trays that hold the soil medium and plants. This system is portable, allowing the tray system to be removed easily in case of any problems with the roof. Though green roofs are very reliable, the tray system allows those who may opt to vegetate a roof to know that the system could easily be removed in case repairs to the roof need to be made. However, green roof technology has improved significantly over the last decade such that repairs are unlikely to be needed. The trays are often "pre-vegetated" so that the roof is immediately covered in green after construction is complete. The other system growing on the County Office Building covers the roof more uniformly and takes up to two years to fill in. This system utilizes a large area to apply the parts of the green roof system: rubber membrane, root barrier, water retention layer, filter fabric, and soil medium. These are applied uniformly over the roof and plants are then introduced to the soil medium. This system is relatively inexpensive compared to an intensive green roof. This system covers a majority of the roof and is expected to take two years to fill in. Thus far, the roof has filled in well and appears to be on track for full vegetation by roughly July of 2007. Each system type comes with a comprehensive warranty should any problems arise.

STRUCTURAL CONSIDERATIONS
The roof that was replaced to make way for the green roof was a conventional membrane-ballast roof. These roofs consist of a rubber membrane that is adhered to the roof and further bonded to the roof by placing river stones on top of the membrane. The structural load of the river stone is replaced by the soil medium, which has a similar, but somewhat slightly less, structural load. At complete saturation, the green roof carries a load that is roughly equal to the original roof.

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