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PATH Adds Green Roofs to Its Technology Inventory
The Partnership for Advancing Technology in Housing (PATH) recently announced that it has added green roofs to its Technology Inventory.
The PATH Technology Inventory lists more than 180 technologies — including many green housing strategies from advanced framing techniques to white LED lighting — that demonstrate great potential for improved housing performance, but have not been widely accepted. The inventory focuses on a range of technologies — from new materials, components and systems to complete houses.
Green roof systems of living plants and vegetation, which are installed on the roof of an existing or a new structure, can reduce storm-water runoff and protect the roof from extreme temperatures, ultraviolet radiation and harsh weather conditions. Reducing daily temperature swings also decreases the amount of expansion and contraction of the roof waterproofing membrane, prolonging its life.
Popular in Europe for decades, appropriate green roof systems are now available in nearly all climates and areas of the U.S. All of the systems consist of four basic components — a waterproofing layer, a drainage layer, a growing medium and vegetation. Some green roofs also include root retention and irrigations systems, but these are not essential.
Thin Soil for Most Residential Roofs
There are two types of green roof systems, PATH says. Extensive systems feature low-lying plants such as succulents, mosses and grasses. They require relatively thin layers of soil — one to six inches — and the plants usually produce a few inches of foliage. Accounting for most residential applications, extensive systems have less of an impact on the roof structure, weighting 10 to 50 pounds per square foot on average, and they are generally accessible only for routine maintenance.
Typically designed to be accessible to building inhabitants for relaxation and harvesting, intensive systems feature deeper soil and can support larger plants — including crops, shrubs and trees. These systems can be harder to maintain, depending on the plants used, and are much heavier than extensive systems, ranging from 80 to more than 120 pounds per square foot.
Growing mediums for the systems include soils, peat and other organic materials, gravel and other aggregates. A drainage layer is required to adequately distribute water and prevent pooling. To minimize the weight of the system, drainage layers are often made from plastic or rubber, but they may also be made of gravel or clay. The drainage layer may or may not include filter media to ensure aeration.
The waterproofing membrane is a critical component of the system and should include a root barrier to ensure that the underlying roof surface is not compromised. If the waterproofing material is not root-resistant, an additional layer must be applied to serve this purpose.
Plants Able to Tolerate Extreme Weather
Plants used in green roof applications must be easy to maintain and tolerant of extreme weather conditions — including heat, freezing and drought — and they must have relatively shallow, fibrous root systems. The plants should also be resistant to diseases and insects, and, in order to protect surrounding plantings, they should not generate airborne seeds.
Available in a variety of colors, in both deciduous and evergreen options, climate-appropriate succulents, mosses and grasses are often best suited for extensive green roof systems. Many nurseries throughout the country specialize in vegetation for green roofs.
PATH says that the structural design of the home must be carefully considered to accommodate the additional load of a green roof system. Roofs do not need to be flat, but different systems have varying pitch recommendations and limitations, which should be considered during the design phase. Flood testing of the roof membrane should also be conducted prior to placement of the green roof system.
Costs for research, design and materials of the green roof system and structural support are higher than a conventional roofing system. Extensive systems, according to PATH, can cost as little as $7 a square foot, though they tend to be in the $10 to $15 range. Intensive systems range from $15 to $25 per square foot.
There will be some additional costs involved with maintaining the rooftop plantings, says PATH, but overall maintenance of the roofing membrane will be reduced. Since planted roof systems increase the life span of the roof, repairs and replacement should be minimal.
 ‘Green Roof — A Case Study’ Delves Into Green Roof Construction
A green roof that combines aesthetics and mechanics has become the goal of many landscape architects. In “Green Roof — A Case Study,” available at BuilderBooks.com, author Christian Werthmann explains the history, methodology and design process of green roof garden construction ― providing a rich source of inspiration and technical knowledge in the process for anybody interested in this solution to many of the environmental challenges faced today.
To view or purchase this publication online, click here, or call 800-223-2665.
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