Q: What is a vegetative or green roof?
A: Green or vegetated roofs seem to be popping up all over the place, particularly on new commercial and institutional buildings, including those of the Seattle Cancer Care Alliance, The Neptune apartments, the Mercer Slough Pond Lily Wet Lab, the Hyatt at Olive 8 and, most recently, the Gates Foundation.
The concept of green or vegetative roofing is nearly as old as humanity and used in a variety of ways. Remember the prairie pioneers and their sod houses? These days, the concept of green roofing has been taken to a whole new level as cities worldwide experiment with green roofing as a way to reduce urban problems.
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Simply put, a green roof layers synthetic roofing and a drainage system, with both topped by soil and special plants.
In America, the vegetated-roof industry is still young, but Europe has been “growing” green roofs for more than 10 years, and Tokyo now requires that at least
20 percent of any new roof on a medium or large building be cultivated.
While these innovative roofs have mostly been installed on commercial-scale buildings, interest in their application for residential construction is growing.
Well-designed vegetated roofs provide advantages beyond the typical insulation and watertight properties of a traditional roof. Green roofs can cool cities as the vegetation reduces roof temperatures in summer, which can lower air-conditioning costs and demand on power plants.
Vegetation planted across a roof also reduces stormwater runoff, easing the burden on sewers and wastewater systems, and can extend overall roof life. They can also add living beauty to commercial areas.
A vegetated roof generally has a root barrier and a water-retention and drainage layer, separated by a thin filter fabric from the growing media and plant material on top.
Interestingly, installers rarely use soil because it is heavy and packs tight after repeated rains, reducing water retention and aeration for plant roots.
Instead they use manufactured materials like granulated clay or shale.
Organic compost and fertilizer are added as nutrients. Water, stored by the substrate layer, is taken up by the plants, then returned to the atmosphere through evaporation.
The main drawback to this fascinating roofing option is cost. Depending on the complexity of the system and types of vegetation, another drawback is the need for maintenance.
Here in the Northwest, a simple three-tab asphalt roof will generally cost around $4 a square foot. A simple green roof could be as much as $35 a square foot, depending upon its size.
With a life span of 50 years or more, vegetative roofs make sense at the commercial and institutional level from an operational, tax and environmental perspective. For residential use, the biggest benefits are aesthetic and environmental.
Another consideration is the added weight of a green roof. If you are tempted to consider a green roof for your home, a structural engineer should be consulted as part of the planning process, since most existing homes are not designed for this additional weight.
Tiny green roofs, up to
300 square feet, are often installed as demonstrations, and even birdhouses and doghouses have been given green roofs — a fun and educational way to begin exploring the possibilities of green roofing.
Wayne Apostolik, of Northwest Homecrafters, is a member of the Master Builders Association of King and Snohomish Counties’ Remodelers Council and provided the information contained in this article. If you would like more information or have questions about home improvement, send them to email@example.com. Sorry, no personal replies. Always consult local codes and contractors.