At the most basic level, the term green roof refers to a layer of vegetation planted over a series of specialized roofing layers that function together to keep the vegetation alive and protect the underlying building. Those layers may differ, depending on the intended function for the roof, but typically include a waterproof membrane, a root barrier, a drainage layer, a specialized lightweight growing medium, and the vegetation itself.
Green roofs are becoming increasingly popular, especially in highly urbanized areas, where their benefits extend way beyond their aesthetics and include improved stormwater management, reduced ambient air temperatures and the effect of urban heat islands, improved air quality, lower energy costs, and more.
Green roofs may also be called living roofs, eco-roofs, sod roofs, and vegetated roofs. In many cases, the terms roof garden, urban agriculture, and rooftop farming also refer to or incorporate certain types of green roofs.
Extensive, Intensive, and Semi-Intensive Green Roofs
The term green roof encompasses several different designs and systems that may be used based on the specific goals and constraints of the project. In general, they fall into three broad categories: extensive, intensive, and semi-intensive, although some changes in nomenclature are becoming more prevalent.
Extensive green roofs are designed to have a modest roof load (often not requiring additional structural support) and may even be suitable for roofs that aren’t completely flat. Extensive green roof systems are designed to have minimal watering and maintenance requirements, so they are usually inaccessible except for periodic maintenance. Because of the setup, extensive systems are limited in the types of vegetation they can support. Drought tolerance, shallow roots, and tolerance to full sun are primary requirements for extensive roofs.
Intensive green roofs are designed to have more soil and are more flexible in the types of vegetation they can support. This means intensive roofs can offer much more biodiversity, including bushes and even trees. These uses correlate to a higher structural load and more intense maintenance requirements, including regular watering. Designers of intensive green roofs often prioritize accessibility and may even choose to create a parklike setting where residents and visitors can enjoy the atmosphere. Since intensive green roofs are so flexible, the depth of the roof system can vary quite a bit and depends on factors like the structure and load capacity of the structure itself, the types of plants that will be grown, as well as goals for supplemental functions like stormwater management.
Not surprisingly, the term semi-intensive green roof covers the middle ground, where systems may incorporate features of both types.
Some alternative terms are occasionally used to replace “extensive” and “intensive”. This seems like an improvement, since the terms can be somewhat confusing to those new to the subject. For example, a “low profile” green roof refers to roof systems that are shallower, lightweight, and simpler in design, rather than the traditional “extensive”. To replace the term “intensive,” “high profile” or “deep profile” more clearly describes a complex, heavier system with deeper soil.
