What a green roof is
A green roof is a roof covered with a growing medium and plants over a waterproof membrane. It can be as thin as a few inches of lightweight soil with drought-tolerant plants, or as deep as a rooftop garden with shrubs and small trees. Green roofs are a form of green infrastructure that uses otherwise unused roof space.
The layers
Although designs vary, most green roofs include several layers from bottom to top:
- Structure. The roof deck, which must carry the added weight.
- Waterproofing. A membrane that keeps water out of the building; often with a root barrier so roots cannot penetrate it.
- Drainage layer. A material that lets excess water flow away and sometimes stores a little.
- Filter fabric. Keeps fine soil from clogging the drainage layer.
- Growing medium. A lightweight mix, usually not ordinary garden soil, designed to drain well and stay light.
- Plants. Species chosen for the roof’s exposure, depth and climate.
Failure of the waterproofing is the biggest worry, so quality installation, and often leak detection during construction, matters.
Two broad types
Extensive roofs have shallow growing medium and hardy plants such as sedums, grasses and herbs. They are lighter, cheaper and need less upkeep, but are typically not designed for regular foot traffic.
Intensive roofs have deeper soil and can support lawns, shrubs, gardens and sometimes trees. They add more weight, cost more and need irrigation and regular care, but can function as usable open space.
Weight and structure
Soil holds water, and a saturated green roof can weigh much more than a conventional roof. Whether an existing building can carry that load must be checked by a structural engineer. This is a common reason green roofs are easier on new buildings than on older ones. Adding a lightweight extensive roof to an existing building may be feasible but should not be assumed.
What green roofs can do
Stormwater. The growing medium and plants absorb some rainfall, and the rest flows off more slowly. Performance depends on depth, plant cover, roof slope and how wet the roof already is. A typical green roof can hold back a large share of rain during small and moderate storms, but when the medium is saturated in a prolonged or intense storm, most additional rain runs off. They reduce and delay runoff, not eliminate it.
Roof protection and insulation. The cover shields the membrane from sunlight and temperature swings, which can lengthen its life. It also adds some insulation, mainly reducing summer heat gain through the roof. The effect on a building’s total energy use is usually modest, and it is larger for single-story buildings with big roofs than for tall ones.
Cooling. Plants cool through shade and evaporation, so a green roof is cooler than a dark conventional roof in summer. Its effect on air temperature at street level is limited, because the roof is far from where people stand. Widespread coverage across many roofs can influence local temperatures more, but results vary and are hard to measure.
Habitat. Roofs can host insects, birds and pollinators, especially when varied plants are used, though they are small, isolated patches.
Amenity. Accessible roofs provide gardens, terraces or community spaces in dense areas.
What green roofs cannot do
They do not substitute for street trees at ground level when it comes to shading pedestrians. They do not solve city-wide flooding on their own. Their energy benefits are modest in most climates. Their cost is higher than a conventional roof, so claims of rapid payback should be treated with caution.
Cost and upkeep
Installation costs more than a standard roof, and the extra cost varies by type and region. Maintenance includes weeding, checking drains, irrigating during establishment and dry spells, replacing dead plants and inspecting the membrane. Extensive roofs need less, but none is maintenance-free. Maintenance contracts and clear responsibility between building owner, tenants and managers help prevent neglect.
Some cities offer incentives, fee reductions or requirements for green roofs on large buildings. These policies change, so check current local rules.
When they make sense
Green roofs are most attractive where roof area is large, stormwater fees or rules create a financial motive, structure allows the load, and someone will maintain them. In renewal projects, they can be integrated during design of new buildings or major roof replacements, when the extra cost of adding the system is lowest. Alternatives, such as light-colored reflective roofs or rooftop solar panels, may serve some goals better, and sometimes the two can be combined with planning.