Roofing system
What is a green roof?
A roof with plants growing on it — and, underneath the plants, a waterproofing system that has to be better than the one on an ordinary roof, because nobody will ever see it again.
Updated September 12, 2026 · Every figure links to its source · Sources
DefinitionSystemAlso called: vegetative roof, living roof, eco-roof
Green roof — a roof on which a growing medium and plants are installed over a waterproofing membrane, a root barrier and a drainage layer; the U.S. Environmental Protection Agency describes the two types — extensive, with a shallow medium and hardy low plants, and intensive, with deeper soil that can carry shrubs and trees — and the IBC's roof chapter regulates them as vegetative roofs.
What it is
From the deck up, a green roof is: the structure (sized for the wet weight of everything above); insulation; a waterproofing membrane, usually a single-ply or modified-bitumen system rated for continuous burial; a root barrier so roots cannot find the seams; a drainage and water-retention layer; a filter fabric; the growing medium, which is an engineered mineral mix rather than garden soil; and the plants. Extensive roofs carry a few inches of medium and sedums, grasses and herbs that look after themselves; intensive roofs carry a foot or more and can be gardens, with the irrigation, access and maintenance a garden needs. The EPA sets out the types and the benefits it has measured.
The code treats the assembly as a roof covering with extra duties: the IBC, Chapter 15 covers vegetative roofs among its roof-covering sections and points to the structural chapter for the loads. On a house the same layers apply, and the structure question is the first one to ask.
Why it matters
The EPA's interest is heat and water: a planted roof stays far cooler than a dark membrane in summer, reduces the building's cooling load, and holds and slowly releases rain that would otherwise run straight to the storm drain — its green roofs page sets out those effects. Owners also cite the longer life of a membrane that is shielded from sunlight and temperature swings, and the amenity of a roof you can use.
The risks are the mirror image. Everything is heavy when wet, so the structure must be checked by an engineer; a leak is hard to find under soil and expensive to reach; and a roof that is not drained and maintained becomes a swamp or a weed bed. The membrane is the part to spend on: it should be tested (usually by flooding) before a single tray of plants goes on.
What it costs and who does it
A green roof is priced per square foot of planted area on top of the roofing system beneath it, with the type (extensive or intensive), the depth of medium, access for materials, irrigation and the structural work as the variables. It is a two-trade job: a commercial or low-slope roofing contractor installs the membrane, root barrier and drainage, and a green-roof or landscape specialist installs the medium and plants; a structural engineer signs off the loads. The directory's guide to flat and low-slope roofing covers the membrane systems that go underneath.
Frequently asked questions
Short answers; the sections above carry the detail and the sources.
What is the difference between an extensive and an intensive green roof?
Can a green roof be installed on a house?
Do green roofs leak?
Next step
Find a roofer who does this
A green roof is only as good as the membrane under it, which is the roofing contractor's part. The directory lists roofing contractors by state and city.
Sources
- Using green roofs to reduce heat islands — U.S. Environmental Protection Agency
- 2021 International Building Code, Chapter 15: Roof Assemblies and Rooftop Structures — International Code Council (ICC)