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Types of green roofs

Green Infrastructure: Green roofs consist of a series of layers that create an environment suitable for plant growth without damaging the underlying roof system. Green roofs create green space for public benefit, energy efficiency, and stormwater retention/ detention.

Structural load capacity, how much weight the roof can hold, is a major factor in determining roof design and construction. Green roofs are therefore typically defined as being either extensive or intensive.

  • Extensive green roof: an extensive green roof has growing medium (engineered media) that is 6 inches or less deep
  • Intensive green roof: an intensive green roof has growing medium that is more than 6 inches deep

Semi-intensive green roofs represent a roof design intermediary between extensive and intensive roof. The following table summarizes characteristics of the different types of green roofs.

illustration showing typical green roof sections

Schematic showing the different components of a green roof. Thicknesses of some layers vary with the design (e.g. extensive vs. intensive roofs).
General characteristics of extensive and intensive green roofs (adapted from Green Roofs for Healthy Cities and the Cardinal Group, 2006).
Characteristic Extensive Semi-intensive Intensive
Growing medium depth 6 inches or less Portions of the green roof above and below 6 inches, with a minimum of 25% of green roof area above or below 6 inches More than 6 inches
Accessibility Often inaccessible May be partially accessible Usually accessible
Fully saturated weight Low: 10 to 35 lb/ft2 (48.8 to 170.9 kg/m2) Varies: 35 to 50 lb/ft2 (170.9 to 244.1 kg/m2) High: 35 to 300 lb/ft2 (170.9 to 1464.7 kg/m2)
Plant diversity Lower Greater Greatest
Cost Low Varies High
Maintenance Varies, but generally lower than for intensive green roofs Varies Varies, but generally higher than for extensive green roofs
Stormwater management Best cost-benefit balance More growing medium and more vigorous plant growth provides marginally greater stormwater volume benefits More growing medium and more vigorous plant growth provides marginally greater stormwater volume benefits

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