Sustainable built environment

Green Roof

Meaning statusEstablishedSource recordDirect document linkedWhy these are different

Definition

A green roof is a roofing system incorporating vegetation, a growing medium, and drainage over a waterproof membrane, designed to capture and retain precipitation on the building. Systems are classed by depth and planting: extensive roofs (typically up to about 15 cm of medium, lightweight, low-maintenance, drought-resistant plants such as sedums) and intensive roofs (deeper substrate supporting shrubs and trees, accessible as gardens but heavier and maintenance-intensive).

References

Penn State ExtensionGreen Roofs: Benefits and Design Considerations (extensive 2–6 in. vs intensive 6+ in.; four layers; 50–60% of rain captured and cycled to atmosphere; ~55% runoff reduction)

typology and performance

Holcim ElevateIntensive vs. extensive green roofs (intensive 15–60+ cm, heavy, accessible; extensive 5–10 cm, lightweight, low-maintenance)

system types

Overview

What it means

Green roofs address several urban problems at once: they retain a large share of rainfall — research suggests around half or more can be captured and returned to the atmosphere — reducing runoff peaks; they insulate and protect the membrane, extending roof life; they mitigate the urban heat island effect; and they provide habitat and amenity in dense areas.

How it is used

Cities incentivise or mandate green roofs in stormwater and building codes; designers specify them as stormwater best management practice and for building certification credits.

Why it matters

Roofs are a large share of urban impervious surface; greening them is multifunctional climate adaptation that needs no extra land.