Green and blue roofs

One-line orientation

Green roofs use a vegetated assembly (extensive = light/shallow vs intensive = deep/heavy) while a blue roof temporarily detains stormwater, and the tested distinction is the structural-load gap between extensive and intensive.

Key points

• Green roof — Extensive:
  • Shallow soil, lightweight, low-maintenance plants.
  • Positive membrane drainage still matters. About 2% is a common design value, not a defining minimum for every extensive green roof.
  • Lighter load → less added structure.
• Green roof — Intensive:
  • Deep soil, diverse plantings, often accessible (gardens/landscape).
  • Heavier → requires additional structural support and higher maintenance.
• Blue roof:
  • Purpose: temporarily retains/detains water to manage stormwater during peak discharge.
  • Benefit: reduces peak runoff by detaining water and releasing it slowly.
  • Water quality: pollutant removal is limited and depends on the complete system; detention alone does not guarantee cleaner water.
  • Challenge: design complexity to prevent leaks; performance varies with climate.
• Why they’re specified (benefits): stormwater management, reduced heat-island effect, and added insulation/thermal performance.

Confusions / comparison

	Extensive green	Intensive green	Blue roof
What it is	Vegetated, shallow growing medium	Vegetated, deep soil, garden-like	Controlled water detention (no required planting)
Soil depth	Shallow	Deep	n/a
Weight / structure	Light → little extra structure	Heavy → more structural support	Adds water load during storms
Maintenance	Low	High	Leak-prevention / outlet upkeep
Primary job	Runoff + thermal, low effort	Usable green space + biodiversity	Temporary stormwater detention

Related

→ Roof slope and drainage (this topic): minimum slope to drain · Roof low-slope membranes (this topic): the waterproofing layer beneath a green/blue assembly · (Cross-module) Site / sustainability: stormwater management and heat-island strategies.