Green roof: ≥ 3 in growing medium with appropriate drainage; retains 50-75% of annual rainfall
EPA Stormwater — Green roof / vegetated roofDescription
Vegetated (green) roofs retain stormwater + reduce roof-related runoff. Extensive green roofs use 3-6 in growing medium with sedums + drought-tolerant plants (~25 lb/sqft saturated); intensive green roofs use 6+ in medium with larger plants (often 50+ lb/sqft saturated, requires structural design). Annual rainfall retention: 50-75% for extensive systems in NY climate. Layers from substrate down: vegetation, growing medium, filter fabric, drainage layer, root barrier, waterproofing membrane.
Why this exists
Green roofs offer triple value: stormwater, building cooling (~20-30% reduction in roof solar heat gain), urban heat island reduction. The structural premium (~10-20% of roof framing cost for extensive) is recoverable through longer membrane life. Architects designing flat or low-slope residential roofs should evaluate green roof for at least part of the area.
Measurements
| Property | Operator | Value | Unit | Note |
|---|---|---|---|---|
extensiveDepth | min | 3 | in growing medium | Minimum depth for extensive green roof |
extensiveSaturatedLoad | min | 25 | lb/sqft saturated | Typical extensive saturated weight |
retentionRate | min | 50 | % annual rainfall retained | Typical extensive green roof annual retention |
Categories
Source
Solver enforcement
Browsable only — the solver does not currently enforce this directive (no spec-level data to check against). This entry exists so the architect personas can cite it in conversation and the user can read what the rule says.
Related directives
- Continuous load path from roof to foundation · HUD RSDG §2.4
- Residential structural reliability targets 1-in-100 to 1-in-1000 annual probability of failure · HUD RSDG §2.5
- Residential floor live load: 40 psf minimum (30 psf sleeping rooms) · HUD RSDG §3.4
- Wind load design uses ASCE 7 basic wind speed for the locality · HUD RSDG §3.6
- Ground snow load for Virginia: 25 psf eastern, up to 40 psf western mountains · HUD RSDG §3.7
Last reviewed 2026-05-15.