Green Roof

National baseline

Among Tier A manuals that mention green roofs (36 of 76 scanned), numeric guidance for green-roof-specific facility parameters is sparse: media depth appears in only two jurisdictions (both 3 inches), and maximum slope in only three (2–25 percent, with mode 2 percent). Jurisdiction-wide water-quality volume depth where cited clusters around 1 inch (n=30, mode=1, median=1, range 0.37–1.61 inches)—context for site sizing, not a green-roof media specification. Site-wide maximum drawdown times are commonly 48–72 hours (n=36, mode=72, median=48, range 4–120 hours). Seasonal high water table separation where stated is often 24–36 inches (n=36, mode=24, median=36, range 12–120 inches), again as general groundwater context rather than a roof assembly detail. For temperate, Type II–style assumptions, committees should treat green roof design as qualitatively supported in many manuals but numerically anchored mainly by broad jurisdictional stormwater sizing defaults until more manuals publish explicit roof-specific depths, slopes, and drawdown expectations.

Matrix summary

In Tier A manuals mentioning green_roof (36/76), site-wide WQv depth (n=30) modes at 1 inch (0.37–1.61 in); site-wide max drawdown (n=36) modes at 72 h with median 48 h (4–120 h); site-wide SHWT separation (n=36) modes at 24 in with median 36 in (12–120 in). Practice-specific green_roof_media_depth_inches is n=2 (3 in only); green_roof_slope_percent is n=3 (mode 2%, range 2–25%). These counts describe extracted numeric consensus, not an adopted national standard.

Cross-state parameters

Regional modifiers

Open issues

Green-roof-specific numeric baseline is not yet established: media depth is documented in only two manuals (both 3 inches, n=2); maximum slope appears in only three (2–25 percent, n=3). Forty-five of 76 Tier A manuals lack extracted WQv depth, and 34 of 36 green-roof-mentioning manuals lack media depth—so committees cannot treat mode values as national design standards. Site-wide WQv, drawdown, and SHWT fields must not be read as green-roof layer thickness or roof drain time without manual-specific green roof chapters. Wide drawdown range (4–120 hours) and SHWT range (12–120 inches) include chapter proxies and pond-oriented language that may not apply to extensive roofs. Tier A coverage is mention-weighted (36/76) rather than design-parameter-complete for this practice.