Guidance

Blue roofs function primarily for stormwater attenuation. Refer to CIRIA C753 Ch 13 for detailed principles.

Structural Assessment: CRITICAL. Roof structure must support dead load (system components, ballast if used) PLUS maximum temporary stored water weight (calculated based on design storm depth and area) + live loads.

Waterproofing: High-performance, robust waterproofing membrane essential, zero-falls often preferred. Integrity testing (flood test) highly recommended before overlaying system.

Storage Volume: Calculated based on required attenuation volume for design rainfall event (e.g., 1 in 100 year + climate change allowance). Storage achieved via void-forming units (crates, gravel), weirs/baffles, or surface ponding.

Restrictor Device: Outlet includes flow control device (e.g., orifice plate, vortex control) designed to limit discharge rate to agreed maximum (e.g., greenfield runoff rate).

Drainage Layer/Void Former: Layer above waterproofing creates storage volume (e.g., modular crates, gravel layer, drainage/reservoir boards). Must allow water flow to outlet.

Filter Layer: Geotextile above drainage layer prevents fines/debris blocking void/outlet, especially if overlaid with green roof substrate or paving.

Overflow: Essential safety feature to manage rainfall exceeding design capacity, safely discharging excess water.

Overlay: Can be finished with ballast, paving (on supports), extensive or intensive green roof (requires integration of layers - see relevant pages). Finishing layer must allow rainfall to reach storage void.

Maintenance: Requires regular inspection/maintenance of outlets, flow restrictor, overflow, and void (if accessible) to ensure no blockage. Check waterproofing integrity.