text: Passive blue roof systems control the rate at which water drains from a rooftop through non-mechanical means. Unlike active systems which inhibit water flow through drainage pipes, passive systems temporarily detain water on the surface of the roof by lengthening the path the water must take in order to reach outlet drains. Blue roofs can include open water surfaces, storage within or beneath a porous media or modular surface, or below a raised decking surface or cover. 
  * Roof‐integrated passive blue roof designs are built to retain water directly on a roof's surface, protected by a waterproof membrane, for extended periods of time. This ponding of water can be done either within a porous media, such as gravel, or free standing on the roof surface. The release rate of the stored water is controlled by weirs on the roof drain. Roof-integrated designs are most effective in new construction as achievable storage volume on existing flat roofs is often quite limited. 
  *Modular tray designs allow existing roofs to be retrofitted for stormwater retention capabilities with the addition of plastic or metal trays. Similarly to roof-integrated designs, water collected in the trays can either be ponded within a porous media or free standing within the tray. Modular tray blue roofs allow for more flexibility in the size and location of detention areas on a rooftop than a roof-integrated design. This selective placement of trays makes avoiding roof areas which cannot support the additional structural load, as well as any roof obstructions easier than other blue roof designs. Trays also have the added advantage of not using the roof material itself as a component of the detention structure and thus decrease instead of increase the hydraulic head on the underlying roofing membrane. As the water drains from the trays, it is released onto the roof surface itself and drains normally. 
  *Roof‐dams or roof‐checks physically interrupt the flow path of the water as it travels towards the roof drain. Similar to roof-integrated designs, the roof surface is the primary location of water detention with these impermeable or slow-releasing dams forcing water to pond behind them. The height of the dam and the size of weep holes can be used to control the detention time of the structures.
  *Blue-green roof designs are aesthetically similar to green roofs in that they are vegetative roofs, but functionally different in that they have additional water storage capacity beneath the growing media to facilitate in stormwater retention.