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. Blue colored roof A different type of "blue roof" has been proposed by researchers at the Lawrence Berkeley National Laboratory, who researched a pigment used by the ancient Egyptians known as "Egyptian blue." This color, derived from calcium copper silicate, absorbs visible light, and emits light in the near-infrared range, helping keep roofs and walls cool. See also Eco-village Energy-efficient landscaping Sustainable city Rainwater harvesting Rainwater tank Greywater References Environmental engineering Hydrology and urban planning Roofs Sustainable architecture Sustainable building