*Do not require ground level space, so particularly well suited where ground space is limited or sites with large flat roofs
,*May not require additional sewer connections , *Easy to retrofit if structural support and waterproofing are adequate ,*Relatively low cost compared to green roofs (no vegetation requirements) ,*Potential for evaporative cooling and reduction of urban heat island effect .
*Must be regularly inspected and maintained,
*Limited application on sloped roofs or sites with small building footprints
,*Do not provide any co-benefits such as habitat creation or air quality improvements associated with vegetated practices ,*Mechanical systems and other rooftop architectural features must not be adversely impacted by ponded water .