With the increasing frequency and intensity of extreme rainfall events caused by climate change, the risk of urban flooding continues to rise. In high-density urban areas such as Seoul, there are practical limitations to securing additional large-scale flood control infrastructure. As an alternative, the introduction of decentralized detention infrastructure utilizing unused rooftop space has gained attention. This study focuses on the Rooftop Detention System (RDS), a facility that temporarily stores rainwater on rooftops. While existing rooftop drainage systems are designed to discharge rainwater immediately, the effectiveness and expansion potential of RDS depend on technical feasibility, public acceptance, structural safety, and legal and institutional support. Based on Seoul’s pilot projects, this study presents a comprehensive analysis of the structural validity, legal improvement plans, and strategies for expanding RDS adoption.
Major cities overseas are actively introducing blue roofs and smart roof systems to mitigate the limitations of impermeable surfaces and existing flood control systems. These facilities have demonstrated multidimensional effects, including reducing surface runoff, alleviating urban heat island effects, and enabling landscape irrigation. Leading examples from cities such as New York, Toronto, and Amsterdam highlight the policy and technical viability of rooftop detention systems, contingent upon structural safety and waterproof performance. While similar facilities can be interpreted within the scope of Korea’s “Water Reuse Act” and “Disaster Prevention Act,” detailed installation standards and support clauses remain insufficient. A simulation conducted in the Gangnam area of Seoul showed that introducing RDS could reduce the maximum inundation area by approximately 3.4% and the inundation depth by about 3.2%, thereby confirming its technical effectiveness.
The study further analyzed the structural and waterproofing impacts of storing up to 10 ㎝ of rainwater on rooftops. When rainwater is stored on rooftops, an additional water load of approximately 100 kgf/m² is applied, which may lead to deformation of the structure or damage to the waterproofing layer. An analysis of common construction defects in Korea revealed that waterproofing issues account for over 30% of all cases, with rooftops particularly vulnerable due to exposure to external environmental factors. Field inspections at Seoul National University and Seoul National University of Education confirmed that the state of waterproofing and structural degradation are critical determinants of RDS applicability. Hence, institutionalizing pre-installation assessments of structural safety and waterproofing performance is necessary.
Additionally, this study reviewed construction and maintenance standards to ensure the stability of buildings when implementing rooftop detention. Six waterproofing materials commonly used in Korea—including asphalt-based, polymer sheets, coatings, and hybrid systems—were compared, and polyurethane-based coatings were found to be especially suitable for RDS. Proper quality control during construction and the use of acrylic or epoxy-based repair materials, along with regular inspections, were emphasized. Institutionalizing the quality assurance and post-installation maintenance of waterproofing is crucial for securing trust in broader private-sector applications.
Public and expert perception surveys were also conducted to assess the acceptance of RDS. More than 70% of citizens agreed on the need for such systems, with a high level of understanding after explanation. However, building owners showed lower acceptance due to concerns about leaks and structural safety. Experts and public officials positively assessed the system's flood mitigation effectiveness but identified “ensuring structural safety” as the most critical prerequisite. Effective incentive measures include subsidies for waterproofing, reductions in water bills, and maintenance support, and these should be combined with streamlined administrative procedures and tailored communication strategies.
Although Seoul has established a legal basis for rainwater management through the “Basic Ordinance on Water Circulation Restoration and Low-Impact Development” and the “Local Subsidy Management Ordinance,” rooftop detention systems are not explicitly specified. Therefore, amendments are needed to clearly define RDS and support installation and maintenance costs through detailed implementation rules. A tiered subsidy system based on building size and age is recommended, with grants preferred over loans. Technical support should include structural and waterproof assessments, reinforcement during installation, and periodic post-installation inspections. Additional policy tools, such as building regulation exemptions, recognition of landscaped areas, and incentives through green building certification, should also be introduced. Furthermore, revisions to the “Disaster Prevention Act” and “Water Reuse Act,” along with municipal ordinance reform, are essential for institutionalizing rooftop detention systems. Comprehensive outreach and promotional campaigns will also be required to encourage expansion into the private building sector.