Terminal Roof Drainage System II

3. Stormwater Inlets

The performance of stormwater inlets directly determines the system’s startup efficiency, drainage capacity, and operational safety.

1) Construction and Functional Requirements: The stormwater inlet consists of a leaf guard, flow-straightening grating, anti-vortex device, and inlet body, and is designed to prevent leaf accumulation, straighten flow, suppress vortices, and prevent clogging. The inlet body is typically made of stainless steel formed through integral stamping, while the grating and anti-vortex device are usually made of silicon-aluminum alloy, offering corrosion resistance, resistance to aging, and a long service life.

2) Hydraulic Performance and Selection: The flow rate of the rainwater inlets should be determined based on the catchment area and the design rainfall intensity. The water level upstream of the inlet should be low and stable to ensure rapid siphon formation, reduce air entrainment, and improve system efficiency. Standard test bench testing must be conducted to ensure that their hydraulic performance meets the startup and operational requirements of the siphon system.

3) Layout and Quantity Configuration: At least one collection well should be installed in each drainage sub-zone gutter, with two rainwater inlets in each collection well to ensure drainage redundancy and system reliability. Rainwater inlets should be positioned at the lowest point of the gutter and designed with a two-way slope to ensure rapid water collection and siphon initiation.

4) Adaptability to Special Environments: In extremely cold regions, insulated or electrically heated rainwater inlets should be selected to prevent snow accumulation or ice formation from affecting drainage. In high-temperature and high-humidity regions, the corrosion resistance grade of materials must be enhanced to ensure long-term stable operation.

5) Coordination with roof construction: The connection between the rainwater inlets and the roof must be tightly sealed to prevent water leakage; the base of the inlet body should be welded or bolted to the roof deck, and a stainless steel clamping ring with a waterproof sealing structure should be installed to ensure structural stability and waterproofing performance.

6) Hydraulic Calculations and System Balancing: Each downspout connection pipe should form a separate section. The difference in head loss from different downspouts to the outlet should be controlled within 1.0 m to ensure system hydraulic balance. Pipe flow velocities should meet the following requirements: horizontal pipes ≥ 0.7 m/s, vertical pipes ≥ 2.2 m/s, with a maximum not exceeding 6 m/s, to prevent cavitation and scouring.

7) Rainwater inlets must be securely installed, and waterproofing measures must be implemented at the connection points with the roof to prevent rainwater leakage. A filter device should be installed at the inlet of each rainwater inlet to prevent debris from entering the drainage pipes and causing blockages. Drainage pipes, rainwater inlets, and other components should be clearly marked with labels indicating the direction and purpose of the pipes.

4. Key Points for Roof Drainage

1) Select an appropriate drainage method based on factors such as the terminal building’s roof structure, area, and rainfall volume. For large terminal roofs, a “siphon + overflow” dual-system should be adopted. Roofs of wings, main buildings, and hotels should be divided into independent drainage zones according to expansion joints, with each zone operating as a self-contained system to prevent cross-zone drainage.

2) Drainage Zones: Divide the roof into multiple drainage zones based on the roof structure and slope. Each zone should have an independent drainage system to prevent rainwater accumulation and overflow on the roof, ensuring smooth drainage. The drainage area is calculated based on the horizontal projection plus the side walls. When the slope of a metal roof is insufficient, use variable-section gutters or secondary slope plates to ensure the longitudinal slope of the gutter is ≥0.3%; When the roof slope is >2.5% and there is a risk of backflow, the gutter storage volume shall be verified based on a 2-minute design flow rate.

3) Drainage Slope: The roof shall have sufficient slope to allow rainwater to flow rapidly toward the drainage outlets. For curved roofs, the drainage slope may be determined by the ratio of the overall roof elevation difference to the horizontal projection distance along the drainage path, while also meeting the minimum drainage slope requirements. The minimum slope for flat roofs is 2%; in regions with heavy rainfall, it shall not be less than 3%; structural grading shall be ≥3%. The transition slope between pitched and flat roofs is 10%.

4) Calculation of drainage pipe diameter: Calculate the drainage pipe diameter based on factors such as rainfall intensity and catchment area to ensure that the drainage system can meet the drainage requirements during maximum rainfall, while also accounting for a safety factor for extreme weather conditions.

5) Drainage Pipes and Fittings: Drainage pipes should be made of materials that are corrosion-resistant, high-strength, and have a long service life. In highly corrosive environments, corrosion-resistant pipe materials should be prioritized. Drainage pipes should avoid crossing settlement joints, expansion joints, and deformation joints whenever possible. If crossing is unavoidable, compensating devices, such as bellows compensators, should be installed to eliminate the impact of relative displacement on the pipes and ensure their integrity and safety.
The drainage system should be equipped with vent pipes to balance the air pressure within the pipes, prevent the breakdown of water seals, and ensure smooth drainage. The outlet of the vent pipe should be located outdoors to prevent odors from entering the interior. On metal roofs, vertical grating perpendicular to the slope should be installed to reduce the impact of snow sliding into the gutter. The bases of equipment such as lightning rods, walkways, lighting fixtures, and solar collectors must not obstruct drainage paths; waterproofing membranes should be installed around the bases, and drainage slopes should be provided.

6) To prevent backflow and overflow, the outlet of the drainage pipe should be higher than the outdoor rainwater pipe, generally by no less than 100 mm, to prevent rainwater from flowing back during heavy rainfall. The drainage system shall be equipped with overflow pipes and outlets. In the event of a blocked drainpipe or rainfall exceeding the system’s design capacity, rainwater can be discharged through these overflow facilities to prevent ponding on the roof. The overflow piping system shall not discharge directly into the outdoor storm sewer network; instead, ground-level stormwater inlets may be installed outdoors to ensure safe discharge of overflow water.

7) Frost Protection and Snow Melting Measures: On terminal roofs in extremely cold, cold, or snowy regions, auxiliary snow and ice melting devices—such as electric heat tracing systems—should be installed at eaves and drainage gutters to prevent the accumulation of snow and ice that could block drainage outlets.

8) Maintenance and Inspection: Terminals should be equipped with maintenance access routes leading directly to the roof. Security measures should be installed at the entrances to these access routes, and the operational status of the drainage system should be inspected regularly.