Selection criteria and design key points for hotel siphonic and gravity rainwater systems

Selection criteria and design key points for hotel siphonic and gravity rainwater systems

I am Daniel Wang, a water supply and drainage specialist. The choice between a siphonic rainwater system and a gravity rainwater system is a core decision in hotel roof rainwater drainage design, directly affecting building safety, drainage efficiency and later operation and maintenance costs. Based on GB50015-2019 "Standard for Design of Building Water Supply and Drainage" and years of hotel project experience, I will analyze in detail the selection conditions and design key points.

Selection Condition Analysis

1. Roof Area and Catchment Conditions

• Gravity system: Suitable for cases where the roof area is small (generally catchment area per outlet ≤500㎡), the roof slope is gentle and there is no obvious large-area catchment zone. For example, the podium roof of a boutique hotel (5 storeys) adopted a gravity system because the roof area was only 300㎡ and the slope was 2%, meeting the requirements of Article 4.9.6 of GB50015.
• Siphonic system: Suitable for large-area roofs (e.g., above 5000㎡), complex shapes (e.g., domes, connecting corridors) or when the number of risers needs to be reduced. For example, the lobby roof of a five-star hotel in Sanya has an area of 8000㎡; after adopting a siphonic system, the risers were reduced from 6 to 2, saving indoor space.

• A gravity system requires more risers, which may affect the appearance of the hotel lobby or facade. A siphonic system, through high flow velocity (about 3-6m/s) and negative-pressure suction, can use fewer risers (e.g., a single DN150 pipe can drain 1000㎡ of rainwater), making it especially suitable for hotel public areas (e.g., banquet halls, entrance canopies). In a Shanghai hotel project, because the facade required no exposed risers, the siphonic system hid the pipes within pipe shafts.

• A siphonic system is sensitive to the storm return period (P), usually designed for P=10 years (Article 4.9.5 of GB50015), but the local storm intensity formula must be verified. For example, a hotel in Guangzhou, due to an annual rainfall of 1800mm, adopted a design return period of 50 years for the siphonic system and added overflow outlets. A gravity system is more adaptable to uniform rainfall, but attention must be paid to the risk of gutter overflow.

Design Key Points

1. Gravity System

• Gutter and outlet spacing: Gutter slope ≥0.5%, outlet spacing ≤12m (Article 4.9.14 of GB50015). In a Beijing hotel, because the gutter slope was less than 0.3%, water ponding occurred, which was later solved by adding drainage channels.
• Number of risers: One riser per outlet, pipe diameter ≥DN100. Inspection openings shall be provided (on each floor or every other floor).
• Overflow measures: Overflow outlets or overflow pipes shall be provided, with overflow flow calculated for P=50 years (Article 4.9.9 of GB50015).

• Pipe hydraulic calculation: Dedicated software (e.g., FlowMaster) shall be used to calculate negative pressure values and ensure the system operates in full-pipe flow condition (flow velocity ≥0.75m/s). In a Shenzhen hotel, because the pipe slope was insufficient, the siphon failed, which was later solved by adjusting the pipe diameter (from DN125 to DN150).
• Gutter design: The gutter depth shall meet the submergence depth of the siphonic outlet (≥0.1m), and the width shall be calculated according to the catchment area (e.g., 0.5m width per 1000㎡). Anti-vortex devices (e.g., baffles) shall be provided.
• Suspended pipes and risers: Suspended pipe slope ≥0.005 (Article 4.9.26 of GB50015); energy dissipation devices (e.g., elbows or pressure-relief pipes) shall be provided at the bottom of risers. In a Chengdu hotel, insufficient energy dissipation caused riser vibration, which was later solved by adding elastic supports.
• Overflow system: Must be provided independently and not shared with the siphonic system. The overflow outlet height shall be 0.3m above the top surface of the siphonic outlet.

Practical Engineering Cases

• Case 1: Gravity system application

• Case 2: Siphonic system application

Concluding Summary