1. Wastewater System

In the past, balcony rainwater was discharged into the outdoor drainage system, but several issues were identified during actual use. As balconies are primarily spaces for outdoor activities and drying laundry, the following situations are common: placing washing machines on balconies or relocating them during renovations, yet without a wastewater pipe. Laundry wastewater is then directly discharged into urban waterways through the rainwater pipes, becoming a new source of pollution. Laundry wastewater contains a certain amount of nitrogen and phosphorus, which can easily lead to eutrophication of water bodies. It is necessary to pay attention to the discharge of washing machine wastewater in the drainage system. Therefore, the drainage of balconies equipped with washing machines should be connected to the municipal wastewater pipeline, not the rainwater pipeline. In other words, cast iron rain gutters should not accept domestic wastewater like laundry during the separation of rainwater and wastewater networks. A water trap should be installed under the balcony drain to act as a seal and protect against odors.

In cases where there is typically only one balcony, either north-facing or side-facing, with windows that are sealed, the balcony drainpipe should be designed as a laundry wastewater pipe, serving both as the balcony's rainwater drainage. No additional balcony rainwater stack or floor drain is required. The drainage then connects to the outdoor sewage pipeline. Only when the outdoor rainwater and sewage are combined can they be discharged into the combined sewage pipeline for intercepting treatment.

When the service balcony of kitchen fittings is equipped with a septic tank, the drainage of the balcony should not be combined with the kitchen's waste pipe. Although the drainage should eventually connect to an outdoor sewage pipe, the regular drainage from the balcony is uncertain, and the water volume is small. Once the floor drain dries up, odor may overflow from the drain. Article 4.9.12 of the "Design Code for Water Supply and Drainage of Buildings" proposes that the bottom of the balcony rainwater vent should be indirectly drained. This is mainly due to concerns that the balcony floor drain may not frequently and timely accept rainwater, and the water seal height cannot guarantee effective water seal strength. Measures are taken to prevent odor from the outdoor drainage pipe from spreading through the balcony. In cases where residential buildings have basements or surrounding open rainwater ditches, when the balcony rainwater is treated with indirect drainage, it is advisable to set up a water seal well outdoors or add a water trap to the drainage pipe leading out of the house. The water seal should maintain a sufficient distance from the water seal of the balcony floor drain to prevent the destruction of a water seal chain. Additionally, the courtyard rainwater pipe of the residential buildings can be directly discharged using a rainwater inlet equipped with an in-built water trap.

2 Discharge Volumes

Rainwater from balconies often utilizes ground drain systems, and instances of balcony rainwater flowing back into the living room have occurred multiple times in engineering designs. This issue is particularly prevalent in split-level (duplex) residential buildings.

Although regulations stipulate that the top-level balcony should be equipped with a canopy or awning not less than the size of the balcony, sometimes the canopy or awning on top-level balconies is positioned much higher than the balcony itself, especially in split-level residential buildings where the height difference may be two or three stories. In such cases, the canopy or awning often fails to provide effective rain protection.

3 Overflow Measures

The reasons for overflow on balconies include clogged drain traps and rainfall exceeding the design flow. Based on engineering design experience, two measures are proposed to address the balcony drainage overflow issue.

Firstly, the balcony floor structure has been lowered by 100-150mm, a method commonly used in residential buildings in southern China. This lowering of the floor acts as a flood retention and regulation mechanism, preventing rainwater from flowing back into the residential units even during heavy rainfall. Secondly, an X50mm overflow outlet has been installed on the side of the balcony to accommodate excessive drainage flow. This minor detail of adding an overflow outlet has resolved the issue of accidental activation of the washing machine, thereby avoiding any overflow from the balcony drainage into the indoor space.

According to the "Design Code for Push-up Houses" (GB50096-2011), waterproofing measures should be taken for canopy roofs and open balconies. Waterproof flanges should also be installed on the drainage floor drains of balconies to prevent interference between residents.

Air Conditioning Condensate and Building Facade Drainage

Although the drainage of air conditioner condensate water is not directly related to rainwater drainage, it can sometimes be related in terms of pipeline layout. Typically, the air conditioner's drainage is located outdoors, and the building's exterior wall pipeline layout must be coordinated. The condensate and defrosting water from air conditioners should be equipped with drainage pipes and discharged indirectly. This indirect discharge can be directed to open ditches, rooftops, rainwater outlets, etc. Due to the seasonal nature of this drainage, it is necessary to prevent the overall drainage pipe odor from connecting with the indoor environment. In difficult cases, the drainage can also be combined with the balcony drainage stack, and a water seal partition should be set. In a residential project designed overseas in Shanghai, the air conditioner drainage was pre-buried in the interior partition walls, which offers the advantage of easy connection for the air conditioner drainage pipes, and the refrigerant pipes do not need to be laid at lower levels indoors along with the drainage pipes. However, this layout must ensure the unobstructed drainage of all levels of branches, prevent the clogging of pre-buried pipes, and minimize factors that may lead to disputes with neighbors.

Residential exterior walls must also consider rainwater drainage issues. The design should integrate with the site's drainage, with a preferable method being to set up open ditches around the house, with rainwater collection inlets directing the water to the drainage network. Open ditches around the house are more effective than ground-level drainage, preventing rainwater accumulation at the residential entrance. The open ditches on the building's exterior ground level also facilitate indirect drainage for balcony rainwater and air conditioner condensate.