from《Design code for water supply and drainage in buildings》GBJ15-88Start，my country has set requirements for overflow openings for roof rainwater drainage systems。《Design code for water supply and drainage in buildings》GB50015-2003Edition starts，Further clarified the design parameter requirements of overflow drainage system，That is, the total drainage capacity of gravity flow roof drainage works and overflow facilities of general buildings should not be less than10Annual return period rainfall；important public buildings、The total drainage capacity of roof rainwater drainage works and overflow facilities of high-rise buildings should not be less than their50Annual return period rainfall。Birds on the Tree Education Online Teaching of Water Supply and Drainage Design Teacher Yang。

Code-specified overflow facilities include overflow ports、Overflow weirs and overflow piping，Among them, the overflow port design is the simplest，Also the most widely used。In recent years，With the large-scale commercial complex、The proliferation of super high-rises，Especially the style of the curtain wall is changing with each passing day，The simple way of opening the overflow directly on the curtain wall is less and less accepted by the owners and architects，Therefore, the use of overflow piping has become the rainwater overflow solution for this type of building.。But about how to reasonably design the overflow piping system，Specifications and manuals are rarely covered，Many designers are also unable to start。The author combines several recent project design practices，Talk about the roof rainwater overflow pipe system（hereinafter collectively referred to as the overflow piping system）how to design。

one、Several types of overflow piping

Distinguish from the way of setting，The overflow piping system can be divided into three types：Decentralized、centralized、Outboard。

Decentralized The usual type is that rainwater risers and rainwater buckets are arranged in pairs，The normal rainwater bucket is installed on the bottom of the ditch，High installation of overflow rainwater bucket，The schematic diagram of the design is shown in Fig.1。Such as general building roof，5Annual return period rainwater is drained by normal rainwater buckets，Exceed5Annual return period rainwater is drained by overflow rainwater buckets。Such as important building roof，10Annual return period rainwater is drained by normal rainwater buckets，Exceed10Annual return period rainwater is drained by overflow rainwater buckets。

Decentralized The usual type is a catchment area with an overflow rainwater bucket，Rainwater is drained from normal rainwater buckets during the recurrence period，Rainwater that exceeds the return period is drained by the overflow rainwater bucket，The schematic diagram of the design is shown in Fig.2。

The usual type of the external discharge type is the deformation of the original overflow type.，That is, the original overflow is replaced with a customized side-entry rain bucket of the same size，Rainwater risers run down the façade or in voids in the curtain wall to the ground，The schematic diagram of the design is shown in Fig.3。

picture1 Schematic diagram of the design of decentralized rainwater overflow piping system

(brass overflow tube 8a)picture2 Schematic diagram of the design of centralized rainwater overflow piping system

picture3 Schematic diagram of design of external drainage rainwater overflow piping system

two、Design points of overflow piping system

for decentralized，The design point of the overflow piping system is to determine the size of the overflow pipe diameter and the installation height of the overflow rainwater bucket.。for centralized ，The design point of the overflow piping system is to determine the number of overflow rainwater buckets，Then the overflow amount is determined according to the catchment area burdened by the overflow rainwater bucket，Select rain bucket model and riser size based on overflow volume，Finally determine the installation height of the overflow rainwater bucket。For outboard，The design points of the overflow piping system are firstly based on the layout of the gutter.，Combine the facade at the same time、The design of the curtain wall determines the number and location of overflows，Calculate overflow based on the catchment area served by each overflow，Determine the size of the overflow port according to the calculation formula of rectangular weir flow，Select or customize side entry rainwater buckets according to the size of the overflow，Finally, choose the diameter of the rainwater riser according to the overflow。

roof rainwater overflow pipe

The amount of overflow rainwater per bucket，Can be based on known design return period，Return period required after adding overflow system，using normative4.9.2strip Calculate。For example, a design return period10Years of Building Roofing，After the overflow system is added, the design return period is not less than50year，Then its overflow rainwater volume is calculated as follows：